The moon is the second-largest natural satellite in our solar system. It orbits around the Earth roughly once every month, and when it does, it appears as a circle in the sky.
Because the moon’s orbit is not perpendicular to the Earth’s rotation, we see the moon as a waxing or waning crescent, a new moon, a first quarter moon, a full moon, and a last quarter moon.
Because of how big the Earth is compared to the size of the moon, it takes approximately 24 hours for the Earth to rotate once. This is why we only ever see one side of the Moon – we only ever see one side of the planet!
The Moon does not always appear to be circular; this depends on which angle you view it from. If you look at it from the side, it will look more like an oval shape. If you look at it from above or below, it will look more like a dot.
The moon orbits the earth
Because the moon orbits the earth, we always see half of the moon. We never see a full moon because the side we always see is never facing us as it orbits.
As the moon orbits, it rotates on its own axis. This axis is perpendicular to the orbit so that it goes through a full rotation as it travels around the earth.
Because of this motion, we always see the same side of the moon as it travels around the earth. The side that faces us is called the near side and the other is called the far side.
The reason we only ever see one-half of the moon shining down on us is because it goes through one orbit while rotating on its own axis once. This means that every time we see a new half of the moon, it’s because it has moved in its orbit enough to show us another part of itself.
The earth orbits the sun
Along with all the other planets in our solar system, Earth rotates on its axis and orbits around the sun. This is what defines a planet as a spherical body that orbits a central body.
As Earth rotates on its axis, it also spins at a constant speed. This is why one side of the earth faces the sun and is constantly illuminated, while the other side is always dark.
The moon, however, does not rotate on its own axis. Because it orbits Earth, one side of the moon always faces Earth. As Earth rotates on its own axis, each part of Earth faces the moon for a certain amount of time before it switches sides. This is why we see essentially the same face of the moon at all times.
The moon does have some rotation due to its irregular shape, but this rotation is very slight.
The sun is essentially fixed in the sky
While the Earth is busy rotating on its axis and circling the sun, the moon is orbiting our planet. The moon orbits us at a fixed distance: about 240,000 miles.
Because of this fixed distance and the precise timing of its orbit, the moon always appears in the same part of the sky, opposite the sun.
At night, we see the moon as it passes through its dark phase. But because of sunlight shining on its surface, we can always see it. Even in daylight, we can see the moon if there is enough cloud coverage or sunlight reflection.
The same goes for any other planet or satellite in our solar system: All of them have a fixed distance from the sun, so they all show essentially the same face to it. Any visible changes in appearance are due to different angles when viewing them.
The direction of the moon’s orbit around the earth changes over time
Because of the way the moon orbits the earth, we see the same side of the moon every time it orbits the earth.
Due to its orbit, we see the moon at slightly different angles every night. Because of this, we see slightly different parts of the moon every night.
The moon takes about a month to go around once, so a few days later we see a bit more of the other side of the moon. A full lunar cycle lasts about a month, and during that time, we see both sides of the moon.
The names for each phase of the lunar cycle are defined according to how much of the sunlit side of the moon is visible. New Moon happens when this side is not visible at all, so it is called such because it is new information about where and what side of the moon it is on.
The plane of the moon’s orbit around the earth is not perpendicular to the plane of the earth’s orbit around the sun
This is why we see the moon at all times: Because of the moon’s orbit being offset from a flat plane, we always see one side of it.
Due to the lunar month being close to 29 days, plus a few extra hours, the moon cycles through its orbits fairly quickly. This is why we see the same face of the moon for a few days at a time; once it has rotated enough to see the other side, we can’t until the next lunar month.
Because of this, there are about two weeks out of every lunar month when we cannot see the Moon due to its position relative to the Sun. This is known as Lunar Limelight and was an important part of early Lunar calendars.
The Moon does not have enough gravity to keep its own light trapped so at new moon, there is no visible illumination.
Light from the sun hits objects at different angles at different times of day and year
Because of this, the shape of the moon seen in the sky is always changing. In fact, you can see a full cycle of shapes in just one night!
Because of the way our brains process visual information, we perceive the moon as having a constant size. We also perceive it as moving very slowly across the sky, taking about a week to go from west to east and back again.
Because of its constant luminosity (it doesn’t go through phases like the sun does) and its consistent size in our perception, we interpret this as “the” moon moving. In reality, it’s just the reflected sunlight shifting position in the sky.
The moon’s orbit is inclined at a slight angle relative to the earth’s axis. Because of this, at certain times during the month, we see part of the far side of the moon — but our brain interprets that as just more “facing” on one side.
The surface shape of objects affects how light hits them
When light hits a surface, part of the light is reflected and part of the light is absorbed. The color of the surface determines which kind of light is absorbed or reflected.
If a surface is black, it absorbs all types of light. If it’s white, it reflects all types of light. Other colors fall in between these two extremes.
The Moon’s surface is not black nor white, but “gray.” This means it does not absorb or reflect any certain type of light in particular; it reflects all of them. All the colors of visible light are reflected in some way when they hit the Moon’s surface.
Because the Moon’s surface reflects all visible lights, almost all of them are reflected back into space and do not hit the Earth’s surface.
Clouds can get in the way of light hitting an object at certain times
While the moon is a beautiful object in the sky, it can be a bit boring at times. The same shape of the moon stays the same as it orbits Earth.
Due to its constant shape, it is easy to identify when it’s rising or setting, and when it’s coming out or going down.
The reason for this is because of how the moon rotates on its axis. Just like any other object in space that has mass, it has what’s called gravitational torque.
This means that if an object rotates on its axis, there will be a force that pushes it toward one side. For the moon, this force is very small due to its lack of mass.
Because of this, the moon only rotates on its axis once every month due to gravitational torque. It does not rotate more quickly or slowly due to rotation on its axis.