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09/21/2021 – Ephemeris – Autumn starts tomorrow

September 21, 2021

This is Bob Moler with Ephemeris for Tuesday, September 21st. Today the Sun will be up for 12 hours and 12 minutes, setting at 7:41, and it will rise tomorrow at 7:30. The Moon, 1 day past full, will rise at 8:23 this evening.

Enjoy the last full day of summer. Summer will last until 3:21 pm (19:21 UTC) tomorrow, when the center of the Sun will cross the celestial equator, an imaginary line above the Earth’s equator, heading southward. At that instant, autumn will begin. Shortly after, the Sun will be up less than half the day. The day and the point in the sky that the Sun crosses is called the autumnal equinox. The word equinox means equal night, implying that day and light are of equal length. Geometrically that’s true, but the Earth’s atmosphere and the definition of sunrise and sunset, prolong daylight by a few minutes. The amount of heat we are getting and will get from the Sun cannot sustain our current temperatures, and it will get a lot colder on average before it gets warmer again.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hours). They may be different for your location.

Addendum

Sunrise on the autumnal equinox

That is not a pumpkin on the head of the motorcyclist. That’s the Sun rising as I’m traveling east on South Airport Road south of Traverse City, MI on the autumnal equinox. This is the east-west section of the road. The Sun is rising over the hills some 6 miles to the east. When the Sun is on the celestial equator, it rises due east and sets due west. Credit: Bob Moler.

Sun's path on the equinox for TC-Interlochen

The Sun’s path through the sky on an equinox day from the Traverse City/Interlochen area in Michigan. The Sun is plotted every 15 minutes. This is a stereographic projection which compresses the image near the zenith and enlarges the image towards the horizon. Created using my LookingUp program.

Earth 3 days before autumnal equinox

Earth 3 days before autumnal equinox, 1:17 pm EDT, September 19, 2021. The mitten shape of Michigan was in the clear that day and is visible above center of the image. The image taken by the Earth Polychromatic Imaging Camera (EPIC) on the DSCOVR satellite in halo orbit around the Earth-Sun L1 Lagrange point, about a million miles sunward of the Earth. At the time, DSCOVR was 892,682 miles (1,436,797 kilometers) from Earth.

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