Copyright: Copyright 1992-2000 © Software Bisque, Inc.
Citation: Copyright 1992-2000 © Software Bisque, Inc.


Welcome
Welcome to Patterns in TheSky

This tutorial is to be used in conjunction with TheSky astronomy software.  It is intended for users who want to further their understanding of basic astronomy concepts.  Many concepts and the related terminology are covered in this tutorial.  To reinforce the understanding of a concept being presented, step-by-step instructions are provided on how to illustrate the concept using TheSky.  These illustrations are referred to as demonstrations throughout the tutorial.

The field of astronomy is both exciting and accessible.  When learning any field of knowledge such as astronomy, you must become familiar with new concepts and new terminology. Computer software provides a unique method for teaching and this is especially true for astronomy.

In the past, the computational power required for computing planetary positions was extremely burdensome, and machines capable of performing these calculations were not available to the average person. Today, high-power computer processing is readily available, giving many more people access to powerful tools for learning about the dynamics of our Solar System. Increased storage capability in the form of hard disk and CD-ROM drives allows programs like TheSky to include information on millions of stars and deep sky objects, accessible at the click of a button.

If you have a personal computer coupled with TheSky astronomy software, you have an extremely effective tool for learning astronomy. Add some actual nighttime observations, and you will be able to create a solid foundation for understanding the heavens, an understanding that all great scientific minds have labored to achieve.


Celestial Objects
Introduction to Celestial Objects

When you look up into the night sky, especially under dark skies away from city lights, you can see thousands of objects, mostly stars.  Interestingly, some of the brighter objects are not stars at all, but rather planets.  Other than being able to identify the Sun and Moon, most people cannot differentiate between the various celestial objects visible in the sky.

If you have a little bit of knowledge about the motion of the solar system objects, and the ability to recognize some of the brighter constellations, you will be able to answer many questions often asked at nighttime gatherings with friends.  With a small amount of practice, you can become the astronomy "expert" within your circle of friends.

The Brightness of Celestial Objects: Magnitudes

Objects that we view in the sky each appear to have a different level of brightness.  During the day, the Sun is so bright we dare not stare directly at it.  In the night sky, the Moon is usually by far the brightest object visible, followed by bright planets, and then stars and other objects such as star clusters, nebulae and galaxies.

Modern astronomers have defined a magnitude scale to quantify the difference in brightness between various objects.  Because the scale is logarithmic (an increase of one in the magnitude means an increase of 2.5 in brightness), it is difficult to approximate a magnitude by simply looking at an object.  It is easier to learn the magnitude of a few common objects and use these for comparisons.

If two objects differ by 5 magnitudes, their intensity differs by 100 times.  For example, a magnitude one star is 100 times brighter than a magnitude 6 star.  It follows that a single magnitude represents about 2.5 times difference in intensity (we get this number by taking the fifth root of 100, which is equal to about 2.51189).  A magnitude 2 star is 2.5 times as bright as a magnitude 3 star.

The brighter the object, the lower the magnitude! The brightest star in the sky (other than the Sun) is Sirius in the constellation Canis Major, with a magnitude of -1.6.  The faintest stars visible to the unaided eye under dark skies are about magnitude six. Polaris, the North Star has a magnitude of 2.1.  Small telescopes allow you to see objects as faint as 13th or 14th magnitude.

Venus, Jupiter and Mars all become very bright at certain times and can be many times brighter than the brightest star, Sirius.

Visual Object(s)	Approximate Magnitude
Our Sun	-26.0
Venus	Variable, as bright as -4.4 (sometimes visible during the day)
Brightest Star (Sirius)	-1.60
Polaris, the North Star	2.1
Dimmest object which can be seen with the unaided eye	6th magnitude, about 2500 stars.
The SAO catalog of stars	Stars to 9th magnitude, about 260,000 stars.
Small Telescope	14th magnitude, millions of stars.
Hubble Guide Star Catalog (catalog created for use with the Space Telescope.)	Objects to magnitude 14.5 (20,000,000 objects.)
U.S. Naval Observatory Catalog (USNO)	Stars to 21st magnitude, about 500 million stars.


Planets

The planets are large spherical masses that orbit the Sun.  Our solar system contains nine planets that are bound by the Sun's gravity.  Mercury and Venus are called the inner planets because their orbits are inside the Earth's orbit.  The remaining planets (Mars, Jupiter, Saturn, Uranus, Neptune and Pluto) have orbits outside the Earth's orbit and are called the outer planets.

Inner and outer planets should not be confused with the inner and outer solar system.  The inner solar system contains Mercury, Venus, Earth and Mars, whose orbits are grouped tightly near the Sun as compared to the orbits of the remaining outer solar system planets.

When viewed with the naked eye (an eye unaided by binoculars or telescopes), planets resemble stars in the night sky, but a closer look reveals important differences. If you compare a planet with a nearby bright star, you will see that planets do not "twinkle" like stars do.  Also, the positions of planets change continuously when compared to the background stars that remain fixed year after year.  The word planet comes from the Greek word planes meaning “wanderer."

When observed through a telescope, the planets have an apparent size whereas stars appear as single points of light. For example, Jupiter has an apparent angular diameter of about 30 arc seconds.


Planetary Arrangements

The following figure shows the terminology for some of the unique positions that inner and outer planets can have with relation to the Sun.

Planetary Arrangements - view directly North of the Sun. (Note: Solar system objects are not to scale.)

If Mercury or Venus is directly between the Earth and Sun, it is said to be in inferior conjunction.  When Mercury or Venus is on the opposite side of the Sun, we say that it is in superior conjunction.

The angle between the Sun and a planet from an Earth-based perspective is called the planet's elongation.  At maximum eastern elongation, Mercury or Venus is as far east as it can be and is visible in the evening sky.  At maximum western elongation, they are visible in the morning hours.

When an outer planet is behind the Sun, the planet is at conjunction (its elongation is 0 degrees).  When it is exactly opposite the Sun in the sky, it is at opposition (its elongation is 180 degrees).  The best time for observing a planet is when it is at opposition, since it is far away from the Sun (that is, near 180 degrees away from the Sun), and it will be high in the night sky.

