During a month or one lunar cycle, the Moon passes through successive phases from New Moon to New Moon as it changes its relative position with respect to the Sun. Thus one lunar cycle is analogous to the synodic period for the other planets. That is, the phases result from the relative position of the Moon and Sun as the Moon falls behind the Sun from day to day, traveling roughly eastward around the zodiac.
That the Moon is intrinsically dark and shines by reflected light was assumed by ancient astronomers (with some exceptions). Indeed, if the Moon were a better reflector it would shine much brighter. The intensity of the reflected light divided by the intensity of the incident light upon the Moon from the Sun (= "albedo") is only 0.07, so that 93% of the sunlight that strikes the Moon's surface is absorbed.
Half of the Moon is always illuminated by the Sun (except during a lunar eclipse), though the illuminated half is not always turned toward the Earth. The far side is not the same as the dark side.
The synodic period of the Moon (or lunar month) ranges from 29 to 30 days; the actual value (known to ancient Babylonian astronomers) is 29.53059 days.
The Moon and the Sun both fall behind the fixed stars from day to day as they move roughly eastward around the ecliptic; this is their zodiacal motion. The Moon completes one trip around the zodiac in one month; the Sun requires a year. So, subtracting out the daily motion to consider only their zodiacal motion, in one month (synodic period) the Moon gains one 360-degree lap on the Sun in their zodiacal race eastward. So how fast does the Moon move along the ecliptic?
360 degrees ÷ 29.53 days
= 12.19 degrees per day
Thus each day the Moon gains an average of 12.2 angular degrees eastward along the ecliptic from the Sun. This means that each day the Moon rises about 50 minutes later, although the actual moonrise delay varies considerably from this average value:
(24 hours/360 degrees) x (60 mins/1 hour) x (12.19 degrees/day)
= 48.8 minutes/day
(Note how the units of hours and degrees cancel out in the above equation, yielding an answer in units of minutes and days.)
Variation: At the latitude of Shawnee, the Moon delays its rising each day between 32 and 61 minutes (the former corresponding to a Full Moon one day after the September equinox and the latter representing a Full Moon two days before the March equinox). Watch the Moon two nights in a row at the same time of night to see for yourself how fast it moves!
Examine the web pages describing each lunar phase, and compare them with Crowe, Theories of the World, pp. 12&endash;13.
Check your understanding: