Encyclopedia
In
astronomy, a
lunar phase is any of the aspects or appearances presented by the
Moon as seen from
Earth, determined by the portion of the Moon that is visibly illuminated by the
Sun. The lunar phases vary cyclically as the Moon orbits the Earth, according to the relative positions of the Earth, the Moon, and the
Sun. Since the Moon appears bright only due to the Sun's reflected light, only the half of the Moon facing the Sun is illuminated.
Lunar phases are the result of our seeing the illuminated half of the Moon at different angles. The Moon exhibits different phases as the relative positions of the Sun, Earth and Moon change, appearing as the
Full moon when the Sun and Moon are on opposite sides of the Earth, and becoming invisible as the
New Moon when they are on the same side: these two phases are examples of syzygies. The time between two Full Moons is about 29.53 days ; it is longer than the time it takes the Moon to orbit the Earth since the Earth-Moon system is orbiting the Sun. The phases are not created by the shadow of the Earth on the Moon ; instead, they are a result of our seeing only part of the illuminated half of the Moon.
- Dark Moon - Not visible
- New Moon - Not visible, or traditionally: first visible crescent of the Moon
- Waxing Crescent Moon - Right 1-49% visible
- First Quarter Moon - Right 50% visible
- Waxing gibbous Moon - Right 51-99% visible
- Full Moon - Fully visible
- Waning gibbous Moon - Left 51-99% visible
- Third Quarter Moon - Left 50% visible
- Waning Crescent Moon - Left 1-49% visible
- New Moon - Not visible
In the southern hemisphere, the above is reversed. For example:
- Waxing Crescent Moon - Left 1-49% visible
- Waning Crescent Moon - Right 1-49% visible
When the Sun and Moon are on opposite sides of the Earth, the Moon appears full: the Moon appears as a wholly-illuminated disk. As the Moon orbits Earth, the Moon wanes, as the amount of illuminated lunar surface reduces, until the Moon effectively disappears at the New Moon, when the Moon is between Earth and the Sun and thus the illuminated half cannot be seen at all.
According to geometry, when a sphere is illuminated on one hemisphere and viewed from an angle, then the portion of the illuminated area visible from that angle will be observed to have the two-dimensional shape of a half-ellipse inscribed within a half-circle, where the major axis of the
ellipse is the same as the diameter of the semicircular arc. If the half-ellipse is convex with respect to the half-circle, then the shape will be gibbous, and if the half-ellipse is concave with respect to the half-circle, then the shape will be a
crescent. If the eccentricity of the ellipse is 1, then a half-circle will be seen; while if the eccentricity of the ellipse is 0, then either a fully-illuminated or fully-dark circular disk will be seen . Of course, actual observations of the Moon's lit area will not completely correspond with these abstract geometrical shapes, due to varying surface reflectivities, the lesser brightness of illuminated areas which are viewed at a low angle, etc.
The different phases of the Moon have different names. As the Moon waxes , the Moon moves through the New Moon, Crescent Moon, First-Quarter Moon, Gibbous Moon and Full Moon phases, before returning through the Gibbous Moon, Third-quarter Moon, Crescent Moon and Old Moon phases. Old Moon and New Moon are interchangeable, although New Moon is used in preference, and Half Moon is often used to mean the First- and Third-Quarter Moons.
It would seem that, once per month every time the Moon passes between Earth and the Sun , its shadow should fall on Earth, causing a
solar eclipse. Likewise, during every Full Moon, we might expect the Earth's shadow to be visible on the Moon — a
lunar eclipse. This is, in fact, how
eclipses happen, but they don't happen at every Full or New Moon. This is because the plane of the Moon's orbit around the Earth is tilted by about 5 degrees with respect to the Earth's orbit around the Sun, meaning that at the New and Full Moons the Moon usually lies to the north or south of a direct line through the Earth and Sun. An
eclipse can only occur when the Moon is positioned at, or very nearly at, one of its
nodes . This happens twice a year , and so . Most of these are quite insignificant; major
eclipses of the Moon or Sun are rather rare and newsworthy events.
