Earth and Moon Viewer:
Specifying Time and Date


This set of controls selects the time and date the image will 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), in either 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 specifies 22:17:40 Universal Time on July 20, 1969, the instant when Apollo 11 landed on the Moon. Press the “Update” button to view the Earth as seen from the Moon at the moment the Eagle landed in the Sea of Tranquility.

If you're entering dates before 1 A.D., note that Earth and Moon Viewer 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 frequently work with events with great regularity in time, and 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 start 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 as a decimal fraction of a day.

You can enter any Julian date between 0.5 (January 1, −4712) and 4346655.5 (December 31, 8000). For Julian dates for years before 1 A.D., note that astronomers and historians use different conventions for those years. In history books, the year that preceded 1 A.D. is called 1 B.C.; zero not having come into use in European culture at the time. Astronomers consider the year before 1 A.D. 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, Earth and Moon Viewer 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.


The form above specifies a the date and time as Julian Day 2440423.42894, the time of Apollo 11's touchdown. Press “Update” to view the Moon as seen from Earth at that moment. Note that the Apollo 11 landing time was chosen so that the Sun would be low in the sky at the time of the final descent—this made it much easier to discern craters and rocks which might upset the Lunar Module were it to alight upon them.

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by John Walker