Astronomical Time Calculations

Julian Day, Equation of Time and Siderial Time


  • Local Apparent Time - Sun Dial Time or hour angle of the real Sun
  • Local Mean Solar Time - The hour angle of a fictitious, mean Sun and used to determine Standard Time.
  • Standard Time - The Local Mean Solar Time at the standard meridian of your time zone. Standard meridian are space approximately 15o apart in longitude around the Earth.
  • Equation of Time - The difference between the apparent time and the mean time. The Equation of Time value for the date is used to convert Sun Dial Time to Local Mean Time.

Local Siderial Time is the 'star time'. It is the hour angle of the vernal equinox. Or in more useful terms, the right ascension of the stars on your local celestial meridian.

Julian Day is a linear count of days starting on January 1, 4713 BC and is commonly used for astronomical calculations. The Julian Day starts at Noon Universal Time (UT) hence at 0h UT the Julian day is some whole number + 0.5.

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Created by L.Bogan - March 1998
Updated with corrections for errors- 7 Nov, 2000

Siderial Time > 24 hr corrected - Nov 20,2002


Enter the Longitude of your location.
[This affects local Siderial Time and use of the Equation of Time.]
Longitude (degrees) =
(west longitude is negative)

Enter the Date and Local Time for which the Julian Day will be Calculated
(The time should hours and fraction on the 24 hour clock)

Year - Month - Day - Time

Local Time Zone hours from UT.
Julian Day
Julian Day 0h UT
Equation of Time minutes
Longitude Correction minutes
Local Siderial Time

Standard Time
= Sundial Time - Equation of Time - Longitude Correction