the position on the celestial sphere towards which the Earth's axis points at a particular epoch, with the oscillations due to precession-nutation removed.

Source: IERS Technical Note 36: Glossary (PDF, 244KB, Not barrier-free file.) of the IERS Conventions (2010)

Original Source: IAU 2006 NFA Glossary

Acronym: MJD

The Modified Julian Day (MJD) is defined as MJD = JD - 2400000.5 , where JD is the Julian Day. Start of the JD count is from 0 at 12 noon 1 JAN -4712 (4713 BC).

MJD is used when a continous numbering of days is more convenient than the use of the civil calendar. Fractions of the day can be added to indicate the exact time of the day. The counting starts at midnight.

Related links:

• Modified Julian Day (extended text from USNO)
• Modified Julian Dates (text from JPL)
• Julian Day (Wikipedia entry, in English, French, German etc.)
• Modified Julian Day Converter
• Julian Day and Civil Date Calculator