There were a couple of questions at last week's meeting about TLEs and Epochs - here is a concise explanation from Ed Morana's transit prediction program:
Two Line Element Sets
The program requires satellite orbital Two Line Elements (TLE) to accurately generate a prediction. By default the TLE files are downloaded from the NASA website http://spaceflight.nasa.gov/.
Each TLE contains an epoch time: the closer the epoch time is to the prediction time, the more accurate the prediction.
NASA normally generates new TLEs about twice a week, which normally span about 12 days.
The TLEs can be imported into various planetarium programs which can display the path of the satellite across the sky.
As the prediction time grows near, the user may desire to download a more recent TLE file than the one provided by NASA. The Space Track website http://space-track.org/ generates TLEs on the order of every few hours; however, the Space Track TLE epoch is always in the past, thus the predictions generated with them are only good for one day or so.
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