Across the Universe: Setting holidays using the night sky

When CFHT staff visits local students, we often use Easter as an example of a holiday where the date is determined by the sky. The same applies to Passover, the major Jewish spring holiday. The date for Easter in 2019 has caused a bit of confusion.

Let us start with Passover. Passover begins on the 15th day of the Hebrew month of Nisan, which falls in March-April on the Western calendar. The Hebrew calendar is a lunisolar calendar, which harkens back to times when people used a combination of moon phases and the position of the sun to measure the passage of time. Because the Hebrew calendar is based on the phases of the moon, the 15th day of the month corresponds to the 15th night of the lunar cycle, the night of the full moon. In the Hebrew calendar, the day begins at sundown. To ensure that the holiday always began in spring, the tradition in ancient Israel was the first day of Nisan did not begin until the barely was ripe.


In the Christian tradition, Easter Sunday falls on the first Sunday after the first full moon after the first day of spring. Scientists now determine the first day of spring based on complex calculations involving the position of the moon.

According to the website, there are more than 600 terms in the equations to calculate the moon’s position. When the Catholic Church set the date for Easter, they also set the date for the first day of spring, known as ecclesiastical spring, for March 21. In most cases, the ecclesiastical spring and “science” spring align well enough that the date of Easter is the same regardless of method.

Not this year. The first day of spring was March 20, with the first full moon occurring at 1:21 a.m. March 21. Because the full moon occurred on the early morning of ecclesiastical spring, that full moon did not “count” in the calculations. Instead, the April 19 full moon is the one that set the date for Easter, Sunday, April 20. The last time a spring versus ecclesiastical spring confusion affected Easter was in 1981.


Using the phases of the moon and position of the sun in the sky was the basis for many ancient calendars and calendars used today, like the Hebrew, Islamic, Chinese, Japanese, and Korean calendars. Lunar New Year, which usually occurs on the second new moon after the first day of winter, is celebrated at the end of January or in February. The Hawaiian calendar is a lunisolar calendar and celebrations like Makahiki are set using the sky.

If you are confused, eat a little extra chocolate and be glad we did not talk about the difference between Western Easter and Orthodox Easter, i.e. the old Julian versus new Gregorian calendars. Calendars are complicated.

Leave a Reply

Your email address will not be published. Required fields are marked *


By participating in online discussions you acknowledge that you have agreed to the Star-Advertiser's TERMS OF SERVICE. An insightful discussion of ideas and viewpoints is encouraged, but comments must be civil and in good taste, with no personal attacks. If your comments are inappropriate, you may be banned from posting. To report comments that you believe do not follow our guidelines, email