# The Hitchhiker's Guide to the Moon -- Selenographic Longitude

(Thanks to Jay Reynolds Freeman for this section.)

This table answers the question "which charts of Rukl's atlas of the Moon lie at the terminator on a given night?" Specifically, it answers "which Rukl charts does a given meridian of (Selenographic) longitude pass through?"

To answer the question originally posed, you must know what the Selenographic longitude of the terminator is on the night in question. That's a messy calculation in detail, but an easy one for rough purposes, or you can get it from ephemerides.

Lunar longitude zero runs straight up and down the middle of the Moon's visible face -- or it would if there were no libration to worry about (that's the major mess in the calculation). The terminator advances at 12.2 degrees per day (approximately -- more mess). So if you know the exact time of new or full moon, or of a quarter, you can offset from the obvious indicated longitudes by 12.2 degrees per day, then fudge for the libration by looking it up and interpolating. In case those indicated longitudes are not so obvious:
Phase of Moon Average longitude of visible terminator(s)
New 90 east and 90 west
First Quarter 0
Full 90 east and 90 west
Last Quarter 0

A handy source of information, including libration and phase times, is the _Observer's_Handbook_ of the Royal Astronomical Society of Canada. Look in the pages for "The Sky Month by Month".

Beware of sign and direction conventions when using lunar longitudes and latitudes. There is a long history of confusion stemming from the use of inverting telescopes, and from the fact that the direction in which a Moon-walking astronaut would face to look at the rising sun -- which most people would call "east" -- is toward the western horizon of the Earth, as seen by an Earth-based observer. In Rukl's convention, longitude 90 east is on the edge by Mare Crisium, toward the Earth's western horizon, and 90 west is over by Grimaldi. Northerly latitudes are toward Plato, southerly ones toward Tycho.

Anyhow, once you have figured out what longitude you are interested in, look down the left edge of the table to find it (to the nearest 10 degrees), then read across to find what charts to look at. Thus for example, if you are interested in Selenographic longitude 10 east, scan down to the "10E" line, which is:

 10E 4 5 13 23 34 45 56 66 73 74

and see that the charts in question are 4, 5, 13, 23, 34, 45, 56, 66,73, and 74.

The Rukl charts cover eight zones of Selenographic latitude, Thus, charts 1-7 cover latitudes 90 north down to 48 north, 8-16 are for 48 north down to 30 north, and so on. Because of the curvature of the lines of longitude in the projection seen from Earth, there are often two or three charts to look at in the same latitude zone, for a given longitude.

Latitude limits: +90
+48
+48
+30
+30
+14
+14
0
0
-14
-14
-30
-30
-49
-49
-90
Longitude
80W 1 2 8 17 28 39 50 61 70 71
70W 1 2 3 8 9 17 18 28 39 50 51 61 62 70 71 72
60W 1 2 3 8 9 17 18 28 39 50 51 61 62 70 71 72
50W 1 2 3 8 9 18 29 40 51 61 62 70 71 72
40W 2 3 9 10 19 29 30 40 41 52 62 63 71 72
30W 2 3 4 10 19 20 30 41 52 53 63 71 72 73
20W 3 4 10 11 20 31 42 53 63 64 72 73
10W 3 4 11 21 32 43 54 64 72 73
0W 4 12 22 33 44 55 65 73
10E 4 5 13 23 34 45 56 66 73 74
20E 4 5 13 14 24 35 46 57 66 67 73 74
30E 4 5 6 14 24 25 36 47 57 58 67 73 74 75
40E 5 6 14 15 25 36 37 47 48 58 67 68 74 75
50E 5 6 7 15 16 26 37 48 59 68 69 74 75 76
60E 5 6 7 15 16 26 27 38 49 59 60 68 69 74 75 76
70E 5 6 7 15 16 26 27 38 49 59 60 68 69 74 75 76
80E 6 7 16 27 38 49 60 69 75 76

The Hitchhiker's Guide to the Moon
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