If you stare into the night sky for long enough, the stars will seem to move. If you’re an astronomer or even a stargazer, you’ve probably noticed this. If you’re shooting a photo or pointing a telescope, this can be quite frustrating, but it also creates star trails – which look pretty cool. I’ve always wondered what causes the movement of the stars, so I did some research and found the answer.
The reason the stars move across the sky is because of the rotation of the earth. The earth rotates around one thousand miles an hour, this causes to stars rise in the east and set in the west, just like the sun.
The Rotation of the Earth
The earth rotates because it was formed with clumps of matter which rotated around each other due to gravity until they collided. After they collided the earth kept rotating. The earth will not stop rotating any time soon because it’s in space, which is a vacuum, meaning there is no air friction to slow it down.
The stars don’t really move across the sky, but the rotation of the earth makes it look like they do. Early humans must have imagined that the stars revolved around the earth, but we now know that this is not the case, the earth is simply rotating.
If you ride through a river on a boat, stationary objects like trees seem to move in the opposite direction you are moving. But the trees are not moving, you are. This is similar to the movement of the stars, the stars are not moving, the earth is. The earth rotates from west to east making the stars appear to rise from east to west.
The Sun is a Star too
If you still have trouble understanding why stars move across the sky, realize that the sun is a star as well, and it moves for the same reasons stars do. Stars rise and set just like the sun, but fewer people notice this because there are many stars, making it hard to keep track. The only difference between the sun and other stars is that it’s the closest star to earth.
The North Star
The earth rotates on a tilted axis. The earth’s axis points directly at the Polaris, also known as the North Star. If you took a time-lapse of the night sky, you would see that the stars move in circles around the North Star. This is directly a cause of the earth’s rotation on its axis. The North Star spins around in circles but does not actually move in the sky.
There is a South Star called Sigma Octantis, but it is very dim and it’s extremely difficult to see due to it being very dim as well as light pollution. Because of this, it can’t be used for navigation like the North Star. The same circular movement happens around the South Star as well.
The Celestial Equator
The movement of the stars form two big circles, where these two circles meet is called the Celestial Equator. At the North Pole, the North Star is directly above you, and in the South Pole, the South Star is directly above you. As you may have guessed, at the equator, the celestial equator is perfectly centered in the sky. You can’t see the North Star in most of the southern hemisphere, and you can’t see the South Star in most of the Northern Hemisphere.
The Revolution around the Sun
The stars don’t only move throughout the night, they also move throughout the year. This is because of the revolution of the earth around the sun. For example. in the northern hemisphere, you can only see the core of the milky way in the summer. In the summer, you can’t see the constellation Orion, but as summer turns to fall, Orion rises over the horizon. As the earth revolves around the sun, the direction to earth is facing relative to the night sky changes. This is why some constellations can only be seen at certain times of the year.
The location of where you view stars matters as well. For example, the southern hemisphere sees all the stars which are below the horizon in the northern hemisphere. The star’s position in the sky change based on location. In the northern hemisphere, the milky way forms an arch in the sky, like a rainbow. But in the southern hemisphere, the milky way is directly over the sky.
You can visualize the movement of the stars using a photography technique called star trails. Star trails are when you keep your camera shutter open for extended periods of time. This captures the movement of the stars for as long as you open the shutter. It’s really helpful to visualize the circular movement around Polaris.
In the far future
Over the span of thousands of years, the constellations change time. There’s a good chance that constellations looked slightly different in ancient times. According to wired, constellations have looked different in the past and will look different in the future. This is caused by the actual movement of the stars, as opposed to visual movement caused by the earth moving. This movement is created by gravity and dark energy. Dark energy is the force that causes the universe to accelerate its expansion instead of slowing down.
The Andromeda Galaxy
In the future, the entire galaxy will change. In around 4.5 billion years, the andromeda galaxy will collide with the milky way galaxy. When two galaxies collide they change shape. The band of silver light we see as the milky way from earth could completely change shape. The tiny and dim andromeda galaxy we see today could look massive in the future.