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Last quarter moon 3rd of 4 this season

Usually there are only 3 last quarter moons in a season. The November 21, 2016 last quarter moon, though, counts as the 3rd of four.

Simulated image of moon on November 21, 2016 via the US Naval Observatory

Simulated image of moon on November 21, 2016 via the U.S. Naval Observatory

Last quarter moon falls on November 21, 2016. This is the third of four last quarter moons to occur in between the September 2016 equinox and the December 2016 solstice. More typically, there are only three last quarter moons in one season, with a season being defined as the period of time between an equinox and a solstice, or vice versa.

This time around, in 2016, there are four last quarter moons in between the September 22 equinox and December 21 solstice:

September equinox: September 22, 2016

September last quarter moon: September 23, 2016
October last quarter moon: October 22, 2016
November last quarter moon: November 21, 2016
December last quarter moon: December 21, 2016 at 1:56 UTC

December solstice: December 21, 2016 at 10:44 UTC

The December last quarter moon barely beats the clock, arriving some 9 hours before the December solstice. Sometimes the third of four full moons in a season is called a seasonal Blue Moon, but we know of no terminology for the third of four last quarter moons in one season.

Image via Wikipedia. A quarter moon takes place whenever the sun-Earth-moon angle makes a right angle in space, with our planet Earth being at the vertex of this right angle.

A quarter moon takes place whenever the sun-Earth-moon angle makes a right angle in space, with our planet Earth being at the vertex of this right angle. Image via Wikipedia.

Only 2 last quarter moons in one season?

Although four last quarter moons in one season are fairly rare, it’s even rarer to have only two last quarter moons in a season. But that’s exactly what will happen in between the December 2016 solstice and the March 2017 equinox:

December solstice: December 21, 2016

January last quarter moon: January 19, 2017
February last quarter moon: February 18, 2017
March equinox: March 20, 2017 at 10:29 UTC

March last quarter moon: March 20, 2017 at 15:58 UTC

Then there will be four last quarter moons once again in between the March 2017 equinox and the June 2017 solstice. After that, normalcy will return for a while, with the seasons harboring three last quarter moons. Four last quarter moons in one season will next take place in between the June 2019 solstice and the September 2019 equinox.

Read more: Only 2 full moons in one season possible?

Resources:

Phases of the moon: 2001 to 2100

Solstices and equinoxes: 2001 to 2100

tomruen

Moon phases animation via Tomruen on Wikimedia Commons.

Bottom line: Usually there are three last quarter moons in a season, with a season defined as the time period between equinoxes and solstices. The last quarter moon on November 21, 2016 is the third of four last quarter moons in the current season (between the September 2016 equinox and the December 2016 solstice).

Bruce McClure

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