Eli5: If we are moving trough the galaxy, how come we see the same stars in the sky every night?


Our entire solarsystem is moving through space yet the same constellations cover our skies every night. Do these stars move with us or are they far away enough that they their movement isnt noticable to us and they look stagnant even though everything is moving?

In: 1

The galaxy is huge, we really aren’t covering that much “ground”.

The nearby stars also move with us, while the distant ones are so far away that they don’t really move much even if we cross the whole galaxy. Only with close attention to detail do we notice any change, and that change is mostly the movement of the near stars compared to the far ones.

The second. If you look at the night sky today and compare it to 2,000 years ago, you’ll see some differences.

Space is REALLY big. That’s why distances are measured in light years.

It’s also worth pointing out that while the universe is expanding, it’s not like an explosion; we are in an arm of the Milky Way galaxy and are orbiting the black hole at the centre. So all the local star systems are roughly orbiting with us. One orbit takes around 250 million years.

I think constellations do change slowly over time (thus just one factor in demonstrating how nonsensical astrology is.) Edit: as you wrote – basically to some extent stars in our galaxy will be moving with us and to a greater extent they are just so far away that our movement doesn’t change the perspective much. I’m trying to think of a similar local example …. when you drive through your local town some distant landmarks won’t change as much as the view of the side of the road and how noticeably is the sun changing in the sky – not at all?

It’s all distance. The solar system is moving at about 140 miles per second in its orbit around the center of the galaxy. The stars near us are moving at similar speeds, but not exactly the same. So there is a lot of relative movement.

The closest star is 4.3 light years away. A light year is ~5,880,000,000,000 miles. The furthest stars you can see with the naked eye are about 4,000 light years distance (the galaxy is roughly 100,000 light years across). They have to move extremely large distances for that movement to be apparent. If a star was one light year away, to move 1 degree across the night sky it would have to travel ~103 billion miles relative to the solar system. At a 1,000 light years that changes to about 17.5 light years of travel needed (~107,200,000,000,000 miles). Even if the star was moving at 140 miles per second relative to us (which it wouldn’t, it would be much slower) it would take a star 1,000 light years away nearly 12,150 years to move just half a degree across the night sky. Half a degree is roughly the size of the moons diameter when you see it in the sky.

We’re moving, but we don’t move far enough to get a new perspective.

It’s like driving a half mile down the road and the mountain 30 miles away still looks the same.

Or if you’re in your front yard and walk 10 feet and your house still looks mostly the same.

We move, but not very far compared to the size and distance of the surrounding galaxy.