Important Planetary Data:

Planet	Equatorial radius (km)	Mass/Earth mass	Surface Gravity	Sidereal Rotation	Tilt of orbital axis
Venus	6,052	0.815	0.89	243 days-R	177.3
Earth	6,378	1.000	1.00	23hrs56min	23.45
Mars	3,397	0.107	0.38	24hrs37min	25.20
Jupiter	71,398	318	2.54	9hrs 50min	3.12
Saturn	60,000	95	1.07	10hrs 40min	26.7
Uranus	26,145	14.6	0.80	12to24 hrs	97.9
Neptune	24,300	17.2	1.2	18hrs	29.6
Pluto	1,150	0.002	Unknown	6days 9hrs	118?
Planet	Mean Distance From Sun (au)	Orbital Period (Years)	Eccentricity	Inclination to the ecliptic (degrees)	Average Density (g/cc)
Mercury	0.39	0.24	0.206	7.0	5.4
Venus	0.72	0.62	0.007	3.4	5.2
Earth	1.00	1.000	0.017	0.0	5.5
Mars	1.52	1.88	0.093	1.8	3.9
Jupiter	5.20	11.86	0.048	1.3	1.3
Saturn	9.54	29.46	0.056	2.5	0.7
Uranus	19.18	84.01	0.047	0.8	1.2
Neptune	30.06	165	0.09	1.8	1.7
Pluto	39.44	248	0.250	17.2	1

The orbits of the inner four planets are crowded close to the Sun when compared to the widely space orbits of the outer planets.  Most of the planets’ orbits are nearly circular, that is they have a very small eccentricity.  The eccentricity of a circle is zero, and looking at the above table you can see that the eccentricities of most of the planets are nearly zero.  Mercury and Pluto are the exceptions.

The Inner Solar System.

The planetary orbits all lie within nearly the same plane, the plane of the ecliptic.  We call the angle between the plane of the planet's orbit and the plane of the ecliptic the inclination of the orbit.  By definition, the inclination of the Earth's orbit is zero since the Earth's orbit defines the plane of the ecliptic.  The graphic of the outer solar system below shows the highly inclined orbit of Pluto.

Two views of the Outer Solar System

The Outer Solar System

The figure above shows two views of the outer solar system.  The first view is from directly north of the Sun, and the second is an oblique view.  These graphics of the outer solar system illustrate two interesting points about the orbit of Pluto.  In the upper view you can see that its orbit is more elliptical than any other planet, so elliptical, in fact, that Pluto is sometimes closer to the Sun than Neptune.  In the lower view you can see that the orbit of Pluto is more steeply inclined to the plane of the ecliptic (with an inclination of 17 degrees) than any of the other planets.  The grid in the above graphic represents the ecliptic plane (the plane defined by the Earth orbiting the Sun).


Comets

In ancient times, comets were only noticed when they became very bright so that they were visible to the naked eye. Today most are discovered with telescopes, either visually or with photographs, while they are still very faint. Interestingly, most discoveries are not made at large observatories by professional astronomers, but rather amateur astronomers who systematically "sweep" the sky with low power telescopes or binoculars. Several comets are discovered each year, but rarely do they become bright, impressive objects.

Comets are thought to be made up of dirty ices, mostly water and carbon dioxide and are only a few dozens of kilometers in diameter.  When a comet is near the Sun, its ice begins to vaporize to form a tail of gas and dust which points away from the Sun. The head of a comet, or the coma, is an immense dust cloud that can become many times the size of the Earth. The actual nucleus of the comet is typically so small that it cannot be resolved with a telescope. The vaporizing effect from the Sun makes them appear larger, and enables us to see them.  The tail of a comet beams from the coma and on the largest tails can extend up to one astronomical unit in length. Seen from Earth, the tails can span up to 30 degrees in the sky.

Periodic comets are those comets which, like the planets, have elliptical orbits bound to the Sun. Perhaps the most popular comet of all time is the periodic comet Halley's, which returns every 76 years. During its return in 1986, the expectations of a large, spectacular comet were not met because the comet did not get as close to the Earth as it had during prior returns and it remained in the southern sky.

Comet Hale-Bopp on the other hand, which was discovered in 1995, surprised many astronomers by its spectacular brightness before it came near the Sun.


Minor Planets

Minor planets are relatively small chunks of rock drifting between the orbits of Mars and Jupiter. They are also known as planetoids or asteroids. The first minor planet was discovered on January 1, 1801 by Giuseppe Piazzi, and was later name Ceres. Ceres has a diameter of about 1,000 kilometers. Although Ceres is the largest of the minor planets, it is considerably smaller than the Earth's Moon, which has a diameter of about 3,500 km. Today there are about 4,000 minor planets on file which have accurately known orbits, and some 6,000 more which have poorly known orbits.


Stars

Stars are large masses of intensely heated gases that shine by their own light.  We have gained a vast amount of knowledge about stars by studying our own star, the Sun.  We study the light received from the Sun to learn its physical characteristics.  Because of its proximity to Earth, the light received from the Sun is billions of times brighter than that from other stars.

Even though the light from a distant star is faint, it contains a wealth of information about the star.  By examining the frequency content of the light, we can determine the chemical makeup and temperature of the star.  This is the science of spectroscopy, and it is one of the most powerful tools of the astronomer.


Galaxies

There seems to be a sort of cosmic togetherness in the known universe.  We find that stars are usually clumped together with other stars, billions of other stars.  These immense groups of stars are called galaxies.  The Sun is one of an estimated hundred billion stars in our own galaxy, the Milky Way galaxy.

Galaxies are not randomly scattered across the universe but are grouped into clusters.  The Milky Way galaxy is part of the Local Cluster, which also contains the Large and Small Magellanic Clouds, and the Andromeda Galaxy.  The Large and Small Magellanic Clouds are nearby companions of the Milky Way galaxy.  Each is visible with the naked eye at southern latitudes.  This Local Cluster of galaxies is further a member of the Local Super Cluster.


Nebulas

A nebula is an immense cloud made of dust and gas. Nebulas do not create their own light. Some are made visible by the light from neighboring stars (called emission and reflection nebulas), while others can be seen because of the light they block from more distant stars (called dark nebulas). When viewed through a small telescope, these objects appear as fuzzy patches of light.  Dark nebulas appear as black areas absent of starlight in an area where otherwise many stars are present.


Clusters

Star Clusters are groups of stars that were formed at the same time. There are two basic types of star clusters: open clusters and globular clusters.

Open clusters usually are contained in a region less than 30 light years across. Some Open Clusters are clusters by appearance only (they just happen to be optically aligned, the stars were formed at different times).

A Sky Chart of the Pleiades open star cluster in Taurus

Globular clusters may contain hundreds of thousands of stars of the same origin.  The name Globular is given due to their globe-like appearance.  Globular cluster M13 in Hercules is one of the most impressive examples of a globular star cluster.  Because of its brightness, it is a good target for small telescopes.

