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. 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. 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. For Julian dates for years before 1 A.D., note that astronomers and historians use different conventions for those years.