The phases of the moon have played an important role in the determination of time. The word month is directly derived from the word moon. Traditionally, one month was equivalent to one revolution of the moon around the earth. As the moon orbits the earth, the visible reflective portion (called the lunar disk) seen from the earth varies in predictable patterns. At the beginning of the cycle, the moon is a very thin crescent that increases in size within hours. Because the full cycle of the moon is not exactly 28 days, a day-length based on the rising and setting of the moon results in inconsistent solar positions during the daytime.
A lunar calendar is divided into even periods between phases. In using a lunar calendar, for example, the solstices and equinoxes are never the same. This calendar, utilized by the Greeks, was devised by the philosopher Metones of Athens and bears his name as the Metonic cycle. The Metonic cycle is equivalent to 19 solar years or approximately 235 lunar months. The lunar calendar was eventually abandoned, as it never remained evenly synchronized with the annual solar cycle. The Romans devised their own solar calendar to replace the Metonic calendar.
Lunar phasing is the visible change in the daily appearance as the moon orbits the earth. There are four major phases: new, first quarter, full, and third quarter. Additionally, the moon passes through phases as it rotates on its own axis. In modern times, the new moon occurs when the moon is positioned relatively between the sun and the earth. Because the moon orbits the earth at an inclined angle with respect to the earth’s revolution around the sun, the moon is not always in a direct line between the sun and the earth (see Figure 1). Occasionally, a solar eclipse occurs when the moon does get into direct position, and the moon’s shadow darkens the daytime skies of earthbound observers. At the new moon, the lunar age is zero days. Lunar age is defined as the number of days from the last new moon.
New Moon to First Quarter
As the cycle progresses, the lunar disk continues to gain luminescence (wax), and a crescent shape appears to the right of the lunar face. This stage is known as the waxing crescent and continues for approximately 7 days. To the observer, the moon shifts approximately 12° per day, an effect that causes the moon to rise 48 minutes earlier each day as it lengthens an apparent gap between itself and the sun. Occasionally, when atmospheric conditions are met, the darkened portion of the lunar disk can be seen as a very dull, gray feature in contrast to the illuminated area. This effect, called earthshine, is caused by the reflection of direct sunlight from the earth projected onto the moon (see Figure 2). Earthshine is best viewed when local weather conditions are clear, and the relative humidity is low. Another factor of waxing earthshine magnitude is cloud cover over a location westward of the observer. Clouds reflect more sunlight than land and water reflect.
First Quarter to Full Moon
At the age of 7 days, the moon reaches a relative right angle with the sun and the earth (earth at the apex); the right half of the lunar disk is visible to the terrestrial observer. This phase is the first quarter, and the term quarter refers to two aspects. First, the moon has completed one quarter of its revolutionary period, and second only one quarter of the lunar sphere is visible. At this location, the
moon is exactly 90° east of the sun. Once the moment of the first quarter has passed, the moon continues to wax; yet, the crescent shape appears on the left, unlit portion of the lunar disk. This stage is a gibbous stage; therefore, the term waxing gibbous is used to describe the luminescent increase from the first quarter. This process continues for 7 days, after which time the moon gets to the halfway point in its revolution.
Full Moon to Third Quarter
The full moon occurs when the entire lunar disk is illuminated at an age of 14 days. Although it is not widely known, the full moon is technically considered second quarter phase. Throughout history, we have applied the term full moon to the visible portion of the lunar disk. However, this is a misnomer, as in reality the moon is a sphere, not a disk, so only half the lunar sphere is truly reflecting the sun’s light. At this point, given the right circumstances on the inclined lunar orbit, we can see the moon passing into the shadow of the earth, resulting in a lunar eclipse. Lunar eclipses are much more common than solar eclipses due to the earth casting a bigger shadow over the moon than the moon does over the earth.
From the moment of the full moon, lunar disk illumination begins to recede (wane). Marking the second half of its revolutionary motion, the crescent face of the moon is again on the darkened side; however, the darkened portion shifts to the right side of the moon. This stage is termed waning gibbous. During the current lunar cycle, 21 days has elapsed.
Third Quarter to New Moon
The moon forms another right angle with the earth and sun and has now reached the third (last) quarter phase. From earth, the left side of the moon is illuminated and is exactly 90° west of the solar position. As lunar disk recession proceeds, the crescent shape returns to the left side of the lunar disk, and the stage is called waning crescent. During this stage, earthshine is prominent again; however, the magnitude of the projection depends on what is eastward of the observer’s location. Figure 3 depicts the entire lunar cycle from start to finish. As it completes the nearly 28-day cycle, the moon is visible in the early morning until it returns to its starting position between the earth and sun, at which point none of the lunar disk is visible, as it is lost in solar glare. The new moon, under the right conditions, will provide a solar eclipse on earth.
The lunar sidereal period is one complete lunar cycle. It is equal to 27.321661 days, or the time needed for the moon to revolve around the earth. However, as the moon moves around the earth, the earth is moving around the sun. Observationally, it takes time for the moon to return to the local meridian over its orbital period around the earth. The earth has proceeded in its own orbit, and as a result, the terrestrial observer’s moon has not quite reached the same location. When it does, it will have completed one lunar synodic period, which is equivalent to 29.53 days.
During the course of a lunar cycle, the positions of the craters tend to change slightly. Lunar libration is the apparent wobble of the moon as it revolves around the earth caused by the difference in inclination between the earth and moon, the shape of the moon’s orbit, and the effect of the earth rotating, creating a parallax shift from the observer’s perspective. Although the moon’s daily rotation is locked to the terrestrial rotation, these factors result in approximately 10% more surface feature visibility. Time-lapse photography of all the lunar phases results in a slight shift in the daily positioning of lunar features such as craters, mountain ranges, and lunar maria or seas.
Timothy D. Collins
See also Calendar, Gregorian; Calendar, Julian; Earth, Revolution of; Eclipses; Moon, Age of; Seasons, Change of; Time, Measurements of
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