Moon Phases

The revolution of the Moon around the Earth makes the Moon appear as if it is changing shape in the sky. This is caused by the different angles from which we see the bright part of the Moon's surface. These are called "phases" of the Moon. Of course, the Moon doesn't generate any light itself; it just reflects the light of the Sun. The Moon passes through four major shapes during a cycle that repeats itself every 29.5 days. The phases always follow one another in the same order.
What you see when you look at the moon depends on its location in relationship to the sun and Earth. We see a different fraction of sunlight being reflected from the moon to Earth

Moon & Earth Phase View The four major moon phases are "New" , "1st Quarter" , "Full" and "Last or 3rd Quarter". These phases have to do with the relative positions of the sun, the moon and the earth in the moon's 29 day monthly orbit of the earth.

Moon Phase Descriptions...
Although this cycle is a continuous process, there are eight distinct, traditionally recognized stages, called phases. The phases designate both the degree to which the Moon is illuminated and the geometric appearance of the illuminated part. These phases of the Moon, in the sequence of their occurrence (starting from New Moon), are listed below
(1) New Moon - When the Moon is roughly in the same direction as the Sun, its illuminated half is facing away from the Earth, and therefore the part that faces us is all dark: we have the new moon. When in this phase, the Moon and the Sun rise and set at about the same time.

(2) Waxing Crescent Moon - As the Moon moves around the Earth, we get to see more and more of the illuminated half, and we say the Moon is waxing. At first we get a sliver of it, which grows as days go by. This phase is called the crescent moon.

(3) Quarter Moon - A week after the new moon, when the Moon has completed about a quarter of its turn around the Earth, we can see half of the illuminated part; that is, a quarter of the Moon. This is the first quarter phase.

(4) Waxing Gibbous Moon - During the next week, we keep seeing more and more of the illuminated part of the Moon, and it is now called waxing gibbous (gibbous means "humped").

(5) Full Moon - Two weeks after the new moon, the moon is now halfway through its revolution, and now the illuminated half coincides with the one facing the Earth, so that we can see a full disk: we have a full moon. As mentioned above, at this time the Moon rises at the time the Sun sets, and it sets when the Sun rises. If the Moon happens to align exactly with the Earth and Sun, then we get a lunar eclipse.

(6) Waning Gibbous Moon - From now on, until it becomes new again, the illuminated part of the Moon that we can see decreases, and we say it's waning. The first week after full, it is called waning gibbous.

(7) Last Quarter Moon - Three weeks after new, we again can see half of the illuminated part. This is usually called last quarter.

(8) Waning Crescent Moon - Finally, during the fourth week, the Moon is reduced to a thin sliver from us, sometimes called waning crescent.

A while after four weeks (29.5 days, more precisely) the illuminated half of the Moon again faces away from us, and we come back to the beginning of the cycle: a new moon. Sometimes, when the Moon is almost new, it is possible to dimly see its darkened disk. The light from the Sun cannot reach this part of the Moon directly; but at this time the Earth (as viewed from the Moon) is at its full and very bright, and what we see is light reflected from the Earth, that then bounces back at us from the Moon. It's a long trip for this light: from the Sun to the Earth, to the Moon, and back to the Earth.

Moon Phase Views...
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.

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.
New Moon - The Moon's unilluminated side is facing the Earth. The Moon is not visible (except during a solar eclipse).The lighted side of the Moon faces away from the Earth.  This means that the Sun, Earth, and Moon are almost in a straight line, with the Moon in between the Sun and the Earth.  The Moon that we see looks very dark.
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. This Moon can be seen after the New Moon, but before the First Quarter Moon.  The crescent will grow larger and larger every day, until the Moon looks like the First Quarter Moon.
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. The right half of the Moon appears lighted and the left side of the Moon appears dark.  During the time between the New Moon and the First Quarter Moon, the part of the Moon that appears lighted gets larger and larger every day, and will continue to grow until the Full Moon.
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. This Moon can be seen after the First Quarter Moon, but before the Full Moon.  The amount of the Moon that we can see will grow larger and larger every day.  ("Waxing" means increasing, or growing larger.)
Full Moon - The Moon's illuminated side is facing the Earth. The Moon appears to be completely illuminated by direct sunlight. The lighted side of the Moon faces the Earth.  This means that the Earth, Sun, and Moon are nearly in a straight line, with the Earth in the middle.  The Moon that we see is very bright from the sunlight reflecting off it.
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. This Moon can be seen after the Full Moon, but before the Last Quarter Moon.  The amount of the Moon that we can see will grow smaller and smaller every day. ("Waning" means decreasing, or growing smaller.)
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. Sometimes called Third Quarter.  The left half of the Moon appears lighted, and the right side of the Moon appears dark.  During the time between the Full Moon and the Last Quarter Moon, the part of the Moon that appears lighted gets smaller and smaller every day. It will continue to shrink until the New Moon.
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. This Moon can be seen after the Last Quarter Moon and before the New Moon.  The crescent will grow smaller and smaller every day, until the Moon looks like the New Moon.

Moon Visibility...
The table gives a summary of approximately when the Moon is visible and where to look (the crescent and gibbous phases are in between the table values). There are times during the moon's monthly cycle that the Moon is sometimes visible in broad daylight!

Phase Ahead or Behind the Sun Rise Time in East Mid-Point In Sky Set Time In West
New Within few minutes Sunrise Noon Sunset
1st Qtr. 6 hrs behind Noon Sunset Midnight
Full 12 hrs behind Sunset Midnight Sunrise
3rd Qtr. 6 hrs ahead Midnight Sunrise Noon
The lighted part of the moon always points towards the sun. If lighted on the right the sun is ahead of the moon. If the left portion of the moon is lighted, the moon is ahead of the sun.

Moon Phase Comparison...
Comparing Moon Phase Moon Phase Comparison
The moon's cycle is a continuous process that is in constant change. The moon never stays at any one phase for more than an instance in time. Starting with a new moon on day one and ending with a waning crescent moon on day 29, the moon's light shape and intensity in always changing.

These phases of the Moon, in the sequence of their occurrence (starting from New Moon) is listed to the left.

As noted the moon reaches a major phase every seven days after the new moon. The first of which is the first quarter moon occuring after 7.4 days. Between the new and first quarter is the time of the waxing crescent moon. 14.8 days into trip around the earth we see a full moon, but not before the waxing gibbous make an appearance. After the full moon a state of waning begins on the 15th day. Along with a last quarter moon both a waning gibbous and crescent moon is visible before a new moon cycle starts a new on the 29th day.


Moon Phase Misconception...
"The most common incorrect reason given for the cause of the Moon's phases is that we are seeing the shadow of the Earth on the Moon! But this cannot be correct: when the Moon passes through the shadow of the Earth, we get a lunar eclipse. Anyone who has seen a lunar eclipse, though, might remember that the Moon actually passes through the Earth's shadow only rarely, so that can't be why the Moon has phases. The real reason for the Moon's phases depends on two things: the Moon is round, and the angle it makes with the Earth and Sun changes over its orbit."
- As Quoted From Bad Astronomy


Copyright © 2001 By Keith Cooley     - eMail:krcool@hiwaay.net - Return To Keith's Moon Page