Lunar X (Lunar Cross)

[Bill Wilson]

I ran across a discussion of this on SpaceWeather.com on April 16,
2008 (you can look in its archives if you go there after that date).
It had links to a rather long but good article from the RASC journal
<http://idisk.mac.com/chapmandave-Public/WernerX2.pdf>. In the article
is a link to an Excel spreadsheet by ALPO lunar observer Keith Burnett
in his online article Libration and Colongitude <http://www.bodmas.org/
kepler/mooneph.html> that allows the user to manually enter dates,
times, and lunar latitudes/longitudes of various features. It then
calculates the colongitude and altitude of the sun for whatever time
and date the user has entered.

To have the best chance of seeing the Lunar X, which a number of us
have done recently, the moon has to be at or near first quarter (50 ±
3% illuminated). The optimal colongitude is 358.0 ± 1.2 degrees, with
a solar altitude of 1.4 ± .07 degrees at crater Werner.

Using the spreadsheet is a trial-and-error process, but if you know
the date of first quarter it goes pretty quickly. The Lunar X is
visible from any location no more often than once every second
lunation. The last one visible around here, according to SpaceWeather,
was April 12. You can try dates in May (won't be visible) and later
using the spreadsheet. June 11 about 0 hours UT (7 PM CDT) is a
possibility, but the solar altitude may not be optimal. August 8 at 23
hours UT (6 PM CDT) looks better, except that on both dates the sun
will still be up.

Anyway, there it is. You can download the article and the spreadsheet
(I checked it for viruses and found it clean) and play with it
yourself.