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Moon, Phases of

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.

Lunar Calendars

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 Phases

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 earth-shine magnitude is cloud cover over a location westward of the observer. Clouds reflect more sunlight than land and water reflect.

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