2021 January, Auke Slotegraaf

As the line of day & night (the terminator) falls across the Moon, sunlight glints off terrain of high relief while leaving lower regions bathed in shadow. This interplay of light and dark can create striking and short-lived features, known as “clair-obscur effects”.

Some of these lunar moments have evocative names, such as “Cassini’s Moon Maiden” and “Gruithuisen’s Lunar City”.
The visibility of these fleeting events depends on the underlying terrain, the angle of the Sun’s light, and, of course, the visibility of the Moon from your location.

In the 2021 edition of the Sky Guide, the astronomical handbook for southern Africa, predictions are given for the visibility of the “Lunar X” (p 6). The effect occurs during each lunar month, but only the event on 2021 January 20 will be visible from South Africa.

Clair-obscur guru Dana Thompson has kindly also supplied information about spotting the “Curtiss Cross”. This effect was first recorded on 1956 November 26, by astronomy enthusiast Robert E. Curtiss (New Mexico, USA). The distinct formation lies near the crater Copernicus (see accompanying photographs).

During 2021, there are three opportunities to witness this spectral sight: 2021 April 06, June 04, and November 28.
Happy hunting!

2021 Jan 07 09:08
2021 Feb 05 23:42
2021 Mar 07 13:44
2021 Apr 06* 02:53
2021 May 05 15:04
2021 Jun 04* 02:28
2021 Jul 03 13:24
2021 Aug 02 00:20
2021 Aug 31 11:42
2021 Sep 29 23:48
2021 Oct 29 12:50
2021 Nov 28* 02:44
2021 Dec 27 17:14
Key: * Visible from South Africa. All times are in SAST.