How is Easter's date determined?

The date has been printed in church bulletins for weeks and on the calendar hanging on your fridge for even longer, but how was it determined? Even the time traveler in the BBC 's long running sci-fi series Dr. Who remarked, "Easter, I can never find it. It's always at a different time."

The simple definition is that Easter is observed on the first Sunday after the first full moon of spring. In reality, Easter's determination is more mathematical than astronomical. This is because the precise definition is "the first Sunday after the ecclesiastical full moon that falls on or next after March 21."

The ecclesiastical full moon is based on tables that usually correspond to the astronomical full moon, but not always. The complexity of lunar motion isn't accurately reflected in these tables, nor do they account for irregularities in the calendar like leap days.

This places the holiday as early as March 22 and as late as April 25. The earliest recently was March 23, 2008 and latest April 24, 2011. We won't see anything later until 2038. The most common date across the 5.7 million year cycle of Easter dates is April 19.

You'll also likely find two dates for Easter on your calendar.

Western religions such as Roman Catholic and Protestants, which make use of the commonly used Gregorian calendar, observed Easter yesterday. Eastern Orthodox churches make use of the Julian calendar. 

Both use similar methods for calculating the date, but that March 21 starting point is shifted by 13 days on the Julian calendar. The moon continues to go through its lunation cycle regardless of how calendars are kept here on Earth. As a result, the two Easters usually vary by several days to several weeks but have fallen on the same day in recent years.

Look to the southwest beginning tonight at 8:37 p.m. tonight. The ISS will appear like a bright, quickly moving star. It should be visible for nearly 3 minutes before setting in the northeast. It will pass again tonight at 10:14, starting in the west.

There are additional opportunities this week beginning tomorrow at 9:17 p.m. starting in the southwest, Wednesday starting in the southwest at 8:21 p.m. and passing directly overhead so brightly that it should be impossible to miss. Thursday's pass will begin at 9:02 p.m. rising in the west and setting in the northeast.