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| Moon Phases on Jul 10, 2012

Understanding last quarter moon

A last quarter moon looks half-illuminated. It rises around midnight, appears at its highest in the sky at dawn, and sets around noon.

A last quarter moon looks half-illuminated. It rises around midnight, appears at its highest in the sky at dawn, and sets around noon.

Last quarter moon against a blue sky, on the morning of July 10, 2012. Photograph by EarthSky Facebook friend Janet Furlong in Culpeper, Virginia. She spotted this moon high in the sky shortly after sunup. Thank you, Janet!

Last quarter moon comes about three weeks after new moon. Now, as seen from above, the moon in its orbit around Earth is at right angles to a line between the Earth and sun. The moon is now three-quarters of the way around in its orbit of Earth, as measured from one new moon to the next.

It’s very cool to see the last quarter moon just after it rises, around midnight. The image at the top of this post illustrates it well. Just after the last quarter moon rises, its lighted portion is always facing downwards, more or less toward the horizon. Why? Because you’re on the midnight portion of Earth – with the sun below your feet – just as it’s below this moon.

After the last quarter phase, the moon will begin edging noticeably closer to the sun again on the sky’s dome. Fewer people notice the moon during the day from about last quarter on, because the sun’s glare begins to dominate the moon.

A last quarter moon can be used as a guidepost to Earth’s direction of motion in orbit around the sun. In other words, when you look at a last quarter moon high in the predawn sky, you’re gazing out approximately along the path of Earth’s orbit, in a forward direction. The moon is moving in orbit around the sun with the Earth. But, if we could somehow anchor the moon in space . . . tie it down, keep it still . . . Earth’s orbital speed of 18 miles per second would carry us across the space between us and the moon in only a few hours.

As the moon orbits Earth, it changes phase in an orderly way. Follow these links to understand the various phases of the moon.

Understanding Moon Phases
Waxing Crescent
First Quarter
Waxing Gibbous
Full Moon
Waning Gibbous
Last Quarter
Waning Crescent
New Moon