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Since no such objects existed prior to October 4, 1957, all satellite-related MJDs are positive.

The Julian calendar was proclaimed by Julius Csar in 46 The Julian calendar differs from the Gregorian only in the determination of leap years, lacking the correction for years divisible by 100 and 400 in the Gregorian calendar.

This date is defined in terms of a cycle of years, but has the additional advantage that all known historical astronomical observations bear positive Julian day numbers, and periods can be determined and events extrapolated by simple addition and subtraction.

Julian dates are a tad eccentric in starting at noon, but then so are astronomers (and systems programmers!

The Hebrew (or Jewish) calendar attempts to simultaneously maintain alignment between the months and the seasons and synchronise months with the Moon—it is thus deemed a “”.

In addition, there are constraints on which days of the week on which a year can begin and to shift otherwise required extra days to prior years to keep the length of the year within the prescribed bounds.

)—when you've become accustomed to rising after the “crack of noon” and doing most of your work when the Sun is down, you appreciate recording your results in a calendar where the date doesn't change in the middle of your workday.

But even the Julian day convention bears witness to the eurocentrism of 19th century astronomy—noon at Greenwich is midnight on the other side of the world.

This differs from the Julian calendar in which there is no year 0—the year before year 1 in the Julian calendar is year −1.If you add the additional rule that years evenly divisible by 4000 are leap years, you obtain an average solar year of 365.24225 days per year which, compared to the actual mean year of 365.24219878, is equivalent to an error of one day over a period of about 19,500 years; this is comparable to errors due to tidal braking of the rotation of the Earth.Astronomers, unlike historians, frequently need to do arithmetic with dates. This page allows you to interconvert dates in a variety of calendars, both civil and computer-related.All calculations are done in Java Script executed in your own browser; complete source code is embedded in or linked to this page, and you're free to download these files to your own computer and use them even when not connected to the Internet.

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