Why the moon exhibit phases

At new moon, the Moon is lined up between the Earth and the Sun. We see the side of the Moon that is not being lit by the Sun in other words, we see no Moon at all, because the brightness of the Sun outshines the dim Moon!

When the Moon is exactly lined up with the Sun as viewed from Earth , we experience an eclipse. As the Moon moves eastward away from the Sun in the sky, we see a bit more of the sunlit side of the Moon each night.

A few days after new moon, we see a thin crescent in the western evening sky. The crescent Moon waxes, or appears to grow fatter, each night.

When half of the Moon's disc is illuminated, we call it the first quarter moon. This name comes from the fact that the Moon is now one-quarter of the way through the lunar month. From Earth, we are now looking at the sunlit side of the Moon from off to the side. The Moon continues to wax. Once more than half of the disc is illuminated, it has a shape we call gibbous.

The gibbous moon appears to grow fatter each night until we see the full sunlit face of the Moon. We call this phase the full moon. It rises almost exactly as the Sun sets and sets just as the Sun rises the next day. The Moon has now completed one half of the lunar month. During the second half of the lunar month, the Moon grows thinner each night. We call this waning. After a full moon, the moon wanes — becomes smaller — into a gibbous moon, a half-moon also called last quarter , a crescent and finally a new moon.

Just before and just after the new moon, when a slim crescent of the moon is lit, you can also see the rest of the moon lit dimly.

The major phases of the moon — new moon, first quarter, full moon, last quarter and next new moon — occur, on average, about 7. If you need some help tracking these phases yourself or if you want to see where the moon was on an important day in history , NASA provides an online calendar of the dates and times of all phases of the moon for the six thousand year period between BCE to CE.

This activity , provided by the Astronomical Society of the Pacific, demonstrates why the moon has phases. Follow LiveScience livescience. Live Science. Elizabeth Palermo. Over the past few decades, the second full Moon has come to be known as a "blue Moon.

The most recent "blue Moon" occurred in August 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. We see the effect of the Moon in the ocean tides. Likewise, Earth's gravity creates a detectable bulge -- a foot land tide -- on the Moon.