eli5: if everything in the universe is constantly revolving at insane speeds, why are the constellations and star patterns the same all the time?


eli5: if everything in the universe is constantly revolving at insane speeds, why are the constellations and star patterns the same all the time?

In: Physics

They’re not; constellations change over time. They just change very slowly because they’re extremely far away.

Say you’re in a car zipping down the highway, and you look out the window. Signs on the roadside pass by in an instant, but if you look at a mountain in the distance then there’s barely any motion at all – even though you’re moving at the same speed in both cases. The bigger something is, and the further away it is, the more it needs to move before you notice a change.

They are just incredibly incredibly far away, like when you look out of car the things right next to the road move really fast, the trees in the background move slowly and the horizon even slower, the stars are so far that no matter how fast you go, they look like they are still

However over millennia constellations do shift slightly, and there is some impact on human history, like how constellations changed from the Egyptians

Insane distances. The closest star to earth is more than 4 light years away. Most visible stars are hundreds or thousands of light years away. They’d have to be moving at pretty close to lightspeed to move an appreciable distance in human timescales. One of the fastest stars in the galaxy is moving at about 1,200,000m/s.
Lightspeed is about 300,000,000m/s

All visible stars in the night sky are all part of a very small part of the milky way, rotating around its centre; we are moving along with the stars so the position of most of them does not change dramatically. Given enough time, there will be some change, however.

They’re not the same all the time – the stars and constellations look different now than they did thousands of years ago, and will look different in thousands of years from now. I should say look noticeably different. With incredibly precise instruments, constellations look different over much shorter time scales.

The reason the stars appear unchanging is because they are so far away and the distances between them so vast that they seem locked in place even as they careen in all different directions. It’s akin to the apparent speed of a jet flying a few hundred feet overhead and one flying a few miles up – they’re moving at the same speed, but the distance changes your perception of movement and speed. Scale that up a billion-fold or a trillion-fold (or more) and you get the unchanging heavens effect you describe.

The objects in the universe are not all revolving around some central point in the universe, Earth or otherwise. They are rotating around their own axes and revolving around the centers of mass of their immediate systems, such as planets revolving around parent stars or stars revolving around the center of their galaxies. In the case of the stars that we can see, they are all revolving around the gravitational center of our own Milky Way galaxy.

Still, star patterns/constellations are NOT always the same. They are constantly changing as the Earth and these stars move relative to each other while revolving around the center of our galaxy. However, these changes are too small for us, observing the night sky with only our eyes, to notice on the scale of human lives. This is because of how far away they are: there is a reason we call very long measurements “astronomical”. In tens of thousands of years and longer, constellations will appear different to how they appear now. They appeared different thousands of years ago too. We’ve only had the means to precisely catalogue and capture star positions for a very short time, though.

They do but even moving at high speed it takes a huge amount of time to get anywhere new in space. The constellations will change over time, but over tens of thousands of years, far far longer than we have recorded history.

Consider driving on a straight road at night with the moon off to your side. You and the moon are moving quite fast relative to each other, but it still doesn’t seem like the moon’s position is changing quickly because it is so far away from you, it changes over hours not minutes from your perspective.

The Moon is 400,000 km away, the closest star (Proxima Centauri) is 40,000,000,000,000 km away and its just 4.24 light years away.

If you look at a constellation like Orion, the outside two stars on the belt are both about 1200 light years away from Earth and are about 2.75 degrees apart in the sky and around 57.6 light years apart. If you want to wait for them to move to be even 3 degrees apart they’ll need to spread apart by at least 5.2 light years.

The fastest star in the galaxy moves at 8% of the speed of light and it’d take it 65 years to pull that off. Most stars move around 0.03% of the speed of light so expanding their gap by 5.2 light years would take over 17,000 years. Humans have only been around for 10,000 years so the stars have remained relatively constant over our existence.