The Moon as Calendar and Timepiece

Note: This article is Northern Hemisphere–centric, and all points of observation and movement are given from this perspective. Also, these points will be most accurate when seen closest to the mid-latitudes, e.g., the 45th parallel.

To us earthlings, the Moon is a strange, inhospitable place. It's airless and gritty with no discernible atmosphere, and what water is on the Moon is either frozen or hidden underground. Since the Moon has no atmosphere, it never has fog or clouds. For that matter, it has no weather. It also has no global magnetic field and very little gravity. Because the Moon lacks an atmosphere, the Moon's "sky"—to someone standing on its surface—would look black, even at midday. If you were able to walk on its surface, you would leave footprints in a surface that—although dry—felt something like dense, wet sand.

Unusual as these physical characteristics seem to us earthly mortals, the Moon is a happily familiar sight when we gaze into the night sky. And thanks to cycles of rotation and celestial motion that are both constant and continuous, we needn't simply admire them from afar; we can also use the Moon to judge time of day and the passing of each month.

The Moon orbits the Earth once every 27.3 days. This is called its sidereal (sye-DEAR-ee-ull, "with respect to the stars") period. However, as seen from Earth, it takes the Moon 29.53 days to complete its phases. This is called the Moon's synodic (sih-NOD-ick, "with respect to the Sun") period, and this is the cycle we follow here on Earth.

Rule number one in tracking the Moon's movements is that it always rises in the eastern sky and sets in the west. However, rising positions may vary by as much as 60 degrees along the horizon; this is due to an 18.6-year cycle of repeated movements between the Sun and Moon. This means that while you may notice a Full Moon rise at a specific point on your horizon, the Full Moon you observe the following month may rise at a different point, north or south of the previous month's position. If you tracked these risings over 18.6 years, the Sun and Moon's constant slow-dance pattern would become apparent.

The Moon also sequences through eight phases over each monthly lunar cycle, which are described in terms of "waxing" (growing) or "waning" (shrinking). Of course, the Moon isn't changing at all; what we see as phases has to do with (a) whether the Moon is visible above the horizon and (b) how much of the Moon's visible surface is lit by the Sun. The latter is determined by the geometrical relationship between Earth, Moon, and Sun. A Full Moon occurs when the Moon and Sun are on opposite sides of the Earth, while a New (dark) Moon occurs when the Sun and Moon are on the same side.

Here is the precession of phases:

• New Moon: This is the completely dark Moon, with the lunar face unlit.
• Waxing crescent: The first appearance of the waxing crescent is a thin sliver, but eventually up to one-fourth of the Moon's surface appears as a lit crescent.
• Waxing half (also called the first quarter): The "right half" of the Moon is lit, with the left dark.
• Waxing gibbous: The Moon approaches being full; three-fourths of the Moon is lit.
• Full Moon: The round, bright Moon is fully lit.
• Waning gibbous: The Moon begins to darken; three-fourths of the Moon is still lit.
• Waning half (also called the third quarter): The "left half" of the Moon is lit, with the right dark.
• Waning crescent: Only a crescent (up to one-fourth of the Moon) remains lit.
• New Moon: A return to the completely dark Moon.

Moon As Timepiece
The Moon phases always rise and set at about the same times:

Do you see the pattern, by which each phase rises and sets about three hours later than the one before it? (6:00 pm, 9:00 pm, midnight, etc.) Of course, there are some Moon phases that we will never or only rarely see. At the mid-latitudes, there is typically enough sunlight available early in the morning that we cannot see the New Moon rise, and it sets before dark; even if it stayed above the horizon after sunset, its completely dark face would make it very difficult to locate. In more northern latitudes, with longer periods of darkness and shorter days, the New Moon and phases closer to the New Moon may be briefly spotted.

Do you see—via the table above—that the Moon always rises and sets in twleve-hour cycles (rises at 6:00 am and sets at 6:00 pm, etc.)? To make these visual calculations even more precise, it helps to know that the Moon rises an average of fifty minutes later each night than on the previous night. Thus, if the Full Moon rises at precisely 6:00 pm, the (waning gibbous) Moon the next night will rise at 6:50, the next night at 7:40, and so forth. In each case, the Moon will always reach its highest point above the horizon halfway (six hours) through each phase.

Once you understand how the phases work and can memorize at least one Moon rise/set time, you can work out the others in your head and can use those to approximate local time. Observe which phase of the Moon you are seeing, then approximately where in its twleve-hour journey it is. Reference the following illustration and these examples:

If you observed a waxing half Moon (with the right side "lit") high in the sky, you'd know it was somewhere around 6:00 pm, give or take an hour. This would be the phase's sunrise (12:00 pm) plus six hours.

If you saw a waxing crescent setting in the west during early evening, you'd know it was around 6:00 or 7:00 pm—sunrise of 9:00 am plus nine hours.

If you were out early in the morning and could see a waning gibbous Moon low in the western sky, you'd guess it was between 6:00 and 7:00 am—rising time of 9:00 pm plus nine hours.

Moon As Calendar
While the lunar phases don’t always show anything about the passing of seasons, per se, those of us deeply attuned to the Moon are able to follow the passing of each month by watching the Moon's phases and appearance. Example: the phenomenon known as the Harvest Moon refers to the Full Moon falling closest to the autumnal equinox, which is on or around September 21 or 22. This Moon received its name because the light it sheds was said to help farmers work longer days when bringing in their crops. Many believe the Harvest Moon appears bigger or brighter than other Full Moons, but that's not actually true. The appearance is a visual effect of the "Moon illusion," which means that a Full Moon close to the horizon always appears larger than one higher in the sky.

The Harvest Moon also behaves differently than other Full Moons; because of the shallow angle between the eastern horizon and the solar ecliptic (the path the Sun appears—to our view—to follow through the sky) occurring around the autumnal equinox, the Harvest Moon rises only thirty minutes later than the Moon of the previous night. Because it stays closer to the horizon, it appears bigger than usual (the Moon illusion again), and because it is low in the sky, it is viewed through particulate matter in the lower atmosphere, often causing it to take on a golden-orange color. This early-rising behavior is repeated by the Hunter's Moon, the first Full Moon that follows the autumn equinox. The Hunter's Moon is so-named because the bright moonlight helps hunters stalk migrating waterfowl.

Because the Harvest and Hunter's Moons rise so quickly—right around sunset—the usual long periods of darkness or semidarkness between sunset and moonrise are absent. Compare that to deep winter, when night falls quickly between 4:00 and 4:30 pm, leaving one or two hours of darkness before the Full Moon rises.

Do you want to have more experience with the Moon as a timepiece? Study the Moon table above until you have it memorized, then practice using Moon phases to tell time. Or, you might do what apprentices used to do long ago: spend weeks, months, or even a year tracking the Moon's rising times, locations, and movements through the night sky, writing down and sketching your observations. There's no better way to deepen your connections to our favorite celestial body.

Resources
Rees, Martin, ed. Universe: The Definitive Visual Guide. DK Publishing, 2005.

Originally published in Llewellyn's 2014 Moon Sign Book. Click here for current-year calendars, almanacs, and datebooks.