Equinox & Solstice Calculator

TIME ACCURACY: The calculated times are in Terrestrial Dynamical Time (TDT or TT), a replacement for Ephemeris Times (ET). Meeus claims the error is less than one minute for the years 1951-2050 for these calculations (the errors increase as you move farther away from these dates). We thank Ken Burrell of the China Lake Astronomical Society who in February 2024 improved the calculation of weekdays to be more accurate and updated the TDT correction table.

CIVIL TIME: TDT is a uniform time used for astronomical calculations. Civil time, such as UTC (popularly known as GMT, but technically incorrect) or your local time, is corrected for the non-linear changes in the rotation of the earth. This is done accurately only through observations. This web page corrects the the calculated TDT to civil time using a table of observations from 1620 thru 2002. Time corrections outside this range are estimated from predictive equations. See Meeus' book for additional details.

LOCAL TIME: Local time zone conversion is done by the JavaScript built-in runtime functions from the corrected UTC (including daylight or summer time rules), based on information it gets from your computer's environment settings. If the local time is in the wrong time zone for you, you need to check and adjust these settings on your computer, there is nothing in this web page or the code behind it to change this. On Windows machines, right click on the clock in the task bar and select "Adjust Date/Time", and then click the "Time Zone" tab.

CREDITS: The algorithms and correction tables for this computation come directly from the book Astronomical Algorithms Second Edition by Jean Meeus, ©1998, published by Willmann-Bell, Inc., Richmond, VA, ISBN 0-943396-61-1. They were coded in JavaScript and built into this web page by the web page author, Ken Slater.

CAUTION: While the output has been spot checked for correctness against known tables, the code has not been thoroughly tested and could contain errors. The underlying algorithms are not exact and times could vary by several minutes. Consult the book credited above for detailed information. Note also that the day-of-week will not be correct when the date is in the Julian calendar (earlier than 1582-Oct-15).