why does the iss move through the sky really quickly yet planets further out barely seem to move?
In: Physics
TLDR: Things that are very far away always generally stay in the same direction from you. If you are in Los Angeles and you point your finger at a person in New York City, it doesn’t matter how fast they move from one end of New York City to the other, your finger really isn’t going to move at all. This is because because relative to how far away New York is from you, that person hasn’t covered very much distance. They could move at the speed of light across new york, but your finger will move much further if it follows your dog slowly walking across the room 10 feet away.
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The further away something is, the longer it takes to move though your field of view, and the slower/less it seems to move.
Compare an airplane overhead vs. a car passing you on the street. The plane is certainly moving faster, but because of how far it is, you can see it for longer, so it seems slower.
The ISS is in orbit of our planet – it’s 400km up, which is a lot more than a plane that averages around 10km up. 40x farther. However, the *nearest* planet (Venus) averages 40 *million* km away. That’s 100,000x farther.
The ISS moves about 27,000 km/h relative to Earth. The planets don’t actually move much faster than that. Venus moves about 126,000 km/h and Jupiter moves about 46,000 km/h around the sun. So the speeds are pretty close, but the distances are hugely different.
Let’s remember those numbers: Venus moves 126,000 km per hour, but it’s also 40,000,000 km away. So in a whole night, Venus moves around 2% of it’s distance from us. And only on rare occasions is that movement entirely “side to side” from our viewpoint – movement towards or away from us woudln’t even be noticed.
**So there’s a simpler way to look at it.** The ISS orbits the Earth several times a day because it’s so close to us. It takes roughly 92 minutes to orbit. If you simplify and say that at any point on Earth, you can see about half the sky at a given time, the ISS has to cross the sky from one horizon to the other in around 46 minutes (ignoring the rotation of the earth)
The planets, on the other hand, don’t orbit the earth at all in a night. They are like the guy in New York – they are moving really fast, but super far away, so always in the same general direction. Throughout a whole night, they aren’t going to move what side of Earth they are on. So the movement we see of planets from Earth with the naked eye is really just the rotation of the Earth – which takes about 12 hours for the planets to go from one horizon to the other.
Thus the ISS takes 46 minutes to cross the sky once while the planets take all night.
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