Solar System Live Help

Time and Date


This set of controls select the time and date the image should represent. The default, “Now”, is the current time and date. You can specify a date in the past or the future (from −4712 [or 4713 B.C.] through A.D. 8000), as either a date and time in Universal (Greenwich Mean) Time or by Julian Date.

Universal Time

To specify a date and time in Universal time, check the “UTC” box and enter the date and time in the text field in the form:

year-month-day hour:minute:second

If you omit minute or second, they're taken as zero. The entry in the example box above represents 22:00 (10 P.M.) Universal Time on September 17th, 1993, the date of the second approach of the “triple conjunction” between Uranus and Neptune in that year. Check the UTC box and press “Update” and observe how Earth, Uranus, and Neptune lined up at that moment. Hold a ruler to your monitor and note how precise the alignment was.

If you're entering dates before A.D. 1, note that Solar System Live follows the astronomical convention where the year historians call 1 B.C. is denoted “year 0”, 2 B.C. “year −1”, and so on. Thus, to specify the year “413 B.C.” you would use “−412” in the Universal Time box. See the discussion below for more details, including the change-over from the Julian to the Gregorian calendar in 1582.

Julian Days

The Gregorian calendar we use today is both exquisitely precise and preposterously eccentric. Astronomers who often work with events with great regularity in time need to be able to calculate the difference between two dates or when a particular phenomenon will recur in the future without checking off boxes on a calendar. The Julian day system meets these needs. A Julian date is simply the number of days elapsed since the beginning of the year −4712. Since optical astronomers of yore worked mostly by night, and observed mostly from Europe, the Julian Day was defined to begin at noon, 12:00 Universal Time. This avoided the inconvenience of having an observing session begin one day and end the next, as would occur were regular civil dates used.

By starting so far in the past, all historical observations can be expressed by positive Julian Day numbers. Time is expressed by a decimal fraction of days. The Julian Day in the box above, 1719286.5, corresponds to midnight, Universal Time, February 25, −5. Notice how the Earth, Mars, Jupiter, and Saturn are close to being along a straight line. Indeed, at this moment, seen from Earth, a rare (not to repeat for 800 years) conjunction among Jupiter (traditional king of the gods), Saturn (ruling planet of Judah), and Mars, shown like a beacon in the Western sky, in the constellation of Pisces the Fish (the House of the Hebrews) where the equinox had just entered, crossing the border from Aries where it had been since the Greeks. Some people believe this was the Star of Bethlehem. You'll recall that the early Christians recognised one another with the sign of a fish.

You can enter any Julian date between 0.5 (January 1, −4712) and 4346655.5 (December 31, 8000). For Julian dates for years before A.D. 1, note that astronomers and historians use different conventions for those years. In history books, the year that preceded A.D. 1 is called 1 B.C.; zero not having come into use in European culture at the time. Astronomers consider the year before A.D. 1 as “year 0”. Thus when an astronomer talks about an eclipse having occurred in the year −412, that's the year historians refer to as “413 B.C.”. In converting Julian days to historical dates, Solar System Live assumes the canonical date for the adoption of the Gregorian calendar, Friday, October 15th, 1582. Many countries shifted to the Gregorian calendar much later; in Great Britain, not until 1752. When investigating events in history, make sure you express all dates after October 15th, 1582 in the Gregorian calendar.

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by John Walker