Sunday, November 29, 2020

updated Lunar X calculator

Extracted the selenographic co-longitude values from the RASC Observer’s Handbook 2021 and updated my Lunar X calculator.

Phil Chow verified the events.

Lunar X table for 2021

Looks like there are 3 good opportunities for year 2021 to view or image the letter-X pattern on the Moon, near the first quarter phase.

  • Sat 20 Mar
  • Tue 18 May
  • Thu 11 Nov

All appear to start while the sky is still light with the peak of the event happening in twilight. Then:

  • 20 Jan
  • 17 Jun
  • 16 Jul
  • 13 Sep

are ops where the Moon is above the horizon for us near Toronto but the sky will be very bright and that may greatly hinder chances. July is borderline; maybe it will be good?

All other events occur when the Moon itself is out of sight.


Chris V check the Dial-a-Moon page for 2021 and reported that my March 20 prediction at ~6:38 pm EDT hits the mark!

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