Globular cluster M13


Hipparcos and Tycho Catalogs

In 1980 the Hipparcos Space Astrometry Mission set out to compile an extremely accurate database of stellar measurements, including position, parallax and proper motion.

The Hipparcos Satellite, short for High Precision Parallax Collecting Satellite, was launched in August 1989.  It spent three years orbiting the earth, constantly scanning the celestial sphere, collecting data and sending it back to earth.  Without interference from earth’s atmosphere, the satellite was able to measure stellar positions with unprecedented accuracy.

The resulting Hipparcos catalog contains 120,000 stars accurate to one milliarc-second. A secondary catalog, coined “Tycho” after the ancient astronomer, consists of more than one million stars with measured positions to about 30 milliarc-seconds.

These stellar catalogs are incorporated into TheSky and are used to accurately determine the positions of other stars using astrometry and show the motions of the stars over long periods of time.

Related Topics:

Hipparcos Mission Information


SAO Catalog

SAO is an acronym for Smithsonian Astrophysical Observatory.  This catalog gives a reasonably complete list of stars to magnitude 9.0.  The complete catalog contains 258,997 stars.  The original catalog was compiled for purposes other than generating computer star charts so there are some attributes of the catalog which are not desirable for this purpose.  For example, in some parts of the sky, the magnitude limit was raised to 10th magnitude due to an absence of 9th magnitude or brighter stars.  Nonetheless, it has become somewhat of a standard and therefore remains a good choice from available stellar catalogs.


NGC Catalog

The New General Catalog provides a list of most of the brightest non-stellar objects that are frequently observed in the sky.  The catalog contains 7,840 galaxies, clusters, nebulae, double stars and other deep-sky objects.  The list was originally compiled by J.L.E Dreyer and first appeared in 1888.  A considerable number of these objects (in the hundreds) have not been verified as being non-stellar objects.  Therefore, many NGC objects are of the type unknown or double and triple stars.


Messier Catalog

The Messier objects are some of the most spectacular deep-sky objects in the sky.  Charles Messier (1730-1817) was the first to compile an extensive list of nebulae and star clusters, and the Messier designation has remained a very popular means of identifying these objects.  The Messier objects are numbered from 1 to 110, though a couple of them were found to be repetitive.  Most Messier objects are included in the NGC catalog and therefore have a corresponding NGC number.  Placing an M prior to the catalog number usually references them.  For example, the Great Nebula in Orion (NGC 1976) is also known as M42.


The Scale of the Solar System

Although the dimensions of our solar system are quite small when compared with the scale of even nearby stars, distances from one planet to another are difficult to comprehend.

Our close neighbor, the Moon, averages about 385,000 km (239,000 miles) from the Earth.  The diameter of the Earth is about 12,900 km so the circumference is 12,900 times pi (3.14159) or about 40,000 km. This means that the Moon is about "10 trips around the Earth" away.

The average Earth-Sun distance is called an astronomical unit (au) and is equal to about 150,000,000 kilometers (93,000,000 miles).  At 300,000 km per second, light takes over 8 minutes to travel from the Sun to the Earth (one astronomical unit, or au). Neptune, which is currently the furthest planet from the Sun at about 30 au, is over four light-hours (the distance light travels in one hour) from the Sun.

Inter-Stellar Scales

To put the solar system dimensions into perspective with inter-stellar dimensions (inter stellar means "between stars"), assume for a moment that the distance between the Earth and Sun is one meter. At this scale, Mercury is about 30 centimeters from the Sun and Neptune is about 30 meters away. The diameter of the Earth is a few thousandths of a millimeter. The entire solar system would fit inside a 60-meter circle.

At this scale of one meter equals one astronomical unit, where would the closest star (other than our star the Sun) Alpha Centauri, which is about 4.3 light years away, be?  First, lets compute the number of seconds in one year:

Light travels at 300,000 kilometers per second so one light year can be calculated as follows:

So Alpha Centauri is about 41 trillion km (4.3 light years x 9.5 trillion km/light year) from our solar system. If we divide this huge number by one astronomical unit in meters, (150,000,000 km/au x 1000 m/km) we get 271 kilometers, that is, the number of kilometers to the nearest star at our scale of one meter equals one astronomical unit.  To put this into perspective, if our solar system was the size of a small yard (60 meters diameter) our nearest stellar neighbor would be 271 km away! It’s almost enough to make you feel lonely.


Demonstration: View of the Solar System

1.	On the View menu, click 3D Solar System Mode.

2.	On the View menu, click Filters.  From the Filters dialog box, click the Planets button then mark the selected check boxes.

3.	Click OK.

This view shows the positions of the planets relative to the Sun for the specified date.  When viewing the entire solar system the inner solar system objects (Mercury, Venus, Earth and Mars) are all so close to the Sun that they appear to plot right on top of the Sun.

Click on any planet in the solar system view to identify it.


Demonstration: Moon and Sun Distance from Earth

1.	On the Edit menu, click Find.

2.	In the Find box, type Moon.

3.	Click Find.

When the Object Information dialog box is displayed, click the Distance button and note the current Earth-Moon distance.  Now repeat the above steps for the Sun. The Earth-Moon distance changes continuously due to the elliptical orbit of the Moon around the Earth and has a cycle time of about 28 days. The Earth-Sun distance also changes due to the elliptical orbit of the Earth around the Sun, and has a cycle time of one year.

In which month is the Earth closest to the Sun?  In which month is it furthest?


Angular Measurement

When we describe the position of a body or the apparent distance between two bodies on the celestial sphere, normal distance terms such as feet or miles will not work.  We must express these positions and distances in angles.

Angles are typically measured in degrees, and one complete circle is 360 degrees. Degrees are further divided into minutes, and minutes into seconds. There are 60 minutes in a degree, and 60 seconds in a minute. This can become confusing since degrees can be expressed in decimal form or degrees, minutes and seconds. For example, 5 degrees 20 minutes is the same as 5.33333 degrees.

When you look into the night sky, objects appear across the entire sky. It is easy to estimate the angular distance between an object just rising due East and another just setting directly West because we know objects at opposite parts of the sky are 180 degrees apart.  When objects are not near known positions in the sky, estimating the angular distance between them is not as easy. Knowing the angular separation of a few common objects in the sky can help you get a feel for estimating angles.

Examples showing the angular separation/size of some common objects


Demonstration: Angular Measurement

1.	On the Edit menu, click Find.

2.	Enter the star name Dubhe in the Find box.

3.	Repeat steps 1 and 2 for the star Merak.

4.	Click the Angular Separation button on the Object Information dialog box.

Whenever two objects are identified in succession, TheSky computes the angular separation between them and displays the result in the Object Information dialog box.

