# Astronauts' time vs. Earth time \[new title\] **URL:** \ **Category:** Factual Questions\ **Created:** [March 21, 2003, 3:01pm UTC](https://boards.straightdope.com/t/astronauts-time-vs-earth-time-new-title/163099 "2003-03-21T15:01:51Z")\ **Posts on this page:** 11\ **Page:** 2
**Author:** ![Bootieman](https://avatars.discourse-cdn.com/v4/letter/b/ecd19e/32.png) [@Bootieman](https://boards.straightdope.com/u/Bootieman)\ **Post date:** [March 21, 2003, 4:03pm UTC](https://boards.straightdope.com/t/astronauts-time-vs-earth-time-new-title/163099/21 "2003-03-21T16:03:11Z")
FYI… It’s spelled Bottieman for a reason… just to avoid possible confusion with the anatomical term for Booty (as in Arse)… Although appearing similar, they have far different meanings and connotations… (i.e. “Bootieman” - A derivative of someones name or alias… “Bootyman” - someone who may lay claim to being an “arse man” or alluding to have significant sexual prowess) ---
**Author:** ![Bootieman](https://avatars.discourse-cdn.com/v4/letter/b/ecd19e/32.png) [@Bootieman](https://boards.straightdope.com/u/Bootieman)\ **Post date:** [March 21, 2003, 4:05pm UTC](https://boards.straightdope.com/t/astronauts-time-vs-earth-time-new-title/163099/22 "2003-03-21T16:05:59Z")
\*Bootieman… My own handle… lol Doh!! ---
**Author:** ![John\_Kentzel-Griffin](https://avatars.discourse-cdn.com/v4/letter/j/ccd318/32.png) [@John\_Kentzel-Griffin](https://boards.straightdope.com/u/John_Kentzel-Griffin)\ **Post date:** [March 21, 2003, 4:12pm UTC](https://boards.straightdope.com/t/astronauts-time-vs-earth-time-new-title/163099/23 "2003-03-21T16:12:26Z")
**Bootieman** , Welcome to the SDMB. As **NYR407** mentioned it helps to have descriptive titles. I’ve changed the title for you. **DrMatrix** - GQ Moderator ---
**Author:** ![Bootieman](https://avatars.discourse-cdn.com/v4/letter/b/ecd19e/32.png) [@Bootieman](https://boards.straightdope.com/u/Bootieman)\ **Post date:** [March 21, 2003, 4:14pm UTC](https://boards.straightdope.com/t/astronauts-time-vs-earth-time-new-title/163099/24 "2003-03-21T16:14:11Z")
Thanks Dr. Matrix… glad to be here! ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [March 21, 2003, 5:28pm UTC](https://boards.straightdope.com/t/astronauts-time-vs-earth-time-new-title/163099/25 "2003-03-21T17:28:25Z")
[utterly frivolous nitpick] Of course you haven’t specified at what height the astronauts were circling the world - if they were three and a bit light days away, powering around the outside of the solar system in a starship capable of nearly lightspeed, they could experience 90 minutes subjective time while travelling on a path 24 hours long. (The OP doesn’t mention freefall…) [/ufn] ---
**Author:** ![Bootieman](https://avatars.discourse-cdn.com/v4/letter/b/ecd19e/32.png) [@Bootieman](https://boards.straightdope.com/u/Bootieman)\ **Post date:** [March 21, 2003, 5:36pm UTC](https://boards.straightdope.com/t/astronauts-time-vs-earth-time-new-title/163099/26 "2003-03-21T17:36:45Z")
LOL… now that WAS frivolous!! ---
**Author:** ![cloudbase](https://avatars.discourse-cdn.com/v4/letter/c/ee59a6/32.png) [@cloudbase](https://boards.straightdope.com/u/cloudbase)\ **Post date:** [March 21, 2003, 6:08pm UTC](https://boards.straightdope.com/t/astronauts-time-vs-earth-time-new-title/163099/27 "2003-03-21T18:08:05Z")
There is a time rate difference between the surface of the earth and orbit. Large enough so that it is factored into GPS calculations, if I recall. ---
