Friday/ happy equinox!

The September equinox* arrived today at 20:02 UTC (1.02 pm here in my outpost on the globe), ushering in autumn.  So for a moment, night and day are each 12 hours long**, sunrises are due east and sunsets are due west, for all creatures on the globe. The sun’s position crosses the celestial equator (an imaginary equator above the real one on Earth’s surface), and this happens no matter where one is on Earth.

*Equinox from Latin equi (equal) and nox (night).
**Precisely speaking, there is more daylight than nighttime on the day of the equinox, an additional 8 or so minutes of daylight, at mid-temperate latitudes.

Earth’s axis is tilted 23.44° to the vertical*, which makes for the four seasons as it makes a trip around our solar system’s Sun every year.  Autumn has started in the northern hemisphere, and spring in the southern hemisphere. The sun has set on the North Pole, and will only appear again in March.   *Fun fact: due to slight changes in the elliptical orbit of Earth around the Sun, the 23.44° oscillates between 22.1° and 24.5° in a 41,000 year cycle called the Milankovitch Cycle. Earth is about 10,000 years away from the lowest tilt of 22.1°.

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