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Spotting the Lunar X  @psychohistorian.org

Spotting the Lunar X

posted: 943 days ago, on Sunday, 2017 Sep 03 at 23:02
tags: astronomy, Moon.

From Table 1 below, the next chance to spot Lunar X is Wednesday night, 2017 September 27. The almost fully-formed X should be visible by 23:18 SAST, and the effect over some 3 hours later. I'd suggest starting observing 2 hours before the predicted time. Observing reports and photos will be most welcome!

The “line” separating the light and dark parts of the Moon, known as the terminator, takes some two weeks to move across the lunar surface. Where it crosses terrain of high relief, a complex interplay of features lit by the Sun and those still bathed in deep shadow can result.

These high-contrast vistas are sometimes known as “clair-obscur effects”, and have been given evocative names, including Cassini’s Moon Maiden and Gruithuisen’s Lunar City. For a comprehensive list, visit the fascinating Moon-Wiki website.

Around the time of First Quarter, as the Moon’s terminator sweeps across the lunar landscape along more-or-less selenographic longitude 0°, a particularly striking clair-obscur effect can be seen: a highlighted X (located at longitude +1.1°, latitude – 25.8°) created by sunlight falling on the ridges between the craters La Caille, Blanchinus, and Purbach. When the Sun has an elevation of about – 0.84° as seen from the lunar surface, the “Lunar X” is a striking feature and can be viewed even in binoculars. It is well shown in the accompanying photograph by Oleg Toumilovich, taken on 2014 September 01 at 19:47.

Lunar X remains visible for about two hours, so some care in observation is called for.

While hunting for the Lunar X, keep a look out for the Lunar V, too (see the inset on Oleg's photo).

(above) Lunar X and Lunar V, imaged by Oleg Toumilovich, 2014 September 01 at 19:47 SAST

Predictions for Lunar X visibility

The two tables below give predictions for the visibility of the Lunar X, kindly supplied by noted Texan astro-mancer Edward Kotapish. His predicted times are for the nearly fully formed feature, so start observing well before this time. The end time is subjective - YMMV.

Table 1. Predictions for 2017

Start (UT)End (UT)
2017 Jan 05, 06:4209:45
2017 Feb 03, 20:3723:40
2017 Mar 05, 09:5112:54
2017 Apr 03, 22:21Apr 04, 01:24
2017 May 03, 10:1213:15
2017 Jun 01, 21:39Jun 02, 00:42
2017 Jul 01, 09:0012:03
2017 Jul 30, 20:3323:36
2017 Aug 29, 08:3611:39
2017 Sep 27, 21:18Sep 28, 00:21
2017 Oct 27, 10:4213:45
2017 Nov 26, 00:3503:58
2017 Dec 25, 14:4117:44

Table 2. Predictions for 2018

Start (UT)End (UT)
2018 Jan 24, 04:3607:37
2018 Feb 22, 18:0421:05
2018 Mar 24, 06:5709:58
2018 Apr 22, 19:1622:17
2018 May 22, 07:0610:07
2018 Jun 20, 18:4021:41
2018 Jul 20, 06:1509:16
2018 Aug 18, 18:0521:06
2018 Sep 17, 06:2609:27
2018 Oct 16, 19:2222:23
2018 Nov 15, 08:5111:52
2018 Dec 14, 22:40Dec 15, 01:41


Image by Oleg Toumilovich, 2017 August 29 at 17:58 SAST. From Table 1, the Lunar X was fully formed at 10:36 and visible until around 13:39. Oleg's photo, taken some 4 hours later, reveals the X more fully lit.

Image by Oleg Toumilovich, 2016 February 15 at 20:00

Image by Oleg Toumilovich, 2010 January 22 at 21:43 SAST.

Lunar X from the Sky Guide.

nothing more to see. please move along.

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