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Think in 3D with the last quarter moon

Moon reaches the last quarter phase on the night of April 18-19, 2017. Watch for it after midnight – or around dawn – and use it to imagine Earth’s motion through space.

Last month’s last quarter moon was near Saturn on the morning of the March equinox (March 20, 2017). Photo via our friend Lunar 101 – Moon Book.

Last quarter moon comes on April 19, 2017 at 9:57 UTC. At North American latitudes, that translates to 6:57 a.m. ADT, 5:57 a.m. EDT, 4:47 a.m. CDT, 3:47 a.m. MDT and 2:47 PDT. That doesn’t mean you should look for the last quarter moon around those times (although you could). The time just indicates when the moon reaches the exact last quarter phase. A last quarter moon always rises around midnight, appears at its highest in the sky around dawn, and sets around noon.

On the night of a last quarter moon, the moon appears half illuminated in our sky.

A last quarter moon can be used as a guidepost to Earth’s direction of motion in orbit around the sun. For example, when you look toward 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.

The moon was almost exactly at last quarter when Jenney Disimon in Sabah, North Borneo captured this photo in July, 2016.

Another fun time to see a last quarter moon is just after it rises, shortly after midnight. Then the lighted portion points downward, to the sun below your feet. Great time to think of yourself in the three-dimensional world of outer space!

Last quarter moon comes about three weeks after the previous new moon and about a week before the next new moon. Next new moon will come on April 26, 2017 at 12:16 UTC; translate to your time zone.

At the last quarter phase, 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.

After the last quarter phase, the moon begins 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 drown the moon from view.

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

Four keys to understanding moon phases

Where’s the moon? Waxing crescent
Where’s the moon? First quarter
Where’s the moon? Waxing gibbous
What’s special about a full moon?
Where’s the moon? Waning gibbous
Where’s the moon? Last quarter
Where’s the moon? Waning crescent
Where’s the moon? New phase

Moon in 2017: Phases, cycles, eclipses, supermoons and more

Deborah Byrd

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