Why does the Moon have phases?
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).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:
What is a Blue Moon and when is the next one?
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 July 2015. The most recent "blue Moon" occurred in August 2012.
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.