Bob Moler's Ephemeris Blog

This is Bob Moler with Ephemeris for Tuesday, September 22nd. Today the Sun will be up for 12 hours and 8 minutes, setting at 7:39, and it will rise tomorrow at 7:31. The Moon, 1 day before first quarter, will set at 11:05 this evening.

Fall is about to a, well… fall upon us and in a few weeks so will the leaves. At 9:31 this morning (13:31 UTC*) the Sun will cross the celestial equator heading south. The celestial equator is an imaginary line in the sky above the earth’s equator. At that point the Sun will theoretically set at the north pole and rise at the south pole. The day is called the autumnal equinox and the daylight hours today is 12 hours and 8 minutes instead of 12 hours exactly. That’s due to our atmosphere and our definition of sunrise and sunset. The reason for the cooler weather now and the cold weather this winter is that the length of daylight is shortening, and the Sun rides lower in the sky, spreading its heat over a larger area, thus diluting its intensity.

The event times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

* UTC – Coordinated Universal Time. Greenwich Mean Time if you haven’t kept up. Zulu if you’re in the military.

The Sun crossing the celestial equator in three steps: 9:31 am Sept 21, 22, and 23 2020. Created using Stellarium and GIMP.

The Sun’s path through the sky on the equinox day from Traverse City, MI. Created using my LookingUp program.

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. Credit: Bob Moler.