When the Object Information dialog box is displayed for the star Merak, the angular distance from Dubhe is displayed.  You will see that the angular separation between the two “pointer stars” in the big dipper (Dubhe and Merak are called the pointer stars because they point to Polaris, the North Star) is 5 degrees, 22 minutes and 27 seconds. (Note: The exact angular separation between stars may change over time due to stellar precession.  The above measurement is for December 1998.)

You can find the angular separation between any two objects on the Virtual Sky using this technique.


Demonstration: Angular diameter of the Sun and Moon

1.	On the Data menu, click Site Information, then click the Date and Time tab.

2.	Uncheck the Use Computer’s Clock option and enter a date of July 11, 1991, any time of day.

3.	On the Tools menu, click Planet Report to display information on the planets in a tabular form. Note the apparent angular diameter of the Sun is equal to 31 minutes and 28 seconds (remember, one half of one degree equals 30 minutes).

4.	Scroll to the information displayed on the Moon. Note the apparent angular diameter of the Moon is slightly less than 34 minutes.

5.	Click Close.

July 11, 1991 is the date of one of the best total solar eclipses of the century. Total solar eclipses occur when the apparent angular size of the Moon is greater (in this case it was about 2.5 minutes or about 8% greater) than the apparent angular size of the Sun. This permits the Moon to completely cover the Sun when passing in front of it. If you look at the Earth-Moon distance when this eclipse occurred, you see that the Moon is near perigee, its closest approach to the Earth, making its apparent size larger than average.

If you change the date to January 4, 1992 and observe the relative diameters of the Sun and Moon on this date, you will see that the Sun is slightly larger than the Moon, so the eclipse that occurred on this date is an annular eclipse. See the solar eclipse section for more information on annular and total eclipses.


Constellations

The celestial sphere is divided into various sections much the same as countries are divided into states. We call these sections in the sky constellations.  Every object in the sky lies within one of the 88 constellations. The most popular constellations are those near the celestial equator known as the zodiacal constellations.

In ancient times, patterns of bright stars were seen to resemble various creatures and were therefore given names and lore.  For the modern astronomer, a constellation is simply a specific area of the sky. When we state the name of an object, we usually append the name of the constellation to indicate where it is.  In some cases, such as "the Great Nebula in Orion" or the "Andromeda Galaxy", the constellation name is the unique part of the description, for there are many nebulas and a great many more galaxies.


Asterisms

Some of the more common groups of stars in the sky are not constellations but have nevertheless become very popular. These groups are called asterisms.  The Big Dipper, for instance, is part of the constellation Ursa Major, the bear, and is one of the most widely recognized groups of stars in the sky. Other examples of asterisms include the Little Dipper in Ursa Minor, the Teapot in Sagittarius and the Northern Cross in Cygnus.


Constellation Figures
Constellation Lines

On the Virtual Sky, you see various patterns of bright stars connected by lines. These “constellation lines” are helpful when you are trying to orient yourself relative the stars and can help you identify stellar patterns.

Sample star chart showing constellation lines for Sagittarius.

Note: The "stick figures" you see when you display the constellation lines are by no means authoritative or "cast in stone."  They were culled from various books and magazines on astronomy but there are many published variations.


Demonstration: Constellation Figures and Boundaries

1.	On the Orientation menu, click Zoom To then click Wide Field 180°.

2.	On the View menu, click Reference Lines.

3.	In the General Lines group, mark the Constellation Figures and Constellation Boundaries check boxes.

4.	Click OK.

The constellation boundaries and figures are now shown on the Virtual Sky.


Constellation Boundaries

The lines in the following star chart are the constellation boundaries.  In the early part of 1900, Eugene Delporte authored a paper in which he divided the sky into areas based in part on accepted constellation figures.  These boundaries are still in use today.  Note how the boundaries divide the sky into regions and how these regions indicate what constellation each object lies within.

Sample star chart showing constellation boundaries and lines


Coordinates on Earth

The coordinate system used to identify a particular point on Earth is based in part on the axis of rotation of our planet. The North and South Poles are located at the intersections of the axis of rotation and the earth’s surface.   These two unique points on a rotating sphere define the orientation of the longitude and latitude coordinate system.

See also:

Latitude

Longitude


Latitude

In the latitude-longitude coordinate system, we define the North Pole as 90 degrees latitude and the equator as 0 degrees latitude.

The Latitude and Longitude Coordinate System


Longitude

Longitude lines run perpendicular to latitude lines and divide the Earth's sphere into 360 degrees. Usually, longitude is specified from 0 to 180 degrees East or West of Greenwich, England (Greenwich is positioned at 0 degrees longitude).  In contrast to latitude lines, which are parallel, longitude lines are furthest apart at the equator and slowly come together towards the poles.

The Latitude and Longitude Coordinate System


Horizon Coordinates
Horizon-based Coordinates

The horizon-based coordinate system is used to specify the location of objects overhead. When you step outside your home and look into the night sky, the celestial objects you see might be low in the eastern sky, high in the southern sky or directly overhead. In a general sense, each of these location descriptions is based on the horizon coordinate system.  In the horizon-based coordinate system, object positions in the sky are described using two coordinates, altitude and azimuth.


Altitude

The altitude of an object is expressed as the number of degrees from the horizon (the ground at a far distance) to the object, and is always between minus 90 and plus 90 degrees.  Objects that have a negative altitude are below the horizon.  For example, the Sun just after sunrise is close to the horizon so it has an altitude of a couple of degrees, and just after sunset its altitude is minus one our two degrees.


Azimuth

The azimuth of an object is generally reckoned from North, increasing in the clockwise direction, and ranges from 0 to 360 degrees. North is 0 degrees, East is 90 degrees, South is 180 degrees and West is 270 degrees.


Demonstration: Understanding the Horizon Coordinate System

1.	On the Orientation menu, click Look, then click South.

TheSky displays all of the bright celestial objects (stars, star clusters, galaxies and nebulas) oriented so that the local horizon line is placed near the bottom of the computer screen, just like you see the horizon of the Earth at your location.

A line representing the horizon (0 degrees altitude) as it is currently positioned in relation to the celestial sphere is displayed for reference purposes. Note that all objects above the horizon line are objects that are visible at the current time while those below are not. As time progresses, objects rise in the East and set in the West.

If you scroll the screen to the left and right, you will see the various planets, and the Sun and Moon plotted on the screen. Note the position of the Sun and compare it to where you might expect it to be at the current time of day.

Sample display using Horizon coordinates, looking South.


