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December 2009: Earliest Sunset, Geminid Meteor Shower & Blue Moon


Year's Earliest Sunset Comes Two Weeks Before Year's Shortest Day

At middle latitudes in the United States, the earliest sunset of the year comes about two weeks before the December 21 winter solstice - the shortest day of the year in terms of daylight. For instance, at Denver, Colorado (40 degrees north latitude), the year's earliest sunset falls on or near December 7 each year, although the shortest day annually falls on on near December 21.
equation of time for sundialHow is it possible for the earliest sunset not to occur on the shortest day of the year? In a nutshell, it's because of the discrepancy between the clock and the sun. The days as measured from midday (sundial noon) to midday (sundial noon) are nearly 1/2 minute longer than 24 hours long in December. (Photo on right showing the disparity between the clock and sundial throughout the year, courtesy of John Carmichael.)
This 1/2-minute discrepancy doesn't sound like much, but it accumulates daily. Yearly on December 7, midday (noon by the Sun) comes at or near 11:51 a.m. local time by the clock. Yearly on December 21, midday comes at about 11:58 a.m. local time - or 7 minutes later by the clock. Because the time of midday comes later by the clock, so do the sunrise and sunset times. The chart below helps to clarify.

For Denver, Colorado

DateSunriseMiddaySunsetDaylight Hours
December 77:08 a.m.11:52 a.m.4:35 p.m.9 hours 27 minutes
December 217:18 a.m.11:58 a.m.4:39 p.m.9 hours 21 minutes
By way of conclusion, the later clock time for midday pushes the sunset time some 6 to 7 minutes further ahead. Yet, the diminishing daylight shortens the day by about 6 minutes. That's only 3 minutes less sunlight in the morning and 3 minutes less in the afternoon. With the later midday pushing the sunset ahead by 7 minutes and the diminishing daylight taking 3 minutes away from the sunset time, that places the sunset at about 4 minutes later on the solstice than on December 7.
The exact date for the earliest sunset actually varies by latitude. Appreciably farther north of Denver, the earliest sunset comes after December 7. Appreciably farther south, the earliest sunset comes before December 7. For more, click here.

Prospects Good for the December 2009 Geminid Shower

The Geminid meteor shower peaks on the weekend of December 11-13. What's more, there is no moon to ruin the show this year. Read this EarthSky article on the Geminids, one of the year's most prolific and dependable showers. If you're up for a wild ride, travel upon the Geminid meteoroid stream by way of the Vis-viva equation.
If you trace all the Geminids backward, they appear to emanate from the constellation Gemini the Twins, as beautifully illustrated on Larry's Shadow & Subtance page.

Blue Moon Brings in the New Year

By popular acclaim, the second Full Moon to fall in a single calendar month is called a Blue Moon. For this to happen the first Full Moon has to come early in the month to enable the second Full Moon to fall at the end of the month. The first Full Moon falls on December 2, and the second one falls on December 31. Read more about it at EarthSky.
In cycles of 19 years, the phases of the Moon realign on or near the same calendar dates. So expect another Blue Moon to bring in the New Year once again on December 31, 2028!
Blue Moons are inevitable, because there are 235 Full Moons but only 228 calendar months in 19 calendar years. That means in a period of 19 years, at least 7 calendar months have to harbor two Full Moons (235-228=7).
But there is a hitch. If a calendar month has no Full Moon at all, that gives us yet another Blue Moon in this 19-year cycle. Because February 2018 has no Full Moon, two Blue Moons are on tap for 2018 - one in January and one in March. That's a total of 8 Blue Moons for the next 235 returns to Full Moon. The first Full Moon of the cycle falls on January 30, 2010.

8 Blue Moons in the next 19-year Cycle

August 31, 2012
July 31, 2015
January 31, 2018
March 31, 2018
October 31, 2020
August 31, 2023
May 31, 2026
December 31, 2028

copyright 2009 by Bruce McClure

November 2009 Feature * January 2010 Feature

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