Milwaukee Astronomical Society


Eclipsing Binary Ephemeris

The MAS Eclipsing Binary Ephemeris provides the predicted time of mid-eclipse for a number of eclipsing binaries. These times appear in Universal Time in the body of the ephemeris table. The time is rounded to the nearest half hour, which provides sufficient accuracy to plan an observing session while, hopefully, leaving sufficient doubt about the exact time in order to eliminate anticipatory bias. The ephemeris is designed for use by observers at American longitudes.

The top rows of the ephemeris table list the name of the eclipsing binary. Directly below the star's name are its approximate maximum and minimum magnitudes. These magnitudes are taken from the 4th edition of the GCVS and may be visual, photographic, or V.

Below that, in the row labeled "DUR", the approximate number of hours required to obtain a time of minimum. This time is typically shorter than the duration of the eclipse listed in the GCVS. We need good coverage of the steep portion of the descending leg through the corresponding portion of the ascending leg of the eclipse to measure the time of mid-eclipse accurately.

The next row, labeled "TOT" indicates the duration of the totality at minimum in hours.

The numbers in the left-most column are the "double date"—the evening and the following morning—for the event times listed in the corresponding row. For example, 5–6 corresponds to the evening of the 5th and the morning of the 6th of the month. January 0–1 is the evening of December 31 and the morning of January 1.

The 'S' in the table heading stands for secondary eclipse. All other predictions are for the primary eclipse. Sometimes a secondary eclipse column may appear where there is no primary eclipse column for a star; this occurs when none of the primary eclipses for that star during that month are observable.

For two stars (SS Boo and AQ Peg) a '2' or '3' may appear below the star name. This indicates the prediction is for the 2nd or 3rd contact of the eclipse. Because both of these stars have a long totality (about 6 hours), it may be required to observe each of these contacts on separate nights to obtain a time of minimum.

Click here for an introduction to the AAVSO: RR Lyrae Observing Program

These ephemerides are available by clicking on the links below. The data is presented in Adobe Acrobat format. The reader can be downloaded for free.

Eclipsing Binary Ephemerides*


Jan 2022
Feb 2022
Mar 2022
Apr 2022
May 2022
Jun 2022

Jul 2022
Aug 2022
Sep 2022
Oct 2022
Nov 2022
Dec 2022


Jan 2021
Feb 2021
Mar 2021
Apr 2021
May 2021
Jun 2021

Jul 2021
Aug 2021
Sep 2021
Oct 2021
Nov 2021
Dec 2021

* Please note: Ephemerides are published as they become available. Some links may not work correctly.