Equatorial Coordinates

The horizon coordinate system (altitude and azimuth) is not convenient for specifying the location of celestial objects because the horizon coordinates of stars and other objects are continuously changing with time (due to the rotation of the Earth).  For example, at Sunrise, the Sun is near zero degrees altitude in the East, but a short 6 hours later it is high in the sky, with a completely different altitude and azimuth.

The exception occurs with objects near the North and South celestial poles. These are unique since they are close to the axis of rotation of the Earth and therefore move only in small circular paths. For example, Polaris, the North Star remains at a nearly constant altitude and azimuth. All celestial objects that are not near the poles change position from hour-to-hour.

In the equatorial coordinate system, the coordinates of all celestial objects remain fixed* from hour-to-hour, day-to-day and so on.  An object's equatorial coordinates remain the same regardless of from where on Earth the object is viewed.  This allows us to create star maps that apply to any place on Earth, or publish the anticipated position of an upcoming comet so that astronomers everywhere know where it is located among the stars.

* Equatorial coordinates change over long periods of time due to precession (wobbling of the Earth).  TheSky computes this change in stars’ position for the input date and time. Precession, however, does not change the relative positions of objects with respect to one another.

The equatorial coordinate system used to specify the positions of celestial objects is directly analogous to the latitude-longitude coordinate system used on Earth.  In fact, if you were to expand the latitude and longitude grid of the Earth so that it is out beyond all stars, you would have a sphere with identical geometry to the celestial sphere.

We suppose that all stars and deep-sky objects are located on a very large sphere (out beyond all stars).  We call this the celestial sphere. For purposes of describing the positions of celestial objects, we consider all stars and deep sky objects to be on the celestial sphere, when actually they are all positioned at varying distances from the Earth.

The geometric relationship of Latitude-Longitude to Declination-Right Ascension.

Related Topics:

Right Ascension

Declination


Declination

The Declination lines on the celestial sphere are similar to the latitude lines on Earth, ranging in value from -90 degrees to +90 degrees.  The "declination" of an object is the angle measured from the celestial equator (0 degrees declination) along a meridian line through the object. Polaris, the North Star has a declination of 89.26 degrees so it is very close to the North Celestial Pole. Mintaka, the western-most star in Orion's belt has a declination of about 0 degrees 17 minutes south so it is very near the celestial equator.

Star chart of Orion's belt near 0 degrees Declination

Objects with a declination below the latitude of the observer less 90 degrees will never rise at that latitude. For example, if you live at 40 degrees North latitude, objects with a declination below minus 50 degrees (40 minus 90) will never rise. If you are located at 90 degrees latitude (the North Pole), you will never see any objects with a negative declination.

We have defined the declination of stars to be the angle measured from the equator, but we need a second coordinate to completely state the positions of celestial objects.


Right Ascension

The geometry of the right ascension (RA) lines on the celestial sphere is identical to the longitude lines on Earth. Longitude lines on Earth divide one rotation into 360 degrees, but RA lines on the celestial sphere divide one rotation into 24 hours. Therefore one hour equals 15 degrees (360 divided by 24). See the definition of Local Sidereal Time for additional information on why 24 hours are used for right ascension instead of 360 degrees.

Zero degrees longitude passes through Greenwich, England and is the designated reference line for longitude. What then is the reference line for zero hour’s right ascension?  Astronomers use the vernal equinox, the location where the Sun crosses the celestial equator during its apparent annual motion against the background stars, as a "starting point" for right ascension.

The term "Right Ascension" comes from the fact that when viewed from the equator, all stars rise (or ascend) at right angles to the horizon, so their times of rising are called their times of right ascension.

Illustration of Right Ascension and Declination relative to the Celestial Sphere


Precession
What is Precession?

If you’ve ever watched a spinning top or gyroscope, you have seen how the spinning object resists forces trying to topple it. Tap the top on its side and it simply moves out of the way but remains spinning and wobbling. The wobble of a spinning object, like the spinning top, or a planet, is called precession.

The Earth behaves like a giant top spinning on its axis. Its large mass and spinning motion keep the Earth rotating about a fairly constant axis. But the Earth is not exactly spherical; it bulges around the middle. The Sun and the Moon constantly pull on the bulge and cause the Earth to wobble as it rotates.

Path of the North Celestial Pole due to precession (26,000 year period)

Since the equatorial coordinate system (right ascension and declination) is based on the axis of rotation of the Earth, and this axis is not constant due to the wobbling of the Earth, the coordinates of the objects in the sky are constantly changing, albeit very slowly. Therefore, the objects which are near the North celestial pole today (that is, the star Polaris) will not remain there. In the above diagram, you can see the magnitude of the wobble, and how the North Celestial Pole will migrate over the next 26,000 years.

Celestial coordinates are usually given at a certain epoch such as the year 1950 AD or 2000 AD.  The text NGC 2000.0 from Sky Publishing contains positions of all of the NGC objects for the year 2000; the coordinates are correct for that year only (NGC is an acronym for New General Catalog, a catalog of 7,840 of the brightest non-stellar objects). Since the effects of precession are relatively small over short periods of time, the year 2000 coordinates are sufficiently accurate for finding objects for plus or minus 50 years from the year 2000.


Projections
Displaying the Celestial Sphere

TheSky uses various projections to represent all or part of the celestial sphere on a flat surface (your computer screen or printer paper).  Different projections can serve different purposes, depending on what you are trying to accomplish.  For example, the Gnomonic projection is good for plotting the paths of meteor showers over wide fields of view, while the Orthographic projection is best suited for matching small fields to photographs or digital pictures.

The Mercator projection is commonly used on world maps you see hanging in classrooms.  Near the Equator, the distortion is minimal, but toward the poles, the distortion of area becomes greater and greater.


Seasons, Solstices and Equinoxes

In the course of one year, the Earth makes a complete revolution around the Sun in an elliptical orbit. The shape of an ellipse is a circle seen at an angle, and the term “elliptical” simply means having the shape of an ellipse.  Whenever the Moon is not full, the curved shape you see (or terminator) is one half of an ellipse, since it is the circle of Sunlight on the Moon seen at an angle.

If you picture the ellipse traced out by the Earth traveling around the Sun as being a large flat surface extending in all directions, you are imagining the Plane of the Ecliptic.  This plane is used as a reference frame for specifying the coordinates of solar system objects. The Heliocentric coordinate system uses the Sun as the origin, (“helio” is Greek for sun), the Plane of the Ecliptic defines zero degrees latitude and the vernal equinox is the direction of zero degrees longitude.

