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The Julian Date (JD) is the Julian day number combined with the fraction of the day since noon at Greenwich, England (any Greenwich time such as Universal Time or Terrestrial Time may be used). The fraction of the day is found by converting the number of hours, minutes, and seconds since noon into a decimal fraction.
In other contexts, Julian date is also used to refer to:
The Heliocentric Julian Day (HJD) is the same as the Julian day, but adjusted to the frame of reference of the sun, and thus can differ from the Julian day by as much as 8.3 minutes, that being the time it takes light to reach Earth from the Sun. The Julian day is sometimes referred to as the Geocentric Julian Day (GJD) in order to distinguish it from HJD.
Another version of the Julian day, introduced by Peter Meyer, is the chronological julian day (CJD), in which the starting point is set at midnight January 1, 4713 BC (proleptic Julian calendar) local time rather than noon UT. Chronographers found the Julian day concept useful, but they didn't like noon as the starting time. So CJD = JD + 0.5. Note that JD may use Universal Time (UT) or Terrestrial Time (TT), and so it is the same for all time zones and is independent of Summer Time or Daylight-Saving Time (DST). On the other hand, CJD is not, so it changes with different time zones and takes into account the different local DSTs. Users of CJD sometimes refer to the Julian day as astronomical Julian Day (AJD) to distinguish it from CJD.
Because the starting point is so long ago, numbers in the Julian day can be quite large and cumbersome. A more recent starting point is sometimes used, for instance by dropping the leading digits, in order to fit into limited computer memory with an adequate amount of precision.
The Modified Julian Day (MJD), introduced by the Smithsonian Astrophysical Observatory in 1957Events January January 2 San Francisco and Los Angeles stock exchanges merge. January 3 Hamilton Watch Company introduces the first electric watch January 4 After 69 years the last issue of Colliers magazine is published January 5 Russell Endean becomes t to record the orbit of Sputnik, is defined in terms of the Julian day as follows:
The offset of 0.5 means that MJD started at midnight at the beginning of November 17November 17 is also the name of a Marxist group in Greece. November 17 is the 321st day of the year (322nd in leap years), with 44 remaining. Events The Leonids are visible each year around this day. 1292 ( Julian calendar) John Balliol becomes King of Sc, 1858Events January 14 Felice Orsini and his accomplices fail to assassinate Napoleon III in Paris but their bombs kill 156 bystanders. Because of the involvement of French emigres living in Britain, there is a brief anti-British feeling in France but the empe, and that every Modified Julian Day begins and ends at midnight UT or TT.
The Reduced Julian Day (RJD) is also used by astronomers and counts days from the same day as MJD, but from noon UT or TT, and thus is defined as:
The Truncated Julian Day (TJD) was introduced by NASAThe National Aeronautics and Space Administration NASA (established 1958) is the government agency responsible for the United States of America's space program and long-term general aerospace research. A civilian organization, it conducts (or oversees) re for the space program. TJD began at 24 May 1968Events Undated Booker Prize for Fiction is established by Booker plc. 1968 is known as the year of the Prague Spring and also the year of the Paris riots. The ASCII character code is standardized as ANSI Standard X3. Nauru adopt his national anthem of the. Since TJD exceeded four digits on 10 October 19951995 was a common year starting on Sunday (see link for calendar). It has a Golden number of 1, and was the first year of the International Decade of the World's Indigenous People (1995- 2005): http://www. org/culture/indigenous . Events January events Ja, some now count TJD from this date in order to maintain a four-digit number. It can be defined as:
or
The Dublin Julian Day (DJD) is used by computer programmers, and is a count of days from midnight of January 1, 1900.
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