Aug 2011
Why does the Moon have phases?
The Moon has phases because it
orbits Earth, which causes the portion we see illuminated to change.
The Moon takes 27.3 days to orbit Earth, but the lunar phase cycle (from
new Moon to new Moon) is 29.5 days. The Moon spends the extra 2.2 days
"catching up" because Earth travels about 45 million miles around the
Sun during the time the Moon completes one orbit around Earth.
At
the new Moon phase, the Moon is so close to the Sun in the sky that none
of the side facing Earth is illuminated (position 1 in illustration).
In other words, the Moon is between Earth and Sun. At first quarter, the
half-lit Moon is highest in the sky at sunset, then sets about six
hours later (3). At full Moon, the Moon is behind Earth in space with
respect to the Sun. As the Sun sets, the Moon rises with the side that
faces Earth fully exposed to sunlight (5).
You can create a mockup
of the relationship between Sun, Earth, and Moon using a bright lamp, a
basketball, and a baseball. Mark a spot on the basketball, which
represents you as an observer on Earth, then play with various
alignments of Earth and Moon in the light of your imaginary Sun.
When is the Harvest Moon?
The
full Moon that occurs closest to the autumnal equinox is commonly
referred to as the "Harvest Moon," since its bright presence in the
night sky allows farmers to work longer into the fall night, reaping the
rewards of their spring and summer labors. Because the equinox always
falls in late September, it is generally a full Moon in September which
is given this name, although in some years the full Moon of early
October earns the "harvest" designation.
In fact, each full Moon
of the year has its own name, most of which are associated with the
weather or agriculture. The most common names used in North America
include:
Because
the time between two full Moons doesn't quite equal a whole month,
approximately every three years there are two full Moons in one calendar
month. Over the past few decades, the second full Moon has come to be
known as a "blue Moon." The next time two full Moons occur in the same
month (as seen from the United States) will be August 2012. The most
recent "blue Moon" occurred in December 2009.
On average, there's a
Blue Moon about every 33 months. Blue Moons are rare because the Moon
is full every 29 and a half days, so the timing has to be just right to
squeeze two full Moons into a calendar month. The timing has to be
really precise to fit two Blue Moons into a single year. It can only
happen on either side of February, whose 28-day span is short enough
time span to have NO full Moons during the month.
The term "blue
Moon" has not always been used this way, however. While the exact origin
of the phrase remains unclear, it does in fact refer to a rare blue
coloring of the Moon caused by high-altitude dust particles. Most
sources credit this unusual event, occurring only "once in a blue moon,"
as the true progenitor of the colorful phrase.
Why do we always see the same side of the Moon from Earth?
The
Moon always shows us the same face because Earth's gravity has slowed
down the Moon's rotational speed. The Moon takes as much time to rotate
once on its axis as it takes to complete one orbit of Earth. (Both are
about 27.3 Earth days.) In other words, the Moon rotates enough each day
to compensate for the angle it sweeps out in its orbit around Earth.
Gravitational
forces between Earth and the Moon drain the pair of their rotational
energy. We see the effect of the Moon in the ocean tides. Likewise,
Earth's gravity creates a detectable bulge -- a 60-foot land tide -- on
the Moon. Eons from now, the same sides of Earth and Moon may forever
face each other, as if dancing hand in hand, though the Sun may balloon
into a red giant, destroying Earth and the Moon, before this happens.
When does the young Moon first become visible in the evening sky?
There
is no real formula for determining the visibility of the young Moon. It
depends on several factors: the angle of the ecliptic (the Moon's path
across the sky) with respect to the horizon, the clarity of the sky (how
much dust and pollution gunks it up), and even the keenness of the
observer's eyesight.
The young Moon becomes visible to the unaided
eye much earlier at times when the ecliptic is perpendicular to the
horizon, and the Moon pops straight up into the sky. In these cases, it
may be possible to see the Moon as little as 24 hours after it was new,
although every hour beyond that greatly increases the chances of
spotting it. When the ecliptic is at a low angle to the horizon, and the
Moon moves almost parallel to the horizon as it rises, the Moon
probably doesn't become visible until at least 36 hours past new.
The
record for the earliest claimed sighting of the young crescent Moon is
around 19 hours, although most experts are suspicious of any claims of
times less than about 24 hours.
http://stardate.org/nightsky/moon
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21  | 22 | 23  | 24  | 25  | 26  | 27  |
28  | 29  | 30 | 31 |
Why does the Moon have phases?
The Moon has phases because it
orbits Earth, which causes the portion we see illuminated to change.
