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. Likewise, Earth's gravity creates a detectable bulge - a 60-foot land tide - on the Moon. We see the effect of the Moon in the ocean tides. Gravitational forces between Earth and the Moon drain the pair of their rotational energy. (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. The Moon takes as much time to rotate once on its axis as it takes to complete one orbit of Earth. The Moon always shows us the same face because Earth's gravity has slowed down the Moon's rotational speed. Why do we always see the same side of the Moon from Earth? Most sources credit this unusual event, occurring only "once in a blue moon," as the true progenitor of the colorful phrase. 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. The term "blue Moon" has not always been used this way, however. 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 timing has to be really precise to fit two Blue Moons into a single year. 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. On average, there's a Blue Moon about every 33 months. Over the past few decades, the second full Moon has come to be known as a "blue Moon." What is a Blue Moon and when is the next one?īecause 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.
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