The axis of rotation of the Earth is tilted with respect to the plane of the ecliptic at an angle of 23.5 degrees (see the figure below).  This tilt gives us our seasons. During the summer months, the tilt acts to cause the sunlight to strike the Earth at very high angles in contrast with the winter months, when the sunlight strikes the Earth at low angles, becoming spread out. From the Earth-based perspective, during the summer the Sun peaks high into the sky and we enjoy the energy we feel from it. In winter months, the Sun is constantly low in the south — even at noon — and the energy we feel is not as intense.

Seasons, Solstices and Equinoxes

There are four distinct positions the Earth passes through during its yearly cycle around the Sun. These are due to the geometry of the plane of the ecliptic and the plane of the celestial equator. These four positions (called equinoxes and solstices) define the beginnings of the seasons. The vernal equinox (March 21) is the direction in the sky where the Sun appears to cross the celestial equator moving northward. From the vernal equinox, the Sun continues to climb higher in the sky every day because the Earth is moving around the Sun towards the summer solstice (June 22), when the Sun is farthest North (its declination on the celestial sphere is 23.5 degrees). After the summer solstice, the Sun sinks lower each day in the sky until the autumnal equinox, the point where the Sun crosses the celestial equator moving southward. Near December 21, the winter solstice, the Sun reaches its lowest position in the sky where its declination on the celestial sphere is minus 23.5 degrees.

If you live in the Southern Hemisphere, the summer solstice and winter solstice will be reversed.


Demonstration: Path of the Sun

1.	On the File menu, click Open.

2.	In the File name box, enter Sun’s Path and click OK.

3.	On the Tools menu, from the Time Skip fly out, click Record Trails.

4.	On the Tools menu, from the Time Skip fly out, click Go Forward.

The path of the Sun against the stars, spring through summer

The above diagram shows the path of the Sun on a rectangular projection of the stars.  The gray area highlights the minimum and maximum declination of the Sun over the course of the year, from -23.5 degrees to +23.5 degrees.

During the course of a year, the Sun follows a path in the sky called the ecliptic. The ecliptic and the celestial equator intersect at only two points, called equinoxes from the Latin root meaning "equal nights." On the dates of the equinoxes, at all points on Earth except the true poles, daytime and nighttime are 12 hours each. The vernal equinox, which occurs near March 21, is included on the above star chart.

When reading about the motion of the Earth around the Sun, don't forget to relate what you already know about the seasons (such as where you see the Sun in the summer versus where you see it in winter and other times of the year).  This will help you better understand the diagrams and computer plots displayed above.


Demonstration: Time Skip of the Sun at noon each day - Horizon based.
Demonstration: Time Skip of the Sun at noon each day — Horizon based.

1.	On the File menu, click Open.

2.	In the File name text box, enter Analemma and click OK.

3.	On the Tools menu, from the Time Skip fly out, click Go Forward.

The position of the Sun each day at noon — the analemma

This time skip plots the position of the Sun at noon each day for one full year. The pattern you will see resembles a figure eight and is called an analemma. This figure shows the declination of the Sun over the course of a year.

The analemma’s unique shape can be attributed to two factors. The 23.5-degree tilt of the earth’s axis relative to the plane of the ecliptic accounts for the up and down (vertical) motion of the Sun.

The “left and right” motion (or the varying distance from a vertical centerline) is due to the changing speed of the Earth as a result of its elliptical orbit around the Sun.

If the Earth's orbit were perfectly circular, the analemma would become a vertical line.  If there were no tilt in the axis of the Earth relative to the plane of the ecliptic, the Sun's path over time would simply be a dot — that is, it would appear in the same location each day at noon.


Time
What Time is it?

Astronomers use a number of different systems of time to aid in keeping track of and forecasting astronomical events. This section discusses some of the different time systems used and relates them to local standard time, the time that is displayed on our clocks and watches.

The day is commonly defined as the time required for the Earth to make one rotation with respect to the Sun, or, the interval of time between consecutive passages of the Sun past a fixed reference line in the sky. This reference line is called the Local Celestial Meridian. The Local Celestial Meridian is a great circle on the celestial sphere which runs from the zenith directly overhead to a point due South on the horizon, continuing around to the nadir (directly below) and back up to the zenith. Remember that the definition of the celestial sphere is directly related to the axis of rotation of the Earth, so these concepts of time are all directly related to the rotation of the Earth.

We define the mean solar day to be the time required for consecutive passages of the Sun across the local celestial meridian. In the figure below, we see that when the Sun is east of the meridian, that is before it reaches the meridian, we are in the a.m. (ante meridiem, or before midday) hours. After the Sun has passed the meridian, we are in the p.m. (post meridiem or after midday) hours.

The Solar Day

The time interval of consecutive passages of the Sun across this meridian is called the solar day, but astronomers find a differently defined day to be more useful when keeping track of star positions. This "other" type of day is defined by the time interval of consecutive passages of a star across the meridian (rather than the Sun) and is called the sidereal day. Looking at it from a slightly different point of view, a sidereal day is the time required for the Earth to make one complete rotation with respect to the stars (which we are not orbiting) rather than with respect to the Sun (which we are orbiting).

Why is the sidereal day different from the solar day? Why is the interval of rotation for a star different than that of the Sun when both are observed from the same rotating body, the Earth?

The sidereal day is shorter than the solar day since the Earth is not only rotating on its axis (causing days), but is also orbiting the Sun (causing years). In the time required for the Earth to spin on its axis one time, it also moves in its path around the Sun about one degree, so the Sun has "moved" during the day. A complete circle is made up of 360 degrees and the Earth requires about 365 days to orbit the Sun, hence about one degree per day. This one-degree movement of the Sun equates to about four minutes of time. This four minutes is the extra time required for the Sun to reach the meridian since the Earth has moved one degree in its path around the Sun.

The coordinates of the stars are based on the orientation of the axis of rotation of the Earth. It makes more sense to use star days (called sidereal days) and star time (called sidereal time) when working with the stars, rather than confusing the issue by including our movement around the Sun.

Sidereal time is directly (and simply) related to the positions of the stars in the sky, whereas local time has no relationship to the positions of the stars.


Local Sidereal Time

Local Sidereal Time (LST) is always equal to the instantaneous right ascension of the local meridian. Therefore, if you know the LST, you can look at star maps and determine which objects are near the meridian (those with a right ascension close to the LST).  For example, if the LST is 6:10:00, this means that stars with a right ascension of about 6 hours are visible along the meridian.  Looking at a star map, you would see that the constellation Orion lies near 6 hours of right ascension.

