Astronomical Julian Day Calculator

Free astronomical Julian Day calculator. Convert dates to Julian Day Numbers for astronomical calculations. Essential for ephemeris work.

Calculate the Astronomical Julian Day Number for any date. Julian Day Numbers provide a continuous day count used in astronomy for calculating celestial events and positions.

Common use cases

  • Astronomical event calculations
  • Ephemeris and orbital mechanics
  • Historical astronomy research
  • Space mission planning
  • Eclipse and conjunction predictions

How to use

  1. Enter any date and time
  2. View the Julian Day Number
  3. See Modified Julian Date (MJD)
  4. Calculate differences between JD values
  5. Convert back to calendar dates

FAQ

When did Julian Day counting start?

JD 0 was January 1, 4713 BC (Julian calendar) at noon, chosen to predate all recorded history.

Why noon instead of midnight?

Astronomers chose noon so observations during a single night share the same Julian Day number.

What is Modified Julian Date?

MJD = JD - 2,400,000.5, starting at midnight November 17, 1858. It uses smaller numbers and midnight epochs.