Circuit of the Moon

The moon’s appearance changes from day to day. The time that it rises and sets changes from day to day. The moon shines by reflected sunlight. For three days every month the moon is bright enough to be called full because almost the whole disk is lit by the sun from our point of view during those three days. This is because the moon is then far from the sun in the sky. The sun is on one side of the sky, and the moon is on the other side.

Until that time the moon has been moving away from the place where we see the sun in the sky. Every month the moon moves around the earth toward the east, in a path that is very close to the path that the sun follows every year. That path is called the ecliptic.

We see the sun rise in the east and set in the west every day. But that is not because the sun is moving. We are the ones who are moving. The earth turns every day on its north-south axis. The earth turns to the east one complete revolution every 24 hours. That makes both the sun and the moon appear to rise and set every day. They seem to us to be moving from east to west daily. But that is an illusion. In fact, both move a little eastward every day against the background of the stars. The moon is in orbit about the earth. It moves eastward once every month. It really is moving, about 13 degrees per day.

Yet for the sun, even its annual movement to the east against the background of the stars is an illusion. We are the ones moving. Every year the earth revolves around the sun. As it orbits the sun, it stays about 93 million miles away from it. It takes just over 365 days to make one orbit.

If we could see the stars near the sun in the daytime, we would see the sun appear to move about one degree to the east every day against those stars. The sun is too bright to let us see those stars. But in the evening, after the sun sets, we can see which stars are to the east of the sun, and in the morning, before it rises, we can see which stars are west of the sun. So we can determine the sun’s apparent annual path against the stars, called the ecliptic.

We have seasons because the “plane of the ecliptic” is tilted when compared to the equator. This is because the axis of the earth is tilted over 23 degrees from this “plane of the ecliptic.” As the earth moves around the sun every year, its axis always points toward the north star. That’s why the north star is always in the same place every night, even while other stars appear to move around it. But the path that we see the sun follow each year carries it north of the equator during summer in the northern hemisphere, and south of the equator in the winter. So the sun is high in the sky on summer days, and low in the sky on winter days.

Every month the moon, in its orbit around the earth, follows a path that is very near the ecliptic. So we see the same kind of effect for the moon that we see for the sun. In the winter, when the sun is low in the sky at noon, the full moon, then on the opposite side of the ecliptic, is high in the sky at midnight. And in the summer, when the sun is high in the sky at noon, the full moon is low in the sky at midnight.

But during the two weeks after those full moon nights, as the moon continues to move to the east every day, it rises later and later. Its light is “waning” every night as it gets closer to the sun in the sky. Finally we see just a thin crescent moon rising an hour or two before the sun rises, and the next morning it is so close to the sun that we don’t see it at all. For two or three days, the moon is too close to the sun in the sky to see, either before sunrise or after sunset. Nearly all the light falling on the moon is on the other side of it, the far side hidden from us.

While the moon is moving eastward at 13 degrees a day, and the sun just one degree a day, the moon passes close by the sun. The time that it does that we call the conjunction. The sun and moon are said to be in conjunction when the moon is at the same place along the ecliptic as the sun is. However, the moon’s orbit is actually tilted just over 5 degrees from the ecliptic, so nearly every month the moon is either north or south of the sun, by an average of 2 or 3 degrees, when the conjunction occurs. In that rare month when the moon’s path crosses the ecliptic at the time of conjunction, we have a solar eclipse.

Astronomers like to use the term “new moon” to refer to the moment of conjunction. But the moon is not new until it has been “renewed.” That is, when we can see its light again. That happens after sunset, about two sunsets after the conjunction. When the conjunctions occurs each month, it can happen any time of day. But we can’t see the new moon until it has moved away from the sun to the east. At first we can only see it after the sun has set and the sky darkens. The first night it is a thin crescent. That is the beginning of the month.

After that, the moon “waxes,” growing brighter every night, and setting later as it moves away from the sun. After a couple of days, it is bright enough to see in the daytime. Before then, it was in the sky near the sun all day, but we could not see it until sunset. At what is called the “first quarter,” about a week after conjunction, the moon rises in the east around noontime and is highest when the sun is setting. We can see it, even on the sunniest of days. This first quarter moon (first quarter of a month) is half in sunlight and half in darkness. For the next week it continues to “wax,” rising later every day, until it is full again. Then it begins to “wane.” About a week after the full moon it is called the “third quarter” moon. It rises about midnight, is highest in the sky as the sun is rising, and sets around noon. Half lit by the sun and half dark, it is still visible until it sets.

From the full moon until the third quarter, it is called a “waning gibbous” moon. After the third quarter, as it draws closer to the sun in the sky, it is called a “waning crescent” moon. After we see the new moon, it is called a “waxing crescent” until the first quarter. From the first quarter until the moon is full, it is called a “waxing gibbous” moon.