If everyone reckoned time based on the Sun crossing the meridian, then each longitude would have a different time. Someone living 60 miles east of you would see the Sun cross the meridian a few minutes before you, so noon would come a few minutes earlier. Only those people living at the same longitude would share a common time.

In the late 1800's time zones were established to minimize the problem of having different times at different parts of related populated areas. The time within these zones is call Zone Time.  Zone Time places all locations on Earth into various time zones. By definition, the zero time zone is at zero degrees longitude, and increases by one with about each 15 degrees in longitude.


Universal Time

Universal Time, also known as Greenwich Mean Time (pronounced "grenich", not like sandwich), is related to the motion of the Sun as observed on the Greenwich meridian (the local meridian at Greenwich or longitude 0 degrees). Astronomers tend to use UT in their work regardless of which time zone they live in. Local time can be converted to UT by adding the time zone to the local time, and making sure the result is within the range of 0 to 24 hours.

Daylight Savings Time further complicates keeping track of time since it is a one hour adjustment during the summer months that is not engaged by all countries.


Rise, Transit and Set Times

The daily rotation of the Earth causes the objects in the sky to appear to move in circular paths. The apparent geometry of these paths depends on the declination of the object and your location on Earth. For Northern and Southern latitudes (where most people live) some objects move in small circles and are always visible, while other objects are visible for just a portion of the night and spend the rest of the time below the horizon, that is, they rise and set. Each of these types is explored below.

If you are positioned at the North Pole, South Pole or on the Equator, the motion of the stars is more consistent and easier to describe.

Because we define the coordinates of the celestial sphere to be an extension of our own latitude and longitude coordinate system used on Earth, it follows that the North Celestial Pole is at an altitude in the sky equal to the latitude of the observer. If you are standing on the North Pole through which the axis of rotation of the Earth passes (90 degrees latitude), the North Celestial Pole, by definition, is directly overhead  (90 degrees altitude).  If you are standing at 40 degrees latitude, the North Celestial Pole appears at 40 degrees above the horizon. The North Star, Polaris, is a fairly bright star that is very near the North Celestial Pole. Therefore, the altitude of Polaris (in degrees) is very close to the latitude from which it is being observed. We cannot state the current altitude of any other bright object in the sky in such general terms since only Polaris lies close enough to the axis of rotation of the Earth that it does not move appreciably over the course of a day.

The transit time of an object is the time at which the object crosses the Local Celestial Meridian. It follows that if an object does rise and set, the transit time is directly midway between the rise time and the set time (barring any apparent motion on their part). For example, if a star rises at 10 p.m. and sets at 2 a.m., the transit time is midnight.


Circumpolar Objects

Objects that are close to the celestial pole appear to move in small circles around the pole and never travel far enough to go below the horizon. These objects are called circumpolar since they appear to simply move around the pole, never rising above or setting below the horizon.  If you were to stand on the North or South Pole, all visible objects (excluding solar system objects whose paths are more complex) are considered circumpolar.

Which objects are circumpolar?

If the declination of an object is greater than "90 degrees less the latitude of the observer" then the object is circumpolar. For example, at 40 degrees latitude, all stars which have a declination of 50 degrees or greater (90-40=50) are circumpolar.


Demonstration: Rising, setting and circumpolar objects

To begin the demonstration

1.	On the File menu, click Open.

2.	In the File name text box, enter Rise, Set and click OK.

3.	On the Tools menu, from the Time Skip fly out, click the Go Forward.

To end the demonstration

On the Tools menu, from the Time Skip fly out, click Stop.

Notice how the stars move in a counterclockwise circle about the North Celestial Pole.  The motion of the star Polaris is very small compared with other stars that are further away from the pole.  Note that some stars, such as those in the Little Dipper, never travel below the horizon.  These stars are called “circumpolar” because they never rise or set at this location on earth.

Stars that are further from the poles cross the horizon moving upwards in the East (rising), then continue in a circular pattern about the pole, then move downward across the horizon in the West (setting). From this display, you can see which stars are circumpolar and which stars actually rise and set.


Moon Phases

As the Moon orbits the Earth, we see a different portion of its illuminated half. In the following diagram we see that the Sunlight always illuminates one half of the Moon's sphere. But as the Moon orbits the Earth, we see a portion of the illuminated half and a portion of the dark half (except at new and full Moon).

If you watch the Moon over the course of the month, you’ll see that when the Moon is in the same part of the sky as the Sun (East in the morning and West in the evening) it must be a crescent phase since its light source is the Sun. When heading toward new Moon, you see the Moon's illuminated phase decrease while the angular separation between the Sun and the Moon decreases.

The phases of the Moon

The Moon orbits the Earth at an average distance of about 384,400 kilometers. Its orbit is slightly elliptical so its distance from Earth varies by as much as six percent. The sidereal period of the Moon is 27.32166 days. That is, the Moon takes 27.32166 days to orbit the Earth and return to the same place among the stars.  From an Earth based perspective, the Moon moves about 13 degrees per day.


Eclipses
What is an Eclipse?

An eclipse occurs when sunlight cast on one solar system object is obscured, or “blocked” by the shadow of another solar system object.

There are two types of eclipses that can be viewed from earth: solar eclipses and lunar eclipses. Solar eclipses occur when the sun is partially or completely obscured by the moon.  They tend to be much more dramatic than lunar eclipses and are much more popular.  Lunar eclipses occur when earth’s shadow obscures sunlight cast on the moon.


Solar Eclipses

Solar eclipses occur when the Moon passes between the observer's location on Earth and the Sun, thereby blocking the Sunlight to the Earth. By coincidence, the apparent angular size of the Sun and the Moon from Earth is about one half of a degree. Because the Moon is much closer to the Earth than the Sun (about 400 times closer) its small size is enough to cover the Sun's disk. Interestingly though, the shadow of the Moon on the Earth is relatively small, making the chances of seeing a Total Solar Eclipse from any particular location small. If you move just a few dozen miles from the path of totality, you will only see a partial eclipse, that is, the Moon will cover only a part of the Sun's disk.

Geometry of a Solar Eclipse


Demonstration: Total Solar Eclipse

To begin the demonstration

1.	On the File menu, click Open.

2.	In the File name text box, enter Eclipse, Total – Solar, 1991.11.07 and click OK.

3.	On the Tools menu, from the Time Skip fly out, click Go Forward.

To end the demonstration

1.	On the Tools menu, from the Time Skip fly out, click Stop.


Lunar Eclipses

Lunar eclipses occur when the Moon moves through the earth's shadow. If you look at the geometry of a lunar eclipse, it is apparent that they can only occur when the Moon is at full phase. The Sun, Earth and Moon fall within a straight line, and since the Earth is between the Sun and Moon, we see the sunlit side of the Moon (full phase) until it falls within the shadow of the Earth.

