Lunation Number

The Moon's continuous count · and where its sunrise has reached

The Moon keeps no clock, and that is the interesting part: a lunar solar day is about 29.5 Earth days, so twenty-four lunar hours would each run nearly thirty of ours. What lunar observers use instead is a count and an angle. The Brown Lunation Number runs continuously from the first new moon of 1923, numbering every lunation since. Selenographic colongitude gives the longitude of the morning terminator — the line of lunar sunrise creeping across the surface at about 12.2 degrees a day. It reads 270 degrees at new moon, 0 at first quarter, 90 at full and 180 at last quarter, which makes it the Moon's own time of day, and it is what tells an observer which craters are catching the low light tonight.

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