why does the iss move through the sky really quickly yet planets further out barely seem to move?
In: Physics
Objects further away always appear to be moving slower. When a bird flys over your head it zips by quickly. The same bird flying past you 100 feet up looks like it is moving slower.
Take your finger and move it at a slow speed like 2 inches in front of your eye. Then take your finger and move it at the same speed as far away from your body as it can reach.
The closer objects are to you, the faster they move across your field of vision. The same principle applies here. The ISS is 254 miles above you. The closest Mars ever comes to earth is 33.9 MILLION miles. That’s the difference between your finger an inch from your eye and your finger being 2.1 miles away.
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.
(more:)
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.
A Minute of Angle (MOA) is an angular measurement. A MOA is 1/60th of a degree. 1 MOA spreads about 1″ per 100 yards. ( actually 1.047″) 1 MOA is a different size at different distances, 8″ at 800 yards is still just 1 MOA.
So 1 MOA at the ISS is around 7 miles. 1 MOA of say Jupiter is 10.14 million miles. So to you they appear to travel the same distance but Jupiter actually travels 1.45 million times the distance. Yet Jupiter doesn’t even travel at twice the speed of the ISS so that 1 MOA takes the ISS 1.47 seconds. The same MOA if Jupiter takes about 346 hours but both appear to travel the same distance to an observer 9n earth.
Someone who knows more will probably correct my math. I did pretty rough math just as an example.
The apparent speed of an object is just how fast your eyes have to move to track it. Things that are closer to you make you have to move your eyes more than things that are far away.
It’s like driving down the highway and you see trees and poles adjacent to the road zip by but the mountains in the distance appear to move much slower. The viewing angle of closer things quickly changes whereas the viewing angle at distant things changes much slower. Likewise, the ISS is many orders of magnitude closer to Earth’s surface than planets, so while the ISS flies by, planets traverse the sky much more slowly.
Distance, if a bee flies past your face 3 inches away it will be out of your sight almost Instantly, but if it flies past 30 feet away it will take longer to travel across your field of view.
Because ISS is in a lower orbit (closer to Earth) than most satellites, it has to move much faster to stay in orbit and not fall on Earth.
Satellites that are in higher orbits appear to move slower, and satellites in geostationary orbit (which is the farthest, I believe) appear to stand still because they spin around the Earth with the same speed the Earth spins.
The scientific term for this is called “motion parallax”
So you know how objects far away seem small? Basically the same principle. (in psycho-physics this is called a monocular cue). It works for regular stuff too. That’s why you can watch boxing and you can see things but if you’re actually the one about to get hit, you just see a blur of movement before you get punched.
The planets are so far away, their movement is almost impossible to tell from the naked eye even though they’re moving thousands of miles per hour. The ISS travels about 4.7 per second, where as the Earth moves around the sun at roughly 18 miles per second. (That’s some 67,000 mph, btw)
But because of the aforementioned motion parallax you perceive closer objects (like the ISS) to move much faster as the farther object (like far distant planets)
Ignore the first commenter.
The ISS is not stationary. That is; its orbiting (flying around earth) at around 17,000 kmph.
The stars are basically stationary. Our frame of reference makes them appear to move as we’re spinning around on this rock earth at around 1600kmph.
However, depending on your location and which stars you’re observing; some stars will hardly ‘move’ at all (such as the north star).
Latest Answers