Full moon and equinox

Two images of last night’s full moon appear below. One was taken a few minutes before the equinox; the other was taken a few seconds after it. Can you tell which is which? (Hint: the moment at which the equinox occurs has no bearing on the appearance of the moon.)

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At the time these pictures were taken, the sun was on the opposite side of the earth. The moon was about 6 hours shy of full (which occurred officially at 5:18 a.m. EDT this morning), but close enough that it would take a [...]

September equinox around the corner

The September equinox is just around the corner. This year it happens at 11:09 PM EDT on September 22. I wrote about the problem of equating “equal day and night” with the date of the equinox last year, and rather than repeat myself, I just link to that article. Suffice to say here that, because our Sun, unlike all the other stars in the universe, is not a point source of light, not many places on Earth experience equal lengths of day and night precisely on the date that the geometric center of the Sun’s disk is above the horizon [...]

March equinox

Today, at approximately 1:32 p.m. EDT, the Sun will be on the celestial equator, in the constellation Pisces. It will then be at the position known as the first point of Aries. (Makes sense, right? If you’re in Pisces, you pretend to be in Aries. Chalk this one up to the effects of precession.)

Here is an illustration from Wikipedia of the orientation of the Earth relative to the Sun’s rays on the equinox:

This alignment occurs, not because the Earth’s axis is any more or less tilted than it ever is (23.5 degrees from the ecliptic plane), [...]

September Equinox

Today, at 5:18 p.m. EDT, the sun will cross the celestial equator in the constellation Virgo. Often called the autumnal equinox, I prefer to name it by month, since neither South Florida, nor the Southern Hemisphere, experiences autumn in September. (I must admit, though, that the phrase autumnal equinox has a much more mellifluous sound, and is much more satisfying to say, than September equinox. Tennyson, though not an astronomer, would probably agree.)

The celestial equator. Image from NASA

On this day, the sun rises directly in the west, and sets directly in the east. Tomorrow, the sun will [...]

Happy equinox!

The September equinox arrives today at one minute before noon, Eastern Daylight Time. One technical definition of the equinox (from the U.S. Naval Observatory) is that day when the geometric center of the Sun’s disk passing through the equator, with that point appearing above the horizon everywhere on Earth for 12 hours. Another definition, one that I like a little bit better, is “the date when the Sun crosses the celestial equator moving southward in the Northern hemisphere.”

One thing many people (including geographers, who really ought to know better) fail to understand is that the astronomical equinox does not [...]