Teaching · For Teachers · Astronomy

One running count
of days, for every
date you need.

Astronomers number days in a single count that started thousands of years ago. Type a Julian Date to find its calendar date, or list calendar dates to get theirs, with the day of the year and days between them.

Convert a dateWhat you get

The Julian Date converter

The worked example converts JD 2461000.5 to a calendar date and lists four moments, from the J2000.0 reference epoch to a first day of class. Nothing you type leaves this browser.

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What you get

Teaching · For TeachersFree

What you finish with
The calendar date and weekday for a Julian Date, and for each calendar date you list its Julian Date, Modified Julian Date, day of the year, days since J2000.0 and weekday, with the days between the first and last. It prints as a table and downloads as a spreadsheet.
Who it is for
Science and astronomy teachers and their students, astronomy clubs, and students working with dated data.
Format
Browser converter for a phone, tablet or computer, by touch or keyboard. Gregorian dates from October 15, 1582, in Universal Time. Up to 40 dates. Prints a table; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The For Teachers space in Caveat Teaching.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Type a Julian Date at the top to see its calendar date, if you have one.
  2. Add calendar dates with a label and a time in Universal Time, or leave the time blank for midnight.
  3. Read each Julian Date, its Modified Julian Date and the day of the year.
  4. Use the days between dates for orbits, light curves or any time series.
  5. Print the table or download the spreadsheet.

Convert a date

How the conversion works

Meeus’s algorithm, worked for J2000.0.

January 1, 2000 at 12:00 UTC (J2000.0), by the Meeus algorithmJan and Feb countY 1999, M 13as months 13 and 14Century fixA 19, B −13B = 2 − A + ⌊A/4⌋Day with timeD 1.5noon is half a day⌊365.25(Y+4716)⌋ + ⌊30.6001(M+1)⌋ + D + B − 1524.52452653 + 428 + 1.5 − 13 − 1524.5each piece of the sumJulian Date2451545.0J2000.0J2000.0 by the Meeus algorithmJan counts asmonth 13of 1999Century fixB = −13A = 19Sum of the pieces2452653 + 428 + 1.5− 13 − 1524.5Julian Date2451545.0January 1, 2000, 12:00 UTC
The conversion for J2000.0, the standard reference moment in astronomy: January 1, 2000 at noon Universal Time is Julian Date 2451545.0.

The arithmetic

The page uses the algorithm in Jean Meeus’s Astronomical Algorithms: January and February count as months 13 and 14 of the year before, a century correction accounts for the Gregorian leap-year rule, and whole-day terms built from 365.25 and 30.6001 days are added up and offset so the count starts at noon on January 1, 4713 BC. The reverse direction undoes the same steps. The Modified Julian Date subtracts 2400000.5 so it starts at midnight and needs fewer digits.

Two different “Julian dates”

Astronomers’ Julian Dates are a running count of days and fractions of a day. The “Julian date” printed on food packages and used in farming and by the military is usually the day of the year, from 1 to 365 or 366, often with the year in front. Neither has anything to do with the Julian calendar that the Gregorian calendar replaced in 1582, apart from the name.

Sources

  1. Meeus, J. (1998). Astronomical Algorithms, 2nd ed. Willmann-Bell, chapter 7: Julian Day, conversion in both directions.
  2. U.S. Naval Observatory, Astronomical Applications Department. Julian Date Converter and notes on Julian dates (aa.usno.navy.mil).
  3. International Astronomical Union. Resolution on the J2000.0 standard epoch: JD 2451545.0, January 1, 2000, 12:00 Terrestrial Time.
  4. NASA Goddard Space Flight Center, HEASARC: Modified Julian Date (MJD = JD − 2400000.5).

The worked example uses published dates and one made-up class date. Times are Universal Time; leap seconds and the difference between Universal and Terrestrial Time are not modeled. Your dates stay in this browser. Support: team@caveatacademic.com.