The Moon takes 27.3 days to orbit Earth, but the lunar phase cycle (from
new Moon to new Moon) is 29.5 days. The Moon spends the extra 2.2 days
"catching up" because Earth travels about 45 million miles around the
Sun during the time the Moon completes one orbit around Earth.
At
the new Moon phase, the Moon is so close to the Sun in the sky that none
of the side facing Earth is illuminated (position 1 in illustration).
In other words, the Moon is between Earth and Sun. At first quarter, the
half-lit Moon is highest in the sky at sunset, then sets about six
hours later (3). At full Moon, the Moon is behind Earth in space with
respect to the Sun. As the Sun sets, the Moon rises with the side that
faces Earth fully exposed to sunlight (5).
You can create a mockup
of the relationship between Sun, Earth, and Moon using a bright lamp, a
basketball, and a baseball. Mark a spot on the basketball, which
represents you as an observer on Earth, then play with various
alignments of Earth and Moon in the light of your imaginary Sun.
When is the Harvest Moon?
The
full Moon that occurs closest to the autumnal equinox is commonly
referred to as the "Harvest Moon," since its bright presence in the
night sky allows farmers to work longer into the fall night, reaping the
rewards of their spring and summer labors. Because the equinox always
falls in late September, it is generally a full Moon in September which
is given this name, although in some years the full Moon of early
October earns the "harvest" designation.
In fact, each full Moon
of the year has its own name, most of which are associated with the
weather or agriculture. The most common names used in North America
include:
- January -- Moon after Yule
- February -- Snow Moon
- March -- Sap Moon
- April -- Grass Moon
- May -- Planting Moon
- June -- Honey Moon
- July -- Thunder Moon
- August -- Grain Moon
- September -- Fruit Moon (or Harvest Moon)
- October -- Hunter's Moon (or Harvest Moon)
- November -- Frosty Moon
- December -- Moon before Yule
Because
the time between two full Moons doesn't quite equal a whole month,
approximately every three years there are two full Moons in one calendar
month. Over the past few decades, the second full Moon has come to be
known as a "blue Moon." The next time two full Moons occur in the same
month (as seen from the United States) will be August 2012. The most
recent "blue Moon" occurred in December 2009.
On average, there's a
Blue Moon about every 33 months. Blue Moons are rare because the Moon
is full every 29 and a half days, so the timing has to be just right to
squeeze two full Moons into a calendar month. The timing has to be
really precise to fit two Blue Moons into a single year. It can only
happen on either side of February, whose 28-day span is short enough
time span to have NO full Moons during the month.
The term "blue
Moon" has not always been used this way, however. While the exact origin
of the phrase remains unclear, it does in fact refer to a rare blue
coloring of the Moon caused by high-altitude dust particles. Most
sources credit this unusual event, occurring only "once in a blue moon,"
as the true progenitor of the colorful phrase.
Why do we always see the same side of the Moon from Earth?
The
Moon always shows us the same face because Earth's gravity has slowed
down the Moon's rotational speed. The Moon takes as much time to rotate
once on its axis as it takes to complete one orbit of Earth. (Both are
about 27.3 Earth days.) In other words, the Moon rotates enough each day
to compensate for the angle it sweeps out in its orbit around Earth.
Gravitational
forces between Earth and the Moon drain the pair of their rotational
energy. We see the effect of the Moon in the ocean tides. Likewise,
Earth's gravity creates a detectable bulge -- a 60-foot land tide -- on
the Moon. Eons from now, the same sides of Earth and Moon may forever
face each other, as if dancing hand in hand, though the Sun may balloon
into a red giant, destroying Earth and the Moon, before this happens.
When does the young Moon first become visible in the evening sky?
There
is no real formula for determining the visibility of the young Moon. It
depends on several factors: the angle of the ecliptic (the Moon's path
across the sky) with respect to the horizon, the clarity of the sky (how
much dust and pollution gunks it up), and even the keenness of the
observer's eyesight.
The young Moon becomes visible to the unaided
eye much earlier at times when the ecliptic is perpendicular to the
horizon, and the Moon pops straight up into the sky. In these cases, it
may be possible to see the Moon as little as 24 hours after it was new,
although every hour beyond that greatly increases the chances of
spotting it. When the ecliptic is at a low angle to the horizon, and the
Moon moves almost parallel to the horizon as it rises, the Moon
probably doesn't become visible until at least 36 hours past new.
The
record for the earliest claimed sighting of the young crescent Moon is
around 19 hours, although most experts are suspicious of any claims of
times less than about 24 hours.
http://stardate.org/nightsky/moon
