A Staff Report from the Straight Dope Science Advisory Board

What time do the astronauts use in space?

August 25, 2000

Dear Straight Dope:

What time zone do astronauts and cosmonauts recognize? I saw the cosmonauts on space station Mir celebrating New Year's, and was wondering at what time this occurs. Is there a "universal" time in space?

There is a universal way of keeping time in space, and you just named it: Universal Time. Universal by the puny standards of earth, anyway.

Bureaucracy being what it is, the reader will not be surprised to learn that there are actually two Universal Times, both of which come from the old Greenwich Mean Time that we remember from our old almanacs (the term Zulu time is also sometimes used). But this time at least the bureaucrats have a decent excuse.

In most civil uses, Coordinated Universal Time (UTC) is the standard. This is the time calculated by atomic clocks and distributed by radio to stock exchanges, news radio stations and anyone else who wants to know "exactly" what time it is. It''s pretty much a human invention, but it is highly precise (see below).

There is also a special variant on Universal Time called UT1. Essentially, this is Universal Time but with a correcting factor thrown in to account for variations (so far all downward) in the earth's rotation. One can easily see how this might be a more useful measure of time for navigators and astronomers. If I need my satellite to be in a certain place over the earth at a certain time, I couldn't care less what people call the time, so long as it's related to my selected place. The difference between UTC and UT1 is continually broadcast, so those who have to can translate between the two.

The rules set up by international agreement don't allow UTC and UT1 to vary by more than 0.9 seconds. In a metaphysical sense UTC is a more accurate measure of time, since it is defined by the highly regular radiation period of cesium rather than the lumbering, slowing revolution of a big hunk of rock in space. But from a practical standpoint, adjusting the rotation of the earth has been a goal that has thus far eluded our best scientists. So when the variance between UTC and UT1 gets too large, we just introduce a leap second to the UTC clocks.

Want more? Try the U.S. Naval Observatory at aa.usno.navy.mil/AA/faq/docs/UT.html .

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