Understanding the Moon's Phases



What Phases Are
The part of the moon that we can see at a given time is the part visible from earth of the side that is lit up. Half of the moon is always lit up, but the amount of that light that we can see depends on where the moon is in its orbit. As the moon travels around the earth, the amount of that light first grows larger, until the moon is full and directly opposite the sun, and then grows smaller again.

A Note on "Waxing" and "Waning"
The moon spends half its 29.5 day cycle waxing, or growing larger, and the other half waning, or growing smaller. As the Moon waxes it is first a crescent, then a half circle, then three quarters full, and rises later and later in the day. As the moon wanes, it is first three quarters full, then a half circle, and finally a crescent again, and rises later and later at night. The moon waxes on the right and wanes on the left, so you can easily tell whether the moon is waxing or waning by which side is lit up. Each night the moon rises about an hour later, although at different times in the cycle it can be more or less.

The New Moon
new The new moon occurs when the moon is between the earth and the sun, when the lit-up part of the moon is facing away from us, so the moon is almost never visible on this day. The new moon rises and sets with the sun.

The Waxing Crescent Moon
waxing crescent The waxing crescent is the period between the New Moon and the First Quarter. During this 6 or 7 day period, the moon appears as a crescent rising later and growing larger each day. The first day it rises shortly after the sun and sets shortly after the sun, and by the last day, it rises shortly before noon and sets shortly before midnight. The best time to view the waxing crescent is in the evening.

The First Quarter Moon
first quarter The first quarter marks the end of the first fourth of the moon's cycle. At this point the moon appears as a perfect half circle, as we can see half of the moon's lit-up side. The first quarter moon rises around sunset, making this the best time to view the moon. It sets around midnight.

The Waxing Gibbous Moon
waxing gibbous Waxing gibbous is the the period between the first quarter and the full moon, and is also 6 or 7 days long. On the first night, the moon appears slightly more than half full, and by the last day, it is nearly full. The first night, the moon rises shortly after noon and sets shortly after midnight, and on the last night, it rises shortly before sunset and sets shortly before sunrise.

The Full Moon
full The full moon occurs when the moon is on the opposite side of the earth from the the sun, when the whole lit-up side is facing the earth, and it appears as a perfect circle of bright light. The full moon rises around sunset and sets around sunrise, and the best time to view it is around nine or ten o'clock.

The Waning Gibbous Moon
waning gibbous The waning gibbous moon is the 6 or 7 day period between the full moon and the third quarter. It starts out slight less than full and ends up slightly more than half full. On the first night it rises shortly before midnight and sets shortly before noon. The best time to view it is between 9 at night and early in the morning, depending on the day.

The Third Quarter Moon
third quarter On the night of the third quarter, the moon appears again as a half circle, but this time on the left side rather than the right. The third quarter moon rises around midnight and sets around noon, and the best time to see it is either late at night or early morning.

The Waning Crescent Moon
waning crescent The waning crescent is the 6 or 7 day period between the third quarter and the new moon. Each night the crescent wanes thinner and thinner. The first day, it rises shortly after noon and sets shortly after midnight, and the last day, it rises shortly before sunrise and sets shortly before sunset. The best time to view it is late night to early morning.

Eclipses

Lunar Eclipse
lunar eclipse A lunar eclipse occurs when the earth's shadow falls on the moon. Because this only happens when the earth is directly between the moon and the sun, the moon is always full on the night of an eclipse. There is not an eclipse on every full moon because most full moons are not exactly on the opposite side of the earth from the sun, but slightly off, so that it doesn't fall in the earth's shadow. Light reflected from the earth, "earthshine", causes the moon to appear red during a lunar eclipse.

Solar Eclipse
solar eclipse A solar eclipse occurs when the moon's shadow falls on the earth. This only happens when the moon is directly between the sun and the earth, so it only occurs on the day of the new moon. Again, this doesn't happen every new moon because most new moons aren't exactly between the sun and the moon, but slightly off.

Other sites that explain the phases:
Phases of the Moon and Percent of the Moon Illuminated
Moonlight Madness - Test your knowledge of the phases
On the Moon's Phases
Phases of the Moon
Phases of the Moon and Percent of the Moon Illuminated
Educator's Guide to Moon Phases - How to explain the phases to children
Phases of the Moon

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