Motion of the Moon
by Ron Kurtus (revised 30 June 2012)
The motion of the Moon is that it revolves around the Earth in an elliptical orbit every 27.3 days. Since one side of the Moon has greater density than the other, the force of the Earth's gravity causes that side to always face the Earth as it moves in its orbit.
The length of a day on the Moon is 29.53 Earth days. The angle of the Sun on the Moon, with respect to the Earth, causes the phases of the Moon. An eclipse of the Moon is when the Earth completely blocks light from the Sun. An eclipse of the Sun is when the Moon completely blocks sunlight from areas on the Earth.
Questions you may have include:
- What is the motion of the Moon relative to Earth?
- What are the phases of the Moon?
- What is an eclipse?
This lesson will answer those questions.
Motion relative to Earth
The Moon travels in an ellipse around the Earth, varying from as far as 405,500 km (252,700 miles) away to as close as 363,300 km (221,500 miles). The Moon's orbit around the Earth takes approximately 27.3 days to complete.
Rotation on axis
Because the mass of the Moon is greater on one side than the other, the gravity of the Earth causes the Moon to keep the denser side always facing the Earth. Thus, the Moon rotates on its axis at the same rate as its orbit around the Earth.
It also means that we only see one side of the Moon. It wasn't until we sent a rocket with cameras aboard to orbit the Moon that we were able to get pictures to see what the other side looked like.
Length of day on Moon
A given point on the Moon will experience a day of sunlight that lasts 29.53 Earth days. The reason is the slow rate of rotation of the Moon when it goes around the Earth, plus the effect of the Earth moving around the Sun.
Phases of the Moon
The relationship between the Sun, Moon and Earth results in the illuminated portion of the Moon to vary according to how the Sun is shining on it.
At different times of the month, the Moon can appear as a crescent, half circle, or full. These are the different phases of the Moon. These phases of the Moon are determined by the position of the Sun and how it is reflecting off the Moon's spherical surface, with respect to our view of the Moon.
(See Phases of the Moon for more information.)
A quarter-Moon or crescent phase of the Moon
Reflection off dark side
In the picture above, the bright crescent is light from the Sun, reflecting off the Moon's surface. But also, the dark portion of the Moon seems to be slightly illuminated. That is caused from sunlight reflected off the Earth and hitting the dark side of the Moon. In other words, the Sun shines on the Earth, some light is reflected and shines on the Moon, and then that light is reflected again from the dark part of the Moon.
When Moon looks larger
An interesting phenomenon that is seen when there is a Full Moon is that the Moon appears much larger when it is just near the horizon. It almost looks twice as big as it does when it is overhead.
The reason is that it is an optical illusion. When the Moon is close to the horizon, your eyes perceive it differently than when there is no horizon in view. The optical illusion is so good that I always thought the size had been magnified until I saw actual measurements proving its size didn't change.
Seeing Moon day and night
You mainly see the Moon at night, although you may see it once in a while during the daytime. In reality, the Moon is in view as many times in the day as it is in the night. The problem is that you just don't notice the Moon as well in the day as in the night.
Obviously, at night you can easily see the brightness of the Moon, even if it is a thin sliver. Another reason is that at night you can see the side facing the Sun more readily.
During the day, you may only occasionally see the bright side of the Moon. More often, the complete dark side is facing your view. In this situation, the glow of the atmosphere masks the Moon such that you don't even notice it. Similarly, all of the stars are still up in the sky during the day, but the scattered light from the atmosphere is much brighter than the light from the stars.
There can be an eclipse of either the Moon or the Sun.
Eclipse of Moon
Although the view of the Moon you see is determined by the angle of the Sun hitting it, there are some rare times when the shadow of the Earth blocks out part or all of the Moon. This is called an eclipse of the Moon.
A full eclipse of the Moon is when the shadow of the Earth completely blocks the light from the Sun from directly striking the Moon. In such a case, the Moon is glows only slightly from Sunlight reflected off the Earth onto the Moon's surface.
Eclipse of Sun
Another more dramatic phenomenon is when the Moon blocks out some or all of the view of the Sun. This is also a rare occasion.
A full eclipse is when the Moon completely blocks the Sun from viewers on Earth.
The Moon blocks the Sun from view in a full eclipse of the Sun
Although the Moon is much smaller than the Sun, it is also much closer to the Earth than the Sun, such that they appear to be about the same size to a viewer on Earth. Thus, the Moon can block out the view of the Sun in some situations.
When an eclipse of the Sun occurs, care must be taken when looking at it, because invisible electromagnetic waves or rays can damage your eyes.
The Moon rotates around the Earth in an ellipse every 27.3 days. The Earth's gravity causes one side of the Moon to always face the Earth. The length of a day on the Moon is 29.53 Earth days. The angle of the Sun on the Moon, with respect to the Earth causes the phases of the Moon. An eclipse of the Moon is when the Earth blocks light from the Sun. An eclipse of the Sun is when the Moon blocks the view of the Sun from observers on Earth.
Observe and learn
Resources and references
Facts About the Moon - From NASA
The Moon - Good details from Nine Planets site
The Moon - Information and statistics from Russian version of American website
From Blue Moons To Black Holes: A Basic Guide To Astronomy, Outer Space, And Space Exploration by Melanie Melton Knocke; Prometheus Books (2005) $19.00
Observing the Moon by Peter T. Wlasuk; Springer (2000) $39.95 - Reference book for anyone seriously interested in the Moon and its geology
Welcome to the Moon: Twelve Lunar Expeditions for Small Telescopes by Robert Bruce Kelsey; Naturegraph Publishers (1997) $11.95 - Well written "how to" for novice astronomers
Questions and comments
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Motion of the Moon