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PHASES OF THE MOON - PART 1 Print E-mail
Written by nazeer   
Sunday, 09 September 2007 14:12

PHASES OF THE MOON

PART-1

 


New Moon

Try though you might, you won't be able to see the Moon at all today. That's because the Moon is "new" at 1:45 p.m. Central Daylight Time. That's the moment it crosses the line between Sun and Earth. Since it lines up so close to the Sun, it's lost in the Sun's glare.

In most cultures, new Moon is the time when we begin counting off the Moon's monthly cycle of phases. In about a week, it'll be at first-quarter -- one-quarter of the way through the cycle. And about a week after that, it'll be half-way through the cycle, so it'll be full. In all, it takes about 29 and a half days to complete a cycle.

Sky watchers have been counting out the days of the lunar cycle for millennia. Archaeologists have discovered many artifacts at sites around the world that may mark the days of the lunar cycle.

One of them was found at Ishango, the site of an African fishing village that was inhabited around 8500 years ago. The Ishango artifact is a bone that's been engraved with several groups of notches. The groups correspond to the number of days in a lunar cycle. Scientists can't say for certain that the bone was a lunar cycle marker, but that's one of the leading ideas.

For those at southern latitudes, the Moon may return to view as a thin crescent quite low in the west shortly after sunset tomorrow. But for much of the United States, it'll take one more day before the Moon returns to view.

Crescent Moon

The night sky is filled with beautiful sights: colorful stars, sparkly star clusters, brilliant planets. Yet few sights are more beautiful than a "fingernail" crescent Moon.

The Moon's appearance changes as it orbits Earth. Every day, the angle between the Moon, Earth, and Sun is a little different, so sunlight illuminates a different portion of the lunar surface.

When it's a crescent, the Moon lines up a little to the side of the Sun as seen from Earth. That means most of the hemisphere of the Moon that faces our way is in darkness -- it's nighttime. Only a sliver is in sunlight.

As twilight begins to fade, though, we can see the entire disk, because sunlight reflecting off of Earth lights up the Moon. Anyone standing on that portion of the Moon would see an almost-full Earth lighting up the night sky like a beacon, shining dozens of times brighter than the full Moon looks from Earth.

In fact, from any given spot on the Moon's nearside, Earth always appears in almost exactly the same spot in the sky. That's because the same side of the Moon always faces Earth. So if you watched over the course of a month, you'd see Earth go through the same cycle of phases that the Moon does -- including a thin crescent.


New Moon - The Moon's unilluminated side is facing the Earth. The Moon is not visible (except during a solar eclipse).
Waxing Crescent - The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.
First Quarter - One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.
Waxing Gibbous - The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.
Full Moon - The Moon's illuminated side is facing the Earth. The Moon appears to be completely illuminated by direct sunlight.
Waning Gibbous - The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.
Last Quarter - One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.
Waning Crescent - The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.


Following waning crescent is New Moon, beginning a repetition of the complete phase cycle of 29.5 days average duration. The time in days counted from the time of New Moon is called the Moon's "age". Each complete cycle of phases is called a "lunation".

Because the cycle of the phases is shorter than most calendar months, the phase of the Moon at the very beginning of the month usually repeats at the very end of the month. When there are two Full Moons in a month (which occurs, on average, every 2.7 years), the second one is called a "Blue Moon". See the article "Once in a Blue Moon" for the story of how the usage of this term has evolved.

The first time that the thin waxing crescent Moon is visible after New Moon (low in the evening sky just after sunset) marks the beginning of a month in the Islamic Calendar.

Although Full Moon occurs each month at a specific date and time, the Moon's disk may appear to be full for several nights in a row if it is clear. This is because the percentage of the Moon's disk that appears illuminated changes very slowly around the time of Full Moon (also around New Moon, but the Moon is not visible at all then). The Moon may appear 100% illuminated only on the night closest to the time of exact Full Moon, but on the night before and night after will appear 97-99% illuminated; most people would not notice the difference. Even two days from Full Moon the Moon's disk is 93-97% illuminated.

