Life on the edge

[Demonperformer]

Attached is the result of a little project I have been working on - a spreadsheet of the 225 named lunar craters that appear/disappear as a result of lunar libration - which, although designed to meet my specific criteria, may be of interest/use to someone else.

The criterion for selection was that a crater had to either:

- be a nominally 'farside' crater (longitude <-90 or >+90), but be visible on occasion due to libration, or

- be a nominally 'nearside' crater which could disappear around the lunar limb on occasion due to libration (strictly speaking, I did this the other way round, considering the moon as viewed from a position 180 degrees away from the earth and identifying the craters that would become visible due to libration from there).

I got the formula for doing this from another Internet forum, but have lost the address. It was provided by one Grant Hutchison and involved using two nested right spherical triangles.  I can provide details of the method to anyone interested, and if anyone happens to know the location of this discussion, please add a link to it.

The first four colums were obtained from the IAU's "Gazetteer of Planetary Nomenclature" website - http://planetarynames.wr.usgs.gov/SearchResults?target=MOON&featureType=Crater,%20craters - the longitude and latitude are the figures for the centre of the crater

Z is either "E" for nominally 'nearside' craters  or "L" for nominally 'farside' craters

d is the distance from the selenographic origin (long=0, lat=0) to the centre of the crater, as a proportion of the lunar radius. The formula for this was discussed in another SGL thread - http://stargazerslounge.com/topic/216213-something-to-do-with-spherical-triangles/

PA is the direction of the crater from the selenographic origin; N=0 E=90

LibLat and LibLong (quoted for 'farside' craters only) give the minimum libration required (in degrees) to move the crater to the limb of the moon and so make it nominally visible

LibIdx is the result of converting LibLat and LibLong into percentages of the theoretical maximum of each (8.14 degrees longitude, 6.89 degrees latitude - Jean Meeus: Mathematical Astronomy Morsels) and using the higher figure.  For nearside craters this was calculated for an observer of the farside of the moon and given a negative value.  This does not take into account how great a libration is required in the other direction. Nor does it take into account the likelihood of good illumination of the crater at the time.  With these restrictions, it provides a first order approximation of how frequently/rarely a crater will be visible.  The theoretical limits for points that meet the selection criterion are -100 (can just disappear at maximum libration) to +100 (can become just visible at maximum libration).  This is the field I have used to sort the craters and the spreadsheet should therefore be roughly in 'easiest to hardest' order.

Thanks for looking.

libration craters.xls