On September 22 comes the equinox: the start of the third quarter of the seasonal year on our planet.
Here is one of the illustrations in the “Sun and Seasons” section of Astronomical Calendar 2024, suggesting how the Sun’s daily arc across the sky comes to a middling height, rising and setting at the due east and west points on the horizon.
The moment when the Sun’s center appeared to cross Earth’s equatorial plane into the southern half of the celestial sphere is 12:42 by Universal Time. This is the same by Standard time in Britain; 6 hours earlier in North America’s Central time zone; another hour earlier for both by clocks still twisted to “summer” or “daylight saving” time, in which the Sun does not reach the middle of the sky at the middle of the day.
The equinoxes and solstices (which in 2024 fall at March 20, June 20, Sep. 22, Dec. 21) divide the year into four natural quarters. In discussion among the expert folk at EarthSky.org who are helping to update the Astronomical Companion, there has been a debate about the best brief way of stating why the March equinox point is used as the zero point for measuring positions in the sky. Would you understand, or approve, if we said it’s the beginning of the “astronomical year,” or the “seasonal year,” or the “natural year,” or the “tropical year”?
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