New Moon, First Quarter, Full Moon, and Last Quarter phases are considered to be primary phases and their dates and times are published in almanacs and on calendars. The two crescent and two gibbous phases are intermediate phases, each of which lasts for about a week between the primary phases, during which time the exact fraction of the Moon's disk that is illuminated gradually changes.

The phases of the Moon are related to (actually, caused by) the relative positions of the Moon and Sun in the sky. For example, New Moon occurs when the Sun and Moon are quite close together in the sky. Full Moon occurs when the Sun and Moon are at nearly opposite positions in the sky - which is why a Full Moon rises about the time of sunset, and sets about the time of sunrise, for most places on Earth. First and Last Quarters occur when the Sun and Moon are about 90 degrees apart in the sky. In fact, the two "half Moon" phases are called First Quarter and Last Quarter because they occur when the Moon is, respectively, one- and three-quarters of the way around the sky (i.e., along its orbit) from New Moon.

The relationship of the Moon's phase to its angular distance in the sky from the Sun allows us to establish very exact definitions of when the primary phases occur, independent of how they appear. Technically, the phases New Moon, First Quarter, Full Moon, and Last Quarter are defined to occur when the excess of the apparent ecliptic (celestial) longitude of the Moon over that of the Sun is 0, 90, 180, and 270 degrees, respectively. These definitions are used when the dates and times of the phases are computed for almanacs, calendars, etc. Because the difference between the ecliptic longitudes of the Moon and Sun is a monotonically and rapidly increasing quantity, the dates and times of the phases of the Moon computed this way are instantaneous and well defined.

The percent of the Moon's surface illuminated is a more refined, quantitative description of the Moon's appearance than is the phase. Considering the Moon as a circular disk, the ratio of the area illuminated by direct sunlight to its total area is the fraction of the Moon's surface illuminated; multiplied by 100, it is the percent illuminated. At New Moon the percent illuminated is 0; at First and Last Quarters it is 50%; and at Full Moon it is 100%. During the crescent phases the percent illuminated is between 0 and 50% and during gibbous phases it is between 50% and 100%. For practical purposes, phases of the Moon and the percent of the Moon illuminated are independent of the location on the Earth from where the Moon is observed. That is, all the phases occur at the same time regardless of the observer's position.

Crescent Moon Visibility

Although the date and time of each New Moon can be computed exactly the visibility of the lunar crescent as a function of the Moon's "age" - the time counted from New Moon - depends upon many factors and cannot be predicted with certainty. In the first two days after New Moon, the young crescent Moon appears very low in the western sky after sunset, and must be viewed through bright twilight. It sets shortly after sunset. The sighting of the lunar crescent within one day of New Moon is usually difficult. The crescent at this time is quite thin, has a low surface brightness, and can easily be lost in the twilight. Generally, the lunar crescent will become visible to suitably-located, experienced observers with good sky conditions about one day after New Moon. However, the time that the crescent actually becomes visible varies quite a bit from one month to another. The record for an early sighting of a lunar crescent, with a telescope, is 12.1 hours after New Moon; for naked-eye sightings, the record is 15.5 hours from New Moon. These are exceptional observations and crescent sightings this early in the lunar month should not be expected as the norm.

Obviously, the visibility of the young lunar crescent depends on sky conditions and the location, experience, and preparation of the observer. Generally, low latitude and high altitude observers who know exactly where and when to look will be favored. For observers at mid-northern latitudes, months near the spring equinox are also favored, because the ecliptic makes a relatively steep angle to the western horizon at sunset during these months (tending to make the Moon's altitude greater).

If we ignore local conditions for the moment, and visualize the problem from outside the Earth's atmosphere, the size and brightness of the lunar crescent depend on only one astronomical quantity - the elongation of the Moon from the Sun, which is the apparent angular distance between their centers. For this reason the elongation has also been called the arc of light. If we know the value of the elongation at any instant, we can immediately compute the width of the crescent.

 
 
 

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