Equinox, more or less

The Sky This Week, 2020 September 15 – 22 — Naval Oceanography Portal

The astronomical season of autumn begins on the 22nd at 9:31 am EDT.  This is the moment when the Sun’s disc reaches an ecliptic longitude of 180 degrees and crosses from the northern hemisphere of the sky into the southern hemisphere.  While the term “equinox” means “equal night”, a glance at a sunrise/sunset table reveals that the difference between sunrise and sunset on this date isn’t exactly 12 hours as one might expect.  Since the Sun subtends a tangible disc and we measure sunrise and sunset by the first and last appearances of the solar limbs, the duration of daylight is 12 hours and 8 minutes on the 22nd here in Washington.  The actual date when day and night are equal falls on the 26th.  From then until March 16th next year our nights will be longer than our days.

[“Hello, darkness, my old friend.”]