**Author:** ![micco](https://avatars.discourse-cdn.com/v4/letter/m/5f8ce5/32.png) [@micco](https://boards.straightdope.com/u/micco)\ **Post date:** [March 21, 2003, 6:28pm UTC](https://boards.straightdope.com/t/astronauts-time-vs-earth-time-new-title/163099/28 "2003-03-21T18:28:58Z")
> [@](#): > > \*Originally posted by cloudbase \* > \*\*There is a time rate difference between the surface of the earth and orbit. Large enough so that it is factored into GPS calculations, if I recall. \*\* I believe you may be thinking of transmission latency, not an actual time difference due to altitude. As **Q.E.D** pointed out, there is a difference in measured clock rates at different altitudes, but this effect is miniscule compared to the time delay between a satellite sending a signal and a ground-based receiver getting it. GPS may actually account for both, but the calculations I’ve seen only accounted for the latter. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 22, 2003, 12:23am UTC](https://boards.straightdope.com/t/astronauts-time-vs-earth-time-new-title/163099/29 "2003-03-22T00:23:49Z")
Special Relativity is part of General Relativity, so the GR time effect does depend on the speed at which you’re orbiting. Further, if you’re in such an orbit that the gravitational effects are significant, I can guarantee you that your orbital speed will be very high. Saying that the orbital speed is much less than the speed of light is equivalent to saying that the gravitational time dilation is very small. And yes, this is an important consideration for the GPS satellites. The time difference may be small, but it accumulates. If you didn’t correct for relativistic effects, then the accuracy of the measurements would get progressively worse with time. ---
**Author:** ![William\_Ashbless](https://avatars.discourse-cdn.com/v4/letter/w/a87d85/32.png) [@William\_Ashbless](https://boards.straightdope.com/u/William_Ashbless)\ **Post date:** [March 22, 2003, 1:22am UTC](https://boards.straightdope.com/t/astronauts-time-vs-earth-time-new-title/163099/30 "2003-03-22T01:22:27Z")
\< gps hijack \> Actually I’ll jump in here because I help develop GPS receivers. We use the travel time of the signal transmission to deduce how far away the satellite is. Without a finite speed of light, GPS as it stands would be impossible. Time of reception minus time of transmission times the speed of light gives range. By working backward in time, and knowing the satellite orbit particulars (ephemerides) we can work out where each satellite is, and so work out where we are (intersection of spherical shells of distance around each satellite). The GPS satellites themselves, though, compensate for the relativistic effects. Down here on planet Earth we measure the passage of time of the satellite clocks by way of how fast the data’s being transmitted and at what times the starts of the messages are synchronized (6 seconds a ‘frame’ of data, for example). However, at the satellite, time’s running a little faster, so the clocks of the satellites are set up so that they run a hair slower such that we end up with nice round numbers for clock rates and bit rates for messages down here. So time dilation does play into this… but its fixed at the satellite by design. This all works nicely as long as you’re using the GPS system from anywhere local. Get a lot closer to the satellites, or go beyond the GPS orbit significantly, and there’s a very very small chance you’d have to compensate for this. As it is, though, down here, minor variations in height off the ground creates errors that are lost in the noise weeds. \< gps hijack \> ---
**Author:** ![William\_Ashbless](https://avatars.discourse-cdn.com/v4/letter/w/a87d85/32.png) [@William\_Ashbless](https://boards.straightdope.com/u/William_Ashbless)\ **Post date:** [March 22, 2003, 1:25am UTC](https://boards.straightdope.com/t/astronauts-time-vs-earth-time-new-title/163099/31 "2003-03-22T01:25:11Z")
The GPS satellites themselves, though, compensate for the relativistic effects _on time_ (that is, time dilation). Just thought I’d clarify. [Previous page](https://boards.straightdope.com/t/astronauts-time-vs-earth-time-new-title/163099.md?page=1)