Solstice full moon is rare, but how rare?

The moon will rise with the setting sun Monday, June 20, as every full moon does. But this full moon is a bit rarer because it falls on the June solstice.

Monday, June 20, at 22:34 UTC (6:34 p.m. EDT) the sub-solar point reached its furthest point North, directly over 23 degrees, 26 minutes north latitude, the Tropic of Cancer. This yearly event is better known as the June solstice. It is the first day of astronomical summer in the northern hemisphere and the longest period of sunlight (14 hours, 35 minutes in Raleigh).

The June solstice falls between June 20 and 22 each year. This variance happens because the Gregorian calendar year is about a quarter day faster than the astronomical calendar year. Gone unchecked, the moment of the solstice would continue to occur about six hours later each year. But leap days help bring the two calendars closer together ensuring the solstice falls between those dates.

About 11 hours earlier, at 11:02 UTC (7:02 a.m. EDT) the moon reached the point in its orbit around the Earth where it was most fully illuminated, a full moon.

Solstice Full Moons are a rare occurrence, but there is some disagreement about just how rare they are. Across the globe, some headlines call it a once-in-a-lifetime event pointing to either 1948 or 1967 as the last occurrence. Another claims it last occurred in 1986. I’ll even throw 2010 into the discussion as well.

So which is it? How could something based in well-understood math produce so such widely varying answers? The reasons are rooted in how the words “solstice" and “day" are defined for these calculations.

The technically correct (the best kind of correct) answer to the question “when did the last Full Solstice Moon occur?” is: December 21, 2010, when the solstice occurred at 23:38 UTC and the full moon at 08:13 UTC. Solstices occur twice yearly in June and December. In the Northern hemisphere the June Solstice (the astronomically, technically correct term) is also known as the summer solstice, marking the beginning of astronomical summer.

If we look just to the June solstice and continue to use the UTC time zone as our guide, the last Full June Solstice Moon occurred June 22, 1967. This is a reasonable definition because astronomers use UTC (Coordinated Universal Time, an abbreviation which compromises between spellings in English and French) to calculate events that occur at the same moment regardless of what time zone you call home.

If you insist on taking a view from the Eastern time zone, the last Full June Solstice Moon occurred June 21, 1986. If your Full June Solstice Moon must fall on the same day in both the UTC and EDT time zones, then June 21, 1948, is the date for you.

So where did this 1948 year come from? The best answer I’ve been able to come up with from my calculations (based on formulae described in Jean Meeus’s seminal book on the subject: Astronomical Algorithms) is that June 21, 1948, is the last time a full moon fell on the day of the June solstice in both EDT and UTC.

I was puzzled why journalists went with this definition until I found their source: The Old Farmers Almanac. No one in the WRAL Weather center will ever recommend this as your best source for weather or astronomy data.