Eye Astronomy #1: The Moon

By Dale E. Lehman

Astronomy starts with the eyes. That’s how the ancients did it, and that’s how I started. I received my first astronomy lessons from my father. He’d grown up on a farm in northwest Ohio, where the Milky Way arched overhead on clear summer nights.

Much of the starlight my dad knew from boyhood is washed out in today’s light-polluted skies. Even on that Ohio farm, you can’t see the Milky Way anymore. Even so, you don’t need a telescope to get started in astronomy. As it’s been for tens of thousands of years, you just need your eyes. Pick a clear night, look up, and see what you can see.

When it’s above the horizon, the moon is the most obvious object in the sky. You can watch it slip eastward from night to night and day to day. Yes, it rises in the east and sets in the west due to the Earth’s rotation, just like the stars and the sun, but it also orbits the Earth from west to east. As a result, its apparent east to west motion as it rises and sets is slower than the stars. Each night (or day), it shifts a fair bit east relative to them.

In fact, the moon moves eastward fast enough that sometimes you can watch it pass in front of a bright star or planet. That’s called an occultation (from the word “occult” which means “covered over”). When that happens, the star or planet vanishes behind the western limb of the moon (which is east on the sky) and later reappears from behind the eastern limb (west on the sky). Such an occultation can last from seconds if the star passes behind one of the lunar poles to about an hour if it passes behind the lunar equator.

You can also watch the moon’s phases change from night to night. As it orbits the Earth, we see it from different angles. During a new moon, we don’t see it at all, because we’re looking at its night side. (The one exception is during a solar eclipse, when we see the dark orb of the moon in front of the sun.) Then a thin crescent of light grows over successive nights. We say the moon is waxing during this time. When it reaches first quarter, we see half of its disk illuminated. It continues to wax into a gibbous phase (from the Latin for “hump”) until it reaches full moon.

During this time, which lasts about 14 days—it varies from a bit under 14 to a bit over 15-1/2—the moon rises later and later, from sunset to midnight. Then it enters its waning phases: gibbous, last quarter, crescent, and finally new moon again. As it wanes, the moon rises between midnight and sunrise and is visible during the daytime. The entire cycle from one new moon to the next is called a lunation and takes about 29-1/2 days.

You might wonder how to tell whether the moon is waxing or waning. After all, one crescent looks pretty much like another. Here’s a trick you can employ to figure it out. The illuminated part of the moon points to the sun. Therefore, if the illuminated part of the moon is on the right (pointing west), it’s waxing. If it’s on the left (pointing east), it’s waning.

You can also study large-scale features on the moon with just your eyes. The dark areas (called “mare” or “seas” because the ancients thought they were) are lava-filled lowlands, while the white areas are mountainous highlands. A few of the larger craters are readily visible to the eye, too.

If you watch the moon from night to night, you’ll notice that while the phases change, the same features face the Earth all the time. That’s because the moon’s period of rotation on its axis is the same as its period of revolution about the Earth. Astronomers say the moon is “tidally locked.” You can find maps of the moon online and identify many of these features just by looking.

In song and poetry, it’s common to speak of “the dark side of the moon,” but you can easily see that, like the Earth, the moon goes through cycles of day and night. Watching the phases of the moon, you’ll notice although the same surface features face us all the time, sometimes they are in daylight and sometimes they are in darkness. Yes, at any given moment, the moon has a day side and a night side, but there is no side that is in darkness all the time, just as there is not on the Earth. The lunar day, though, is a lot longer: about 29-1/2 of our days!