Calendars

From d54fe7c1f704a63824c5bfa0ece65245572e9b27 Mon Sep 17 00:00:00 2001 From: Joseph Hunkeler Date: Wed, 4 Mar 2015 21:21:30 -0500 Subject: Initial commit --- src/slalib/sun67.htx/node222.html | 113 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 113 insertions(+) create mode 100644 src/slalib/sun67.htx/node222.html (limited to 'src/slalib/sun67.htx/node222.html') diff --git a/src/slalib/sun67.htx/node222.html b/src/slalib/sun67.htx/node222.html new file mode 100644 index 0000000..7cc8f58 --- /dev/null +++ b/src/slalib/sun67.htx/node222.html @@ -0,0 +1,113 @@ + + + + +Calendars + + + + + + + + + + + + +

+ +

+
+ Next: Geocentric Coordinates +
+Up: EXPLANATION AND EXAMPLES +
+ Previous: Dynamical Time: TT, TDB +

+Calendars +

+The ordinary Gregorian Calendar Date, +together with a time of day, can be +used to express an epoch in any desired timescale. For many purposes, +however, a continuous count of days is more convenient, and for +this purpose the system of Julian Day Number can be used. +JD zero is located about 7000 years ago, well before the +historical era, and is formally defined in terms of Greenwich noon; +Julian Day Number 2449444 began at noon on 1994 April 1. Julian Date +is the same system but with a fractional part appended; +Julian Date 2449443.5 was the midnight on which 1994 April 1 +commenced. Because of the unwieldy size of Julian Dates +and the awkwardness of the half-day offset, it is +accepted practice to remove the leading `24' and the trailing `.5', +producing what is called the Modified Julian Date: +MJD = JD-2400000.5. SLALIB routines use MJD, as opposed to +JD, throughout, largely to avoid loss of precision. +1994 April 1 commenced at MJD 49443.0. +

+Despite JD (and hence MJD) being defined in terms of (in effect) +UT, the system can be used in conjunction with other timescales +such as TAI, TT and TDB (and even sidereal time through the +concept of Greenwich Sidereal Date). However, it is improper +to express a UTC as a JD or MJD because of leap seconds. +

+SLALIB has six routines for converting to and from dates in +the Gregorian calendar. The routines +sla_CLDJ +and +sla_CALDJ +both convert a calendar date into an MJD, the former interpreting +years between 0 and 99 as 1st century and the latter as late 20th or +early 21st century. The routines sla_DJCL +and +sla_DJCAL +both convert an MJD into calendar year, month, day and fraction of a day; +the latter performs rounding to a specified precision, important +to avoid dates like `94 04 01.***' appearing in messages. +Some of SLALIB's low-precision ephemeris routines +(sla_EARTH, +sla_MOON +and +sla_ECOR) +work in terms of year plus day-in-year (where +day 1 = January 1st, at least for the modern era). +This form of date can be generated by +calling +sla_CALYD +(which defaults years 0-99 into 1950-2049) +or +sla_CLYD +(which covers the full range from prehistoric times). +

+ +

+
+ Next: Geocentric Coordinates +
+Up: EXPLANATION AND EXAMPLES +
+ Previous: Dynamical Time: TT, TDB +

+SLALIB --- Positional Astronomy Library
Starlink User Note 67
P. T. Wallace
12 October 1999
E-mail:ptw@star.rl.ac.uk +

+ + -- cgit