002.4 Predicting Eclipses

The plane of the Earth's orbit around the Sun corresponds to the apparent path of the Sun in the heavens, the ecliptic. The small angle between that plane and the Moon's orbit can be seen on this star chart from Sky & Telescope:

For example, you can see that the moon crosses the ecliptic on about Jan 25, 2002 when it is in the waxing gibbous phase and is crossing from the constellation Taurus to the Constellation Gemini. The full moon on Jan 28, 2002 is not on the ecliptic.

For an eclipse to occur, we need to have the Moon exactly on the ecliptic when it is either Full (for a lunar eclipse) or New (for a solar eclipse). That is the reason the path of the Sun is called the ecliptic.

The Moon orbits the earth once every 27 days relative to the distant stars. In that time, the Earth's motion changes the angle of the Sun by 27° so that the Moon must travel 27° farther to get back to the same phase. A full cycle of lunar phases takes about 29 1/2 days.

• The Moon travels eastward by about 12° per day. In a sidereal month, it comes back to the same place on the ecliptic and must then travel for 2 1/2 more days before it returns to the same phase. Thus, a given phase will shift eastward by about 30° each lunar month. (If we calculate it exactly, it will not be exactly 30°.) Eventually a Full Moon will shift into position for an eclipse.