Casual observers will not typically notice a waxing Crescent Moon until about 60 hours after it has passed conjunction with the Sun ; but some individuals have crafted a hobby out of attempting to view the Moon after a much shorter interval than this. Informal "records" and their confirmability vary; some have claimed to have seen the Moon as little as 12 hours after the moment of conjunction. Three factors increase the likelihood of spying a very "young" Moon: First, the angle the Moon makes with the
ecliptic must be favourable for the applicable side of the Earth — the optimum scenario for this would be a New Moon that falls in mid-March in the
Northern Hemisphere or mid-September in the
Southern Hemisphere; second, the Moon should be at or near
perigee, causing it to appear to move faster ; and third, the New Moon must be at or near its maximum separation from the
node in a favourable direction based on the hemisphere of the observer. These same principles can be applied to sight a very "old" Moon just before conjunction , but this is far less commonly pursued.
A First-Quarter Moon follows a daily path in the sky corresponding to that of the Sun after three months. Hence it comes at the highest altitude — or "runs high" — at or near the
vernal equinox. Similarly, a Full Moon comes highest at the
winter solstice, a Last Quarter Moon at the
autumnal equinox, and a New Moon at the
summer solstice . This also means that a First-Quarter Moon will not necessarily set at midnight, nor must a Last-Quarter Moon rise at midnight; both would do so at the
equator, but north or south of this the time of rising or setting will vary by a progressively wider margin as the latitude increases; indeed, a First-Quarter Moon in the late winter or early spring would remain constantly above the horizon in the polar regions .
Mnemonics
In the
northern hemisphere, if the left side of the Moon is dark, the light part is growing, that is, the Moon is waxing . If the right side of the Moon is dark, the light part is shrinking: the Moon is waning . Simply, assuming you are in the northern hemisphere, it's always the right part of the Moon that is growing. The acronym mnemonic "DOC" represents this .
In the
Southern hemisphere, this is reversed, and the mnemonic is "COD". Since these two mnemonics are equal and opposite, they may be easily confused; however, one can remember "DOC" since the story of the Seven Dwarves was created in the Northern Hemisphere.
A
French mnemonic is that the waxing Moon at its first "premier" quarter phase looks like a 'p', and the waning Moon at its last "dernier" quarter looks like a 'd'. The southern hemisphere equivalent for 'p' and 'd' is that the Moon is 'past it', or 'doing it'. The most popular Italian mnemonic for lunar phases is the rhyming proverb "Gobba a ponente, luna crescente. Gobba a levante, luna calante" , where
hump is a common metaphore for the Moon's crescent.
One more mnemonic, which works for most
Romance languages, says that the Moon is a liar: it spells "C", as in
crescere when it wanes, and "D" as in
decrescere when it waxes. Nevertheless, to Argentines, Brazilians, and other southern speakers of Romance languages, the Moon is honest.
For Polish it is easy to remember that C stands for "cofa sie" and D - for "dopelnia sie" . In
German, one mnemonic uses the cursive forms of the capital letters A for "abnehmend" and Z for "zunehmend" . In
Russia, the 'C' stands for "?????????" or "[a moon] becoming old", while a line is added to the waxing crescent to form 'P', which stands for "???????????" or "[a moon] getting born". In
Hungarian C stands for
csökken , while D stands for
dagad .
Calendar
The average month, based on the seasonal cycle is about 30.5 days, while the moon's synodic period repeats every 29.5 days. Therefore the timing of the moon's phases shifts by an average of one day in successive months.
If you photographed the moon's phase every day for a month, starting in the evening at sunset, repeating approximately 25 minutes later each day, you could create a composition like the example calendar below from May 8, 2005 to June 6, 2005.
Note: May 20th had no picture because a picture would be taken before midnight on May 19th, and after midnight on May 21st.
Lunar phase calculation
,
where
t = [UT] - [12AM, January 1, 2001], days
such that New Moon=.0, First Quarter=.25, Full Moon=.5, Last Quarter=.75
or a .
Warning: the tables below are only correct for a place which follows Greenwich time . Other localities may see any phase a calendar day earlier or later depending on the exact time of the lunar phenomenon.
See table of lunar phases for dates and times.
Patent
-
Moon dial for clocks - Clark - Nov., 1893
-
Astronomical wrist-watch - Oechslin - Dec., 1987
-
Display of changing moon on watch face - Galison - Sept., 1993
See also
External links
