Data sources. Ephemerides, eclipse duration and depth for short-period systems from
Gaia DR3 (the
vari_eclipsing_binary table), which carries an explicit frequency error from which the uncertainty is computed. Detached systems with a sharp eclipse from
GCVS 5.1, whose periods rest on a long time baseline and are therefore more accurate than Gaia for narrow eclipses. Names and variable classifications from
AAVSO VSX.
Updated ephemerides from VarAstro. For stars marked
VarAstro in the uncertainty column, the period and the time of minimum were taken from an export of
VarAstro, which is maintained from a database of hundreds of thousands of observed minima. These are the freshest data available, and they include the actually measured phase of the secondary minimum instead of assuming it is 0.5. VarAstro does not publish a period error, so their uncertainty is computed from a conservative assumption of 10⁻⁵ of the period — about a minute and a half after three months, about five minutes after a year. Because the reference epoch is recent, the drift over the near term is negligible.
The timing uncertainty is also given as a number for stars whose ephemeris comes from Gaia, where the frequency error was measured and published. It is recomputed for every date, as the period error times the number of cycles elapsed since the reference epoch, and so grows the further you are from it. It does not include real period changes — mass transfer, or a light-time effect from a third companion.
What is not included. Stars whose only ephemeris comes from GCVS were removed from the database entirely. No period error is published there, and calibrating against Gaia showed the real error to be 6.8 times the published precision at the median and 120 times at the 90th percentile — meaning any number shown for them would have been fiction. An example: FP Sge, whose epoch is from 1934; after 52,524 cycles the deviation reaches two hours. Every row in the table rests on an ephemeris whose error can be quantified.
To verify. Cross-reference against
VarAstro's minima predictions — the link on each star's name searches for it there directly. The tool does not account for cloud, a local horizon, or the Moon.
The local clock is the one your own computer is set to. It is read from the browser, so daylight saving is handled for you wherever you are, and the offset in use is printed above the summary. The UTC column is there when you would rather not rely on it.