The moon, Earth's celestial companion, undergoes a fascinating cycle of phases, each with its own distinct name and appearance. Just as different stages of life have names, so do the moon's phases. Among these, the "waxing gibbous" phase stands out, inviting curiosity about its origins. This article delves into the etymology of "waxing gibbous," unraveling the meanings behind these descriptive terms and exploring the broader context of lunar phases.
A lunar phase, or Moon phase, refers to the varying shapes of the Moon's illuminated surface as viewed from Earth. This cycle of phases, which takes approximately one lunar month to complete, is a consequence of the Moon being tidally locked to Earth. This means that the same side of the Moon always faces our planet. As the Moon orbits Earth and the Earth orbits the Sun, the relative positions of these celestial bodies shift, causing the amount of sunlight reflected off the Moon's surface to change.
The four major phases are the new moon, the first quarter, the full moon, and the last quarter. Intermediate phases, occurring between the principal ones, are either crescent or gibbous.
The terms "waxing" and "waning" describe whether the Moon's illuminated surface is increasing or decreasing, respectively. "Waxing" signifies the period when the Moon is progressing from new to full, while "waning" indicates the period after the full moon as it heads toward the new moon phase.
The word "waxing" comes from the verb "to wax," which means to increase in size gradually. It was once a common synonym for "grow". Conversely, "waning" means to decrease in size gradually. These terms appeared around the same time (970 AD) and in the same place, highlighting their close relationship in describing the lunar cycle.
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The adjective "gibbous" describes a shape that is more than a semicircle, or rounded and convex. Specifically, a gibbous moon is one that appears between half full and full.
The term "gibbous" has its roots in the Latin noun "gibbus," meaning "hump." It was adopted into Middle English to describe rounded, convex shapes. While "gibbous" can describe rounded body parts of humans and animals (like a camel's back) or the shape of certain flowers (like snapdragons), it's most commonly used to describe the Moon. An interesting gibbous moon fact is that this term originates from the Latin word gibbosus, meaning humpbacked.
Combining these terms, "waxing gibbous" describes the lunar phase when the Moon's illuminated surface is increasing and the shape is more than a semicircle. As the moon waxes (the amount of illuminated surface as seen from Earth increases), the lunar phases progress through the new moon, crescent moon, first-quarter moon, gibbous moon, and full moon phases. This phase occurs between the first quarter and full moon. During this time, the Moon appears to be growing larger and more illuminated each night, gradually approaching the full moon. The angle of the waxing gibbous moon changes from when you might first spy it in late afternoon, until after midnight, when it sets.
Archaeologists have found evidence of timekeeping methods dating back to prehistoric times, at least to the Neolithic period. The day, the solar year, and the lunation are all natural units for timekeeping that have been used by most historical societies. The first crescent of the new moon is a clear and regular marker in time, and pure lunar calendars (such as the Islamic Hijri calendar) rely completely on this metric. However, because a year of twelve lunar months is ten or eleven days shorter than the solar year, a lunar calendar drifts out of step with the seasons.
Lunisolar calendars address this issue with a year of thirteen lunar months every few years, or by restarting the count at the first new (or full) moon after the winter solstice. The Sumerian calendar is the first recorded to have used the former method; the Chinese calendar uses the latter, despite delaying its start until the second or even third new moon after the solstice. The Hindu calendar, also a lunisolar calendar, further divides the month into two fourteen day periods that mark the waxing moon and the waning moon. The ancient Roman calendar was broadly a lunisolar one; on the decree of Julius Caesar in the first century BCE, Rome changed to a solar calendar of twelve months, each of a fixed number of days except in a leap year.
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The number of days counted from the time of the new moon is the Moon's "age". The approximate age of the Moon, and hence the approximate phase, can be calculated for any date by calculating the number of days since a known new moon (such as 1 January 1900 or 11 August 1999) and reducing this modulo 29.53059 days (the mean length of a synodic month). The difference between two dates can be calculated by subtracting the Julian day number of one from that of the other, or there are simpler formulae giving (for instance) the number of days since 31 December 1899. However, this calculation assumes a perfectly circular orbit and makes no allowance for the time of day at which the new moon occurred and therefore may be incorrect by several hours. It is accurate enough to use in a novelty clock application showing lunar phase, but specialist usage taking account of lunar apogee and perigee requires a more elaborate calculation.
The waxing gibbous moon is visible in the sky when darkness falls, lighting up the early evening. In the Northern Hemisphere, if the left side of the Moon is dark, then the bright part is thickening, and the Moon is described as waxing (shifting toward full moon). Closer to the Equator, the lunar terminator will appear horizontal during the morning and evening.
When the Moon seen from Earth is a thin crescent, Earth viewed from the Moon is almost fully lit by the Sun. The dark side of the Moon is dimly illuminated by sunlight reflected from Earth, called earthshine, which is bright enough to be easily visible from Earth.
It's important to note that the phases of the Moon are not caused by the Earth's shadow falling on the moon. They are caused by the moon's shadow on itself, just as the Earth's shadow makes it night on one side of the Earth.
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tags: #waxing #gibbous #etymology
