Astronomy Ch02.8: Visual Activity: Conditions for Eclipses

Part A Suppose that instead of being inclined to Earth's orbit around the Sun, the Moon's orbit was in the same plane as Earth's orbit around the Sun. (Click "Show Moon with flat orbit" to see this situation.) In this hypothetical situation, approximately how many solar eclipses would occur each year? - 12

If the Moon orbited Earth in the same plane that Earth orbits the Sun (the ecliptic plane), we would have a solar eclipse at every new moon. Because there are about twelve new moons in a year, we would have twelve solar eclipses. Of course, this isn't what really happens. Continue to Part B to study the real situation.

Part D What conditions must exist for a lunar eclipse to occur? - The phase of the Moon must be full and the Moon must be passing through Earth's orbital plane

The two points at which the Moon's orbit crosses the Earth's orbital plane are called the nodes of the Moon's orbit. So another way to state the conditions for a lunar eclipse is that (1) it must be full moon, and (2) the Moon must be at or quite near one of the nodes of its orbit.

Part C What conditions must exist for a solar eclipse to occur? - The phase of the Moon must be new and the Moon must be passing through Earth's orbital plane

The two points at which the Moon's orbit crosses the Earth's orbital plane are called the nodes of the Moon's orbit. So another way to state the conditions for a solar eclipse is that (1) it must be new moon, and (2) the Moon must be at or quite near one of the nodes of its orbit.

Part B In reality, the Moon's orbit about Earth is tilted (by about 5°) with respect to Earth's orbit about the Sun. As a result, the actual number of solar eclipses that occur each year is approximately _____. - 2

There are generally two periods of time each year when eclipses are possible — the "eclipse seasons" — and there is a solar eclipse of some type (partial, total, or annular) during each of these eclipse seasons. (Because the time between eclipse seasons is less than six months, in some years there is a third eclipse season and hence a third solar eclipse.)

Part E If you could change the layout of the solar system, which of the following would cause a lunar eclipse to occur at least once every month in this hypothetical situation? - Change the Moon's orbital plane so it is in the same place as Earth's orbit around the Sun

You correctly visualize that the Moon's orbit has to be in line with Earth's orbit in order for a lunar eclipse to happen just about every month.