The umbra is the darkest part of the Earth's shadow, from which no portion of the Sun's surface can be seen (viewing the Sun from the Moon). In the penumbra, only part of the Sun's surface is blocked out.

There are three types of Lunar Eclipses, depending on how the Moon travels through the Earth's shadow.

If the Moon passes only through the penumbra, it is called a penumbral lunar eclipse. This type of eclipse can easily go by unnoticed since there is no change in the appearance of the Moon other than a slight dimming of the light.

When it starts to pass into the umbra, the full phased Moon appears to have a "bite" taken out of it. If the only a portion of the Moon passes through the umbra, we have a partial lunar eclipse. If the entire Moon travels into the umbra, it is called a total lunar eclipse. During a total lunar eclipse, the Moon remains visible by sunlight refracted (bent) through the Earth's atmosphere. Because the path of the lower frequency red light is bent sufficiently to strike the Moon, but the higher frequency blues are not, the totally eclipsed Moon appears red. The totality can last for up to 1 hour 42 minutes.


Conjunctions
What is a Conjunction?

When two or more planets appear near one another — within a few degrees — we say the objects are in conjunction.  There is something particularly pleasing about seeing more than one bright celestial object close to another in the night sky. The actual distances between the objects may in fact be quite far, but from an Earth-based perspective they appear to be next to each other.

Because the motion of the orbiting planets can be quite complex, predicting conjunctions is not an easy task. The computational speed of computers allows us to iteratively compute planetary positions at small increments of time, then compare the angular distances between the various planets. This allows us to find conjunctions and prepare for observations of them.


Occultations
What is an Occultation?

An occultation occurs anytime one celestial object passes in front of a second object, causing the second object to disappear from view.  Occasionally, for example, the Moon or one of the planets passes in front of a bright star or other planet.

Astronomers can learn much about the physical characteristics of a planet when they study the effects the planet has on a far away light source, such as a star. The rings around Uranus were discovered when the planet occulted a bright star in 1977.


Demonstration: Moon occulting Neptune
Demonstration: Moon occulting Jupiter and Venus (from Ascension Islands, United Kingdom)

1.	On the File menu, click Open.

2.	In the File name box, enter Occultation - Moon X Jupiter & Venus, 1998.04.23 and click OK.

3.	On the Tools menu, from the Time Skip fly out, click Go Forward.

In this spectacular event, the Moon passes in front of Jupiter first, then Venus. The first contact with Jupiter is at about 05:04, and both planets remain behind the Moon for about 18 minutes.


Parallax

Parallax is defined as the apparent motion of an object when the location of the observer changes.  If you hold up your thumb at arm’s length and alternately open and close each of your eyes, your thumb appears to move left and right compared to the objects behind it. When you switch from one eye to another, you have effectively moved the observation location by a few inches (the distance between your eyes). Relative to the background, your thumb has “apparently moved” when actually your point of observation has moved.


Retrograde Motion

One of the more interesting results of parallax appears in the Retrograde Motion of the planets. Mars shows the largest retrograde motion due to its smaller distance from Earth. The Earth moves in its orbit faster than Mars and therefore overtakes Mars about every 26 months. This causes the apparent motion of Mars across the background stars to actually change direction for a short period of time. The apparent motion of Mars is normally eastward due to its orbit about the Sun. But, while the Earth passes Mars due to its smaller and faster orbit, Mars appears to progress westward for a couple of months. Once the Earth overtakes Mars in its orbit, the motion of Mars returns to the normal eastward motion.

The retrograde motion of Mars

In the above figure, the Earth moves half way around the Sun in its orbit, which takes about six months. During the same time period, Mars only moves about 27% of its orbit around the Sun (its orbital period is 687 days). While the Earth and Mars are moving through the positions labeled 1, 2 and 3, Mars appears to be moving eastward in relation to the background stars, which is normal. But, while moving through 4, 5 and 6, the faster Earth is at its closest approach to Mars, so its relative speed is greatest. This causes the appearance of Mars moving westward (retrograde) against the starry background for a couple of months. Once the Earth overtakes Mars, the apparent motion of Mars returns to an eastward motion for approximately two years, until once again the Earth passes Mars.

The effects of parallax on the apparent positions of solar system objects can be seen in the following demonstrations.


Demonstration: Retrograde motion of Mars

1.	On the File menu, click Open.

2.	In the File name box, enter Retrograde – Mars, 1999 and click OK.

3.	On the Tools menu, from the Time Skip fly out, click Go Forward.

Near February 28, 1999, the eastward motion of Mars starts to slow, and after March 20, Mars moves westward against the starry background until around June 3, when its eastward motion continues.

The retrograde motion of Mars in Virgo

To watch the motion of Earth and Mars from an overhead view of the solar system:

1.	On the File menu, click Open.

2.	In the File name box, enter Retrograde - Mars, SS View and click OK.

3.	On the Tools menu, from the Time Skip fly out, click Go Forward.

Watch as the Earth overtakes Mars in late April and try to relate the observed motion to the above diagram.


Lunar Motion

Because we are located on the surface of the rotating Earth, our actual position relative to the Earth-Moon-background star system changes continually during the day. The next demonstration shows how this motion affects the apparent path of the Moon.


Demonstration: Parallax due to earth's rotation

1.	On the File Menu, click Open.

2.	In the File Name text box, enter Daily motion of the moon then click OK.

3.	On the Tools menu, from the Time Skip fly out, click Go Forward.

The daily motion of the Moon against the starry background

As in the above star chart, you see a pattern whereby the relative speed of the Moon appears to change during the course of a day. Note: we would need a transparent Earth to be able to see the Moon for a full 24 hour period, unless we were situated near one of the poles.  For a few hours, successive Moon positions are very close to one another, then for a few hours there is considerably more spacing between successive positions.

The reason for this apparent change of speed is as follows.  A point on the surface of the rotating Earth moves in one direction relative to the Moon for 12 hours (from point A to B in the diagram below), then it moves in the opposite direction for the next 12 hours (from point B to A).

Overhead view of the Earth-Moon system

The Moon is close enough to Earth that it appears to move among the background stars when we move a few of thousand kilometers due to the rotation (from A to B or B to A above) of the Earth.  Compare this to the thumb at arm's length example above.

When this motion is superimposed with the motion of the Moon around the Earth, it appears that the Moon is changing speed as it orbits the Earth. Would you expect to see the above pattern if you were situated at the North Pole?