Code written by Rahul Mittal, Villanova University, Villanova, PA
Visit my homepage at http://www.rasteroid.com/.
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Calendar Date to Julian Date converter
Julian Date to Calendar Date converter
Times of Minima Table generator
Everyone knows how to use the conventional date system with seven days a week, about 30 days a month, and 12 months a year (or at least I hope everyone does!). Such a system is very confusing - some months have 31 days, some 30, and the number of days in February change almost every four years! Most people learned and starting using this complicated date system at a very young age, so they face little or no problems with it. However programming a computer with such variable dates is a very difficult task indeed! Which is why we had to choose the simple way out: the Julian Date! In fact, the whole of science is about choosing the easy way out! But I digress... my apologies. On this web page I shall explain and apply Julian Dates (JD) in context with astronomical dating methods, although a JD is (I am told) a more general concept.
In astronomy, a JD is defined as the contiguous count of days from January 1, 4713 B.C., Greenwich Mean Noon (equal to zero hours Universal Time). The fraction of each day is represented as a decimal number. Hence noon (GMT) on January 2, 4713 B.C. would have JD 1.00000, 6.00 pm GMT on the same day would have JD 1.25000 and 6.00 hours Universal Time would have JD 2443509.75. Simple, right?
Also you may ask, what's so special about January 1, 4713 B.C.? Well,
according to Kevin Bourque, this day was chosen because, amongst other reasons,
it is the most recent day in which the year began on a Sunday with a full moon.
In addition, Kenneth Silverstein points out that the Julian Day count was:
Example (with time specified): December 25, 1990
Example (without time specified): April 1, 1993
Several objects seen in the night sky have been found to vary periodically with time. For example binary star systems can be pulsating, or simple stars like Polaris (the Pole Star) can vary in brightness (magnitude) with time. To observe these objects using telescopes, astronomers have to know at precisely what part of its cyclical phase the star is in so that they can calibrate their observations accordingly.
Often it is necessary to observe such objects over their entire cycle. Luckily, one doesn't have to observe these objects continuously over the complete period of the cycle. This is a great relief, especially since many objects have a period of more than one day! (Imagine the poor astronomer locked up in the tiny observatory room for days on end unable to take his eye off the instruments!) The observing can be split into several sessions, but to do so one must know exactly how to time these sessions such that one knows precisely when that particular phase of the cycle occurs. A Times of Minima table is simply a table that, given the cyclical period of the star, lists a set of dates and times between the specified starting and ending dates. An astronomer can then observe the sky at these times and find that particular phase of the cycle occurring for that star. Try it below and see how easy it is!
Contact me by e-mail at firstname.lastname@example.org if you have questions, comments or suggestions.Last updated on November 28, 1997.