I mentioned the Worm Moon/Full Sap Moon in yesterdays post. It so happens that in this case it was also a Super Moon: about 14% larger than the typical full moon because right now the moon is close to the earth in its orbit. In point of fact, we haven’t seen a full moon as large as this since March 1983. The graphic to the right compares the size of this Super Moon (or supermoon) to the more normal full moon, the Super Moon being on the right, of course.
What had slipped my mind is the fact that the equinox is also occurring: on the 20th, whereas the full moon occurred on the 19th. Actually, the true moment of the equinox will be early on the 21st in the Eastern time zone, but measured by UTC (Coordinated Universal Time, which is for all intents and purposes the same thing as Greenwich Mean Time) the equinox will occur just slightly before midnight. Nevertheless, the near synchronicity of the two events is quite the unusual event. I wish I could find out just how unusual. Is it measured in hundreds or thousands of years, I wonder.
When I looked up the timing of the equinox in Wikipedia and elsewhere, I discovered a brand new term — new to me anyway: Equilux. I had always thought and taught that the equinox was the day that brought us twelve hours of light and twelve hours of darkness. While that’s close enough for the non-pedants amongst us, it’s apparently not wholly accurate.
The equinox is really a point in time: the instant that the sun crosses the equator. The equilux is the day of roughly equal amounts of light and dark, and occurs a few days before the vernal equinox and a few days after the autumnal equinox: apparently on March 18 of this year (2011).
The simplified explanation is that the equinox is measured by the midpoint of the sun, but we actually see the top of the sun earlier. In addition, due to refraction, we also see light slightly before the sun is actually visible. Therefore, there is a slight discrepancy in time between the equinox and the equilux.
Please join me in filing this under the You’re Never Too Old To Learn Something New category.
Below, I have embedded a three minute video of NASA’s explanation of the supermoon.
Addendum: You would think that the combination supermoon and equinox would portent good weather. Not! The next week does look look very good at all. They predict ice pellets tonight and about 5 – 10cm or more of some sort of snow-rain combination on Monday. The highs for the next week will barely break the freezing point. Sigh.