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 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. 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 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.
In spite of the phrase, there actually is no dark side of the Moon! Just like Earth, our Moon rotates on its own axis and experiences daylight and dark cycles. Our Moon's day and night cycles are a little longer than Earth's — the Moon spins on its axis once every Our Moon's period of rotation matches the time of revolution around Earth. In other words, it takes our Moon the same length of time to turn once on its axis as it takes it to go once completely around the Earth!
Do other planets have moons? Several of the planets in our solar system have natural satellites that orbit them. Some are so recently discovered that they have not yet been named. Mars has Phobos and Deimos, two small moons that circle very close to the martian surface. Jupiter has more known satellites than any other planet — 61! Saturn has at least 31 satellites, Uranus has 27, and Neptune has 13 — and more are being discovered all the time!
Pluto has one moon — Charon — the largest moon with respect to the size of the planet it revolves around. Only Mercury and Venus do not have any known satellites. In the days following new moon, the Moon emerges from the Sun's glare to the east of the Sun, rising and setting a few hours behind the Sun. This means that it is visible for much of the day, but also for a short time after dusk. A week after new moon, the Moon rises and sets roughly six hours behind the Sun, having completed a quarter of a circuit through the constellations.
The Moon is said to reach first quarter. Continuing eastwards, a week later the Moon lies almost opposite to the Sun in the sky. This is full moon , when the Moon is visible for almost all of the night.
It rises as the Sun sets, and sets as the Sun rises. The Moon's eastward journey then carries it closer to the Sun again, approaching the Sun from the west. At this time it rises in the middle of the night, and is visible in the morning sky. The Moon is said to be waxing in the period between new moon and full moon, when it is visible in the evening sky and the illuminated portion of its disk is growing from one night to the next.
Conversely, the Moon is said to be waning in the period from full moon to the next new moon, when the illuminated portion of its disk is shrinking.
At the beginning of this stage in the Northern Hemisphere, we see the right half of the Moon illuminated plus a tiny fraction more extending into the left side. In the Southern Hemisphere, the same happens, only from left to right. This phase is named as such because, from our perspective, the full disk is illuminated.
Learn more about lunar eclipses here! This phase occurs between the full and last quarter and describes the Moon when it is more than half lit, but not fully. At the beginning of this stage in the Northern Hemisphere, we see a disk almost fully lit except for a tiny sliver on the right side that is in darkness. In the Southern Hemisphere, the same happens, only the light shrinks from left to right.
This stage is sometimes also called Third Quarter. In the Northern Hemisphere, the left side is illuminated; in the Southern Hemisphere, it is the right side. During a last quarter phase, the Moon is said to be at west quadrature, meaning that it is 90 degrees west of the Sun when viewed from Earth.
This phase occurs between the last quarter and new Moon phases. Eventually, the entire disk will be in darkness, at which point it will be the new Moon phase and another lunar cycle will have begun. In the Southern Hemisphere, the same thing happens, only the lit area would have started on the right side and shrunk from left to right, until a thin crescent remained on the right.
Once the Sun rises, it is not easy to see this slim phase; the best time is before the glare of sunrise.
0コメント