Dec 21 ’10 — an astronomy biggie!


On December 21st, here on the northwest coast of N.Am., we’ll have the Extremely Rare coincidence of:

  • solstice
  • full moon
  • total lunar eclipse

Wowee! OK, how rare is this, you ask?

  • Previous solstice full moons were Dec. 22, 1999 & Dec. 21, 1980. (No eclipses.)
  • The next winter solstice full moon here will be Dec. 21, 2094. A lunar eclipse will accompany that, but will not be visible in this part of the world.
  • The last time all 3 events above happened together was Dec. 21, 1638! BUT the eclipse was not visible in our part of the world. The solstice arrived in Greenwich timezone 2 hours after the full moon eclipse but not until the next day over here.

So really, for our part of the world, this is pretty much a never-before-seen event and a not-again-in-this-lifetime event.

“On the West Coast, it begins around 9:30 p.m. PST Monday.
Maximum eclipse is at 12:17 a.m. PST.”


[photo credit: thanks to the Southern Maine Astronomers website]
[info credit: facts above are from]

A geologist friend-of-a-friend in Calgary sent the following factoids & comments after reading my post.

  • probability of full moon on winter solstice = 1/28 = 0.036
  • probability of full moon during eclipse = 1.000
  • probability of total lunar eclipse in a year = 0.712 (average over last 5000years)
  • if there is an eclipse the probability of it occurring on the full-moon nearest solstice = 1/13 = 0.077
  • probability of eclipse occurring during night-time hours in Calgary Winter ~ 0.65
  • probability of year with total eclipse on solstice at night in Calgary =
    0.036 x 0.712 x 0.077 x 0.65 = 0.0013 or about once in 800 years.

So yes that’s pretty rare.

Despite being a rarity, the winter solstice is actually the most probable day of the year to observe a total lunar eclipse, simply because there is the most night-time. Run the same calculation for any other day and it will be even more improbable, with the Summer solstice the most improbable of all.


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