Thursday, November 13, 2008

The Cockeyed Moon & the Listing Earth: Part I

Question posed by a reader: I’ve noticed that the Moon doesn’t always rise in the same spot on the horizon. Is there a pattern to this over the course of a month and over the course of a year?

Answer: Yes!

You could say that our Moon marches to the beat of a different drum. Any self-respecting natural satellite would orbit directly above its planet’s equator, ensuring no change from day to day in the position of the moonrise on the eastern horizon. Not so with our cockeyed Moon.

Instead, the Moon’s path as it orbits Earth is much more closely aligned with the ecliptic, the imaginary line that represents the path the Sun appears to take across the sky, as seen from Earth.

The Cockeyed Moon
Image source: NASA

Hopefully, you’ve noticed that the Sun rises at different spots on the eastern horizon over the course of a year. It rises incrementally farther to the north as we approach the summer solstice in June. Then it reverses course, rising incrementally farther to the south as we approach the winter solstice in December. This occurs because Earth lists a bit. That’s right, our home planet is permanently tilted with respect to its orbit around the Sun. In other words, the Sun is not above the Earth’s equator all the time. If it were, we would see no change in the position of each sunrise.

The Listing Earth
Image courtesy of Tau’olunga

Because it roughly follows the path of the ecliptic, the Moon has a rise cycle with a northern and a southern extreme, just like the Sun does. The Moon, however, goes through its cycle in one month, not one year. In any given lunar month, the moonrise travels back and forth between an extreme northeast position on the horizon and an extreme southeast position. This is observable. Check it out!

First, find a location with a clear view of the eastern horizon. You’ll need to use this same location on several dates over the course of one lunar month. A lunar month or lunar phase cycle is the time it takes the Moon to orbit the Earth and reach the phase it started from, as seen from Earth (for example, Full Moon to Full Moon or 8% waxing crescent to 8% waxing crescent). The average length of a lunar month is 29.5 days.

Second, decide how you will mark moonrise positions on the horizon. If you have a horizon with a lot of topographical and/or manmade landmarks, you could simply make notes describing where the Moon rose with reference to specific landmarks. You may want to sketch the horizon and its landmarks and mark the different moonrise positions and their dates on your sketch. You may even decide to capture the moonrises photographically. It’s up to you.

Third, use the named phases of the Moon (New Moon, First Quarter, Full Moon, Third Quarter) to streamline your data capture. The dates of these phases for any given month can be found on most calendars. If you simply record the moonrise position on the dates of five consecutive named moon phases, you’ll have what you need. A First Quarter Moon rises a couple hours before or after 12 noon, Full Moon rises around sunset, and a Third Quarter Moon rises around midnight or later. Check this website for moonrise times in your area.

Phases of the Moon
Image source: NASA StarChild

New Moon is a bit tricky. Because the Moon travels so close to the Sun in the daytime sky at New Moon, you won’t be able to observe the moonrise at New Moon. However, you can note the approximate location of sunrise on the eastern horizon at New Moon and use that for your moonrise position. It’ll be very close to the Moon’s actual position. (WARNING: Never look directly at the Sun, as this will permanently damage your eyes!)

When you complete this project, your five position markers will show you that the moonrise does indeed swing back and forth between a northeast and a southeast extreme (not necessarily in that order).

We’ll revisit this topic at a later date in Part Two, when we’ll take a look at moonrise positions and the pattern that emerges over the course of a year.