The Moon revolves counterclockwise around Earth (viewed from the direction of the north celestial pole). The Moon’s orbit is tipped about 5 degrees from the plane of Earth’s orbit; its path is always near the ecliptic. The Moon is about 0.5 degrees in angular diameter, and it moves eastward a bit more than 0.5 degrees per hour. The lunar phase cycle (new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter, and waning crescent) is the changing appearance of the Moon as it revolves around Earth. As the Moon orbits Earth, it rotates so that the same side always faces Earth. The Moon orbits Earth in 27.3 days (the Moon’s sidereal period). A complete cycle of lunar phases takes 29.5 days (the Moon’s synodic period). The fact that our calendar divides the year into months about 30 days long originates from the 29.5 day synodic period.
A lunar eclipse can occur at full moon (and only full moon) if the Moon’s path carries it through the shadow of Earth. The umbra is the region of total shadow. The penumbra is the region of partial shadow. A total lunar eclipse occurs when the Moon is completely within the umbra. While in the umbra, the Moon is illuminated by some sunlight that is refracted (bent) through Earth’s atmosphere. A total lunar eclipse can take as long as six hours from start to finish, with totality lasting as long as 1 hour 45 minutes. A partial lunar eclipse occurs when the Moon only partially enters the umbra. A penumbral lunar eclipse occurs when the Moon passes only through the penumbra and not the umbra.
A solar eclipse occurs when the Moon moves between Earth and the Sun. A total solar eclipse occurs if the Moon covers the disk of the Sun completely. A partial solar eclipse occurs if the Moon covers only part of the Sun. The small-angle formula can be used to find the angular diameter of an object from its linear diameter and distance. The Sun is 0.5 degrees in angular diameter. To see a solar eclipse, you have to be in the Moon’s shadow. The combination of Earth’s rotation with the Moon’s orbital motion causes the shadow to rush across Earth at speeds of at least 1,120 mph, sweeping out a path of totality. When the Moon is at its farthest point from Earth, it is at apogee. When the Moon is at its closest point to Earth, it is at perigee. An annular eclipse occurs if the Moon crosses in front of the Sun when the Moon’s disk is smaller in angular diameter than the Sun’s. Sometimes at the beginning or end of the totality phase of a total solar eclipse, a small piece of the Sun’s photosphere can peek out through a valley at the edge of the Moon and produce a diamond ring effect.
There are two conditions for an eclipse: The Sun must be near one of the two nodes of the Moon’s orbit (the points when the Moon crosses the ecliptic), and the Moon must pass near either the same node (solar eclipse) or the other node (lunar eclipse). These periods are called eclipse seasons. Solar eclipses can occur only when the Moon is new, and lunar eclipses can occur only when the Moon is full. An eclipse season occurs each time the line of nodes points toward the Sun. An eclipse year is the length of time (about 347 days) it takes the Sun to go from a node around the ecliptic and back to the same node (less than a full year due to the precession of the Moon’s orbit in the retrograde direction). Eclipses occur following certain patterns. The most important of these is the Saros cycle. After one Saros cycle of 18 years 11 1/3 days, the pattern of eclipses repeats. One Saros cycle contains 6,585.321 days, which is equal to 223 lunar synodic months. Therefore, after one Saros, the Moon is back to the same phase it had when the cycle began. But, one Saros is also equal to exactly 19 eclipse years. After one Saros cycle, the Sun has returned to the same place it occupied with respect to the nodes of the Moon’s orbit when the cycle began. Although the eclipse geometry repeats almost exactly, it is not visible from the same place on Earth. The Saros cycle is 1/3 of a day longer than 18 years 11 days. When the eclipse happens again, Earth will have rotated 1/3 of a turn farther east, and the eclipse will occur 1/3 of the way westward around Earth. That means that after three Saros cycles (54 years and 34 days), the same eclipse occurs in about the same part of Earth.
—January 2023
Lance Jones 