# The ISS, acceleration and space time

This question/theory/thought has been stuck in my head for months and would welcome comments.

Firstly, from my understanding, looking at NASA's website, the ISS cycles the earth 26 times in one 24 hour period. Time on the ISS is also set as GMT +0

As this is the case, how did the time on earth and the ISS stay synced or is that way it goes that many times around earth in 24 hours?

Also I know a single engine burn would cause speed and orbital change but what would a multi-engine burn do? So as an example, assume the orbit is a circle. If the multiburn could be keep the orbit in shape while accelerating what would happen to time on the ISS and Earth?

• Unless the speed became a very substantial fraction of the speed of light, Time on the ISS and Earth would look very similar. You have to be going really fast for relativistic effects to get noticeable without sensitive instruments. – zeta-band Mar 27 '18 at 18:46
• Even the GR contributions are negligible, because the ISS, being in low-Earth orbit, experiences basically the same gravitational field as on the surface. – probably_someone Mar 27 '18 at 18:52
• To me, it seems that you are overly confused. Would have been all fine if the ISS would go around Earth just 24 times in 24 h? Sorry but you are mixing things here. Imagine your watch is GMT x. You cross the border and find yourself at GMT x+/-1. Nothing physical is going on here, except your moving at a certain speed. Of course to follow the local TV news better you adjust the watch. – Alchimista Mar 27 '18 at 20:09

Firstly from my understanding looking at NASA website the ISS cycles the earth 26 times in one 24 hour period.

Not quite that fast. Objects in low earth orbit (including the ISS) can complete an orbit in around 90 minutes.

Time on the ISS is also set as GMT +0. As this is the case, how did the time on earth and the ISS stay synced or is that way it goes that many times around earth in 24 hours?

Think of that as the "timezone" of the ISS. When it's noon in GMT +0 on earth, the clocks on the ISS are set to noon as well. Unfortunately since they're moving around the planet so quickly, the position of the sun and their clocks are not synchronized. Instead, everyone uses the clocks, and just shades the windows to keep from confusing their body clocks too much.

The clocks on the ISS and clocks on the ground tick at (almost exactly) the same speed. So it's no different from being in a cave and setting your clock by a radio station thousands of kilometers away. It's not the same clock as other people nearby use, but it doesn't make much difference to you in the cave.

...theoretically if the earth stopped spinning there would be no time.

The earth's rotation can be considered a clock. But there exist untold others. Galaxies, pendulums, chemical reactions, and molecules all exist and can interact with their environment in ways that display periodic behavior over time. Deep in your cave, you may be unaware of the earth's rotation. But your cells still run biochemical reactions that make you alert and tired periodically.

• thank you for all the replies and for correcting the post at the start. – DarthGremlin Mar 27 '18 at 23:54
• Time and Space Time. The idea of Time from my understanding is man made back from the ages of planting crops and seasons. So therefore time doesnt exisit does it? i mean it is to do with one earth rotation cycle. So based on that, theoretically if the earth stopped spinning there would be no time. So if that is the case why do we consider space time as combined? – DarthGremlin Mar 28 '18 at 0:13
• And what about other planets? Man didn't make time. Man made clocks. – Tom B. Mar 28 '18 at 0:31