Frequently Asked Questions
Why does the Julian day start at noon?
When the Julian Date system was established for astronomy, observations were made at night. Starting the day at noon meant that a single night's worth of observations fell within one Julian Date, avoiding a date rollover mid-observation.
Is the Julian Day the same as the Julian calendar?
No. The Julian calendar is a historical calendar system introduced by Julius Caesar. The Julian Day Number is an unrelated counting system used in astronomy, named after Julius Caesar Scaliger, not the calendar.
What is Modified Julian Date used for?
MJD starts at midnight (not noon) and produces smaller numbers, which are more convenient for modern computing and record-keeping. MJD = JD − 2,400,000.5.
How do I find the number of days between two dates using JDN?
Simply subtract the two Julian Day Numbers. For example, if date A has JDN 2,460,000 and date B has JDN 2,460,100, there are exactly 100 days between them, with no calendar arithmetic required.
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