Julian Date

The continuous day count system used in astronomy

What is Julian Date?

Julian Date (JD) is a continuous count of days since noon Universal Time on January 1, 4713 BCE in the Julian calendar. This date was chosen because it's far back enough that all historically recorded astronomical events occurred after it.

Why Use Julian Date?

Astronomers use Julian Date because:

• Continuous count - No gaps from calendar reforms
• Worldwide standard - Same JD regardless of location
• Simple calculations - Easy to find time differences
• No leap years - Each day is exactly 1 unit

Modified Julian Date (MJD)

Modified Julian Date (MJD) is a simplified version:

MJD = JD - 2400000.5

The 0.5 shifts the start from noon to midnight, making it more intuitive for daily use.

• MJD 0 = November 17, 1858 (midnight)
• MJD 60000 = November 17, 2024

How to Calculate Julian Date

For any date, you can calculate JD using this formula:

JD = (1461 × (Y + 4800 + (M - 14) / 12)) / 4 
    + (367 × (M - 2 - 12 × ((M - 14) / 12))) / 12 
    - (3 × ((Y + 4900 + (M - 14) / 12) / 100)) / 4 
    + D - 32075

Where Y = year, M = month, D = day (with decimals for time).

const jd = (date.getTime() / 86400000) + 2440587.5;

Common Uses

• Astronomy - Recording observations of celestial events
• Space missions - Mission timeline coordination
• Satellite tracking - Orbital calculations
• Scientific research - Time-sensitive experiments

Example Julian Dates

See Also

• Unix Timestamps - Epoch time for computers
• ISO 8601 - International date format
• Time Measurement - History of measuring time