Motion of the Earth
Rotation about our axis
The Earth rotates west to east, or counterclockwise if you were looking down from the North Pole. The Earth actually moves at a speed of kilometers per hour. One revolution of the Earth is experienced by us, the inhabitants of the Earth, as hours.
As we rotate on a twenty four hour time scale, we see the sun ‘rise’ and ‘set’ in our local sky. Because we are spinning west to east, the sun appears in the east of our horizon (as we first begin to see it out of the “corner of our eyes”), and disappears in the west of our horizon, moving east to west.
Rotation around the Sun
It takes the Earth around days or year to complete a full rotation on this ecliptic plane around the sun, at the staggering speed of km/h.
Though the Earth is on average around AU ( million km) from the sun, we actually rotate around the sun on an ecliptic plane, or in the shape of an ellipse, which means we are not always the same distance away from it.
The Earth is on axis of about degrees pointing towards the star Polaris, which ensures it is always seen in the night sky. This axis tilt is actually what causes the Northern and Southern hemisphere to experience different seasons at different times of year.
Rotation in our Local Group
Stars in our local group move randomly relative to one another at speed of about kilometers an hour, including the Sun. Even though these stars move staggering distances at extremely fast speeds, star systems usually don’t collide because of the sheer size difference between the size of the star, the speed the star system is moving at, and the distance between stars in space.
Rotation in our Galaxy
Our local group is part of the Milky Way Galaxy, where we are carried around its galactic center at even more insane speeds of km/hr. Our local group orbits the center of the Milky Way due to its intense gravitational pull, kind of like how we rotate the Sun due to its intense gravitational pull.
The Expanding Universe
Our galaxy’s nearest major space ‘neighbor’ is the Andromeda Galaxy, which just so happens to be hurling through space at us at speeds of around km/hr.
But why is it that we haven’t collided with it or any other galaxy yet? It’s not just the giant size of the distance between us and other galaxies, though that’s part of it. We’ve actually observed that most galaxies outside our local group are moving away from us, and the farther away a galaxy, the faster it is moving away. This indicates that the entire Universe must be expanding!
How is the Universe expanding?
The Universe is not expanding in the sense that the amount of matter in it is increasing, but in the sense that the empty space between galaxies is increasing. This increase in empty space is uniform across the Universe, but from our perspective we see the galaxies that are farther away from us moving faster.
If a galaxy is light year away from us at first, then after a certain period of time it is light years away from us, we say that it travelled at a rate of light years for that period of time. However, if a galaxy is light years away from us from the start of our time period, and there is still that galaxy from before that is light year away from us, we see that the space between us and the first galaxy increases by light years, but the space between the first and second galaxies also increases light years, causing the distance between us and the second galaxy to not be light years but light years.
From our perspective, it looks like this galaxy has travelled at light years for our time period, but it has only moved from the perspective of the first galaxy, and from its own perspective it has no moved at all.