An Animation of the Motion of the Moon around the Earth
Illustrating the Occasions of Solar and Lunar Eclipses

Instructions :

The animation (an applet written in Java) is at the bottom of this page. I recommend you read these instructions sometime before, during or after you play with the animation. The Java source code for this animation is written so that it should run (as of January 2002) on Netscape 4.5 or newer web browser or Internet Explorer 5.5 or newer web browser. In order to have it run on these older versions of these browsers I had to avoid using some of the newer (Swing) features of Java.

Controlling the Direction of the Flow of Time:

The animation has the following look. Don't starting clicking away; this is only a picture.

The five buttons at the lower right of the control panel control the direction of the flow of time in the animation. The animation starts at the pause setting, || . You can press the >| button (not on this figure but on the real animation at the bottom of this page) to move forward one step in time, the |< button to move backward one step in time. The >> and<< buttons will allow time to move continuously forward or backward.

Controlling the Time Increments:

The menu box near the center of the bottom row of controls allows you to select the size of the time steps. The animation starts with 3 hour steps. If you would like to move forward or backward in larger or smaller steps you can choose between 10 days and 15 minute steps. After the time steps are changed the animation is put into the pause setting, and you must select one of <<, |<, >|, or >>buttons to get things rolling again.

Setting the Starting Date:

The animation starts with today's date, read from the clock of your computer. You can enter another date (between 0 AD and 3000 AD) and press the "to Date Entered" button to start at the date of your choice. If you want to return to today's date press the "Set to Today" button.

The Orbit of the Moon:

The animation starts with a view of the orbit of the Moon around the Earth. The Earth is at the center of the circular orbit. There is a line drawn in the direction of the Sun to indicate where the Sun is at any time. When the Moon crosses the "To Sun" line there is a New Moon, when it is opposite the Sun there is a Full Moon.

Orienting the Orbit of the Moon:

You can use the scroll bars at the right and bottom to change the orientation of the orbit and get a somewhat three dimensional perspective. Notice that the line to the Sun is not always in the plane of the orbit of the Moon ; this is most easily seen when you tilt the orbit of the Moon (in blue) so that it appears as a straight line and you are looking parallel to the plane of the orbit of the Moon.

The orbit of the Moon is tilted about 5 degrees with respect to the plane of the orbit of the Earth about the Sun. In this animation the two black lines which cross at the Earth are in the plane of the orbit of the Earth. The line to the Sun is also in this same plane (obviously). The intersection of the plane of the orbit of the Moon and the plane of the orbit of the Earth in called the "line of nodes" and is shownin green.

Eclipses:

One can have an eclipse of the Sun or Moon (seen from the Earth) only when the Sun is close to the plane of the Moon's orbit. Since the Sun is always in the plane of the Earth's orbit (represented by the two black lines) the Sun is only close to the plane of the Moon's orbit when the Sun is close to the line of nodes or the intersection of the two plane (orbit of Earth and orbit of Moon). When this happens it is said to be an "eclipse season".

Once the Sun is close to the line of nodes (which occurs roughly every 177 day) then one only has to wait for the Moon to pass through the shadow of the Earth (on a Full Moon) to get a Lunar eclipse or for the Earth to pass through the shadow of the Moon (on a New Moon) to get a Solar eclipse. Lunar eclipses will occur when the Moon is opposite (as seen from the Earth) to the line to the Sun and the Sun is near the line of nodes. Solar eclipses can occur when the Moon is near the line to the Sun and the Sun is near the line of nodes.

Summary: In order to get eclipses the Sun must be close to the line of nodes and the Moon must be close to the line to the Sun or opposite the line to the Sun.

For massive and detailed information about eclipses (when and where) visit the authoritative NASA/Goddard Space Flight Center Fred Espenak's Eclipse Home Page.

Viewing the Nodes from the Earth:

You can click on the "Sky view of Nodes" button to view the sky in the directions of the two nodes of the orbit of the Moon. In particular when the Sun and/or the Moon are close to the nodes you can see how visually close they are to each other and the node (as seen from the Earth). The "Sky view of the Nodes" also shows the shadow of the Earth (always opposite the Sun). When the Moon is in the shadow of the Earth there will be a lunar eclipse. When the Moon passes in front of the Sun there will be a solar eclipse. (For the more advanced reader: the simulated view of the nodes are as seen from the center point of the Earth. Thus, even if it appears as though the Moon just misses the Sun on the "Sky view of the Nodes" display, it is still possible that when seen from the northern parts of the Earth the Moon might pass directly in front of the Sun producing a solar eclipse.)

I recommend that you switch back and forth from the "3D view of orbit" to the "Sky view of the Nodes." With the "3D view. . . " get the Moon and Sun close to one of the nodes (near the line of nodes), then switch to the "Sky view . . . " and watch to see if indeed you get an eclipse.

The positions of the Sun and Moon are calculated from algorithms presented in Astronomical Formula for Calculators, 4th ed. Jean Meeus, Willmann-Bell. inc. Richmond VA, 1988. This simulation can be used to predict the times of maximum solar and lunar eclipse to an accuracy of about 15 minutes.

Try this Example:

Set the date for 29 March 2006 with the time set for 0.0. Run the simulation forward until it appears on the "3D view . . . " that the Moon is very close to the Sun. Switch over to the "Sky view . . ." Set the time increments to 15 minutes and step back and forth until the Moon is close to covering the Sun. If you are successful the time should read (on the display screen-- not on the text boxes used to input the time) close to 10.5 (indicating 10:30 Universal Time). You have just discovered a solar eclipse visible from the Earth on this date.

Here be the real thing: