question about Epoch

[Commander David]

can somebody tell me more about this word in the .ssc file called: "epoch"? I have same tables of data and there is the same word but in the ssc-file there are more numbers. In my tables: Epoch 1981 Jul. 1.00 TT how can i convert this timeformat to ssc-file epoch?

[selden]

Epoch is the date of the elements -- the time when they are valid.

One definition of the various astronomical time measurements can be found at http://archive.stsci.edu/fits/fits_standard/node87.html Note that "TT" is about 63 seconds ahead of UT. This difference may cause problems for some precise mutual events, but usually can be ignored.

If you're using Celestia v1.2.4 or eariler, Epoch has to be specified as a Julian day: a common astronomical time specification. There are quite a few Web sites that explain it, and several that can convert from Gregorian year/month/day into Julian. One is at http://aa.usno.navy.mil/data/docs/JulianDate.html

If you're using Celestia v1.2.5pre7 or later, Beginning, Ending and Epoch all can be specified as a Gregorian date. Here's what Chris wrote in another thread:

It's already in 1.2.5pre7 . . .

Beginning, Ending, and Epoch can all be specified as calendar date strings instead of Julian dates.

They should appear in this format:

yyyy mm dd hh:mm:ss

where hours, minutes, and seconds are optional.

Example:

Beginning "2001 3 21 10:15"
Ending "2002 11 8 9:45:30"

--Chris

I hope this helps.

[Commander David]

yes, thanks.

[selden]

OOPS: I updated my posting to make a comment about "TT" at about the same time you were responding.