Earth's Motion

Although astronomy is most often associated with the night sky, we will begin our study with the Sun. The Sun and Earth are the two most important objects in the Universe for our existence. The Sun in many ways is the ruler of our daily lives. There is a constant "cosmic dance" between the Sun and Earth, and we need to understand the motions involved. We will see what is physically occurring and how it observationally appears to us.


There are two motions at work. Let's look at each one individually. First, there is rotation – the spinning of the Earth on its axis. The rotation of the Earth gives us night and day. Although the Sun appears to be the one in motion across the sky, it is actually the rotation of the Earth that causes the Sun, Moon, and stars to rise in the East, move across the sky, set in the West, and reappear the next day in the East.


The second motion is that of the Earth's orbit around the Sun. Motion around another object due to gravity is called revolution. [Note that we do not always use the terminology properly. For example, the motion of a car engine is called RPMs – revolutions per minute – but actually the crankshaft is rotating.] The Earth's revolution around the Sun gives us our year. A year is much longer than a day so the changes that are associated with the Earth's revolution, such as the seasons, take longer to occur than the changes from rotation (day to night and back).


Both of these motions have an effect on the heavenly bodies. The daily, or diurnal, rotation of the Earth gives us night and day, and this moves the Sun and stars across the sky. Let's put both motions together now and pick a date, say January 1. At midnight we are viewing stars that are in the opposite direction from that of the Sun (we are looking straight overhead). At noon, while looking straight overhead, we see the Sun. Now those stars we saw 12 hours ago are directly behind us. (You might want to refer to the diagram above.) Let's jump ahead to July 1. What has happened? The Earth has revolved halfway around in its orbit. At noon, the Sun is still straight overhead, but we are now looking in the direction of those stars we saw back on January 1 at midnight. The stars we see this midnight are in the opposite direction. We did not see these stars six months ago because they were behind the Sun. During this half-orbit our seasons have changed and so have the visible stars.



We mentioned before that the Earth has two motions: revolution around the Sun and rotation around its axis. We now add a third motion – that of the Moon's orbit (revolution) around the Earth. First, though, we will pause the revolution and rotation of the Earth and look at only the Moon's revolution. To get a better observational perspective, let's move to our position above the Solar System. We have the Sun to the right and the Earth in the center. From this vantage point, the Moon's orbital motion is in a counterclockwise direction around the Earth.


Our view of the Earth is straight down onto the North Pole, and we see half of the Earth in daylight, the other half in darkness. The Moon is drawn at four locations in its orbit. Note the Moon's shadow pattern. Half of the Moon is always illuminated by the Sun, whereas the other half is not. Let's look at position 1 of the Moon's orbit (New). Notice that all three bodies are in line: the Sun, Moon, and Earth. In order to see the Moon, a viewer must be in daylight. Furthermore, the Moon is in the direction of the Sun. Imagine for a second that a lamp is the Sun, you are the Earth, and a baseball is the Moon. Held at arm's length, the ball is difficult to see because of the lamp's bright glare, and the same is true of the Moon when it is near the Sun in the sky. In addition, you are looking at the unlit, dark side of the Moon. This phase is New Moon.


Now we skip to position 3 (Full). Although the Sun, Moon, and Earth are again in line, the Earth and Moon have switched positions. An observer on the night side of the Earth sees the illuminated side of the Moon. Let's think again about viewing a baseball held at arm's length, but now the bright lamp is behind you. In this orientation you see all of the sunlit side of the baseball. This is Full Moon.


Positions 2 and 4 are times when half of the Moon's bright disk is visible to the Earth. These are the First and Third Quarter Moons. The nomenclature is poor. At any one time, half of the Moon is always illuminated by the Sun. During the Quarter Moons, "half" of the disk is seen. The expression of Quarter Moon has to do with its position in its orbit. New Moon is considered the start of the lunar cycle. First Quarter Moon is one-fourth of the orbital period, Full Moon is half, and Third Quarter Moon is three-fourths.


This is a good time to stress a key point – the phase of the Moon has no correlation with the time of day, or for that matter, day of the week or time in the year. It is purely a function of the positions of the Sun, Earth, and Moon. As the Moon revolves, the amount of the illuminated side that is visible to the Earth changes. The phase of the Moon is not due to the Earth's shadow obscuring part of the lunar disk.


At the start of this section, we stopped the Earth's revolution and rotation, but now let us allow the Earth to spin again. Although we are standing on a spherical planet, due to its large size our visible world appears flat, the edge of which is the horizon. For our idealized observer, he/she will see the Sun rise in the East, be straight overhead on your meridian six hours later at noon, and set in the West about six hours later still. In the following diagram, at position 1 the observer is oriented so the Sun is directly above – this is noon. At position 3 the Sun is directly below the ground, which is midnight. Obviously, at positions 2 and 4, the time is six o'clock, but which is dawn (sunrise) and which is dusk (sunset)? [Hint: From this vantage point above the plane of the Solar System, all of the planets orbit counterclockwise.] The Earth's rotation is in the same direction as its revolution, so the observer goes from position 1 (noon) to position 2 (sunset). Therefore, position 4 is sunrise.


The next diagram is a combination of the previous two. Let's first think about when the Moon is overhead at particular times of the day. We'll start with New Moon. This phase is the easiest to understand because the Moon is in exactly the same direction as the Sun. The Sun and New Moon are directly overhead at noon. The Sun and New Moon rise at sunrise and set at sunset. At Full Moon the situation is exactly opposite. The Full Moon rises at sunset, is directly overhead at midnight, and sets at sunrise.


Look at the First Quarter Moon at position 2. Here the Moon is on the meridian at sunset. When did the First Quarter Moon rise? We know it is overhead at sunset and that it rose six hours previous. Backup six hours from sunset – that would be noon. Therefore, the First Quarter Moon rises at noon. To confirm that deduction, look at the Earth's noon position. At this time, the Sun is straight overhead and the First Quarter Moon is on the horizon 90° away.


In summary,

1. The phase of the Moon is due entirely to the orientation of the Sun, Earth, and Moon.

2. The Moon rises in the East, (approximately) six hours later it is on the meridian, and six hours later still it sets in the West.

3. Although the phase of the Moon is not a function of the time of day, for a given phase the location of the Moon on the sky is dependent on the time. For example, the Full Moon always rises at sunset (sorry all you mystery writers, but you cannot change it.)


Before we continue, it is time for you to do an important observing exercise. It is best to start this a couple of days after New Moon, so check a calendar. Pick a consistent time of the evening to view the Moon, say 7 pm. Now, for a week or two, notice the following three changes: (1) the phase of the Moon, (2) its position on the sky, and (3) the brightness change. Realize that you are literally seeing the Moon orbit the Earth!



The Naked-Eye Sky (copyrighted) by James Sowell, 2013