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Last quarter moon is July 16

Watch for the last quarter moon tonight after midnight, or around dawn Monday.

The moon was almost exactly at last quarter when Jenney Disimon in Sabah, North Borneo captured this photo. The terminator line, or line between light and dark on the moon, appears straight.

Last quarter moon comes on July 16, 2017 at 19:26 UTC. On Sunday morning, the moon appeared nearly last quarter; that is, the terminator line, or line between light and dark on the moon, appeared only slightly convex. Watch for the last quarter moon again late at night on Sunday, or around dawn Monday, July 17. A last quarter moon always rises in the middle of the night, appears at its highest in the sky around dawn, and sets around midday.

On the night of a last quarter moon, the moon appears half illuminated in our sky, and this half-a-pie-shaped 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 toward a last quarter moon high in the predawn sky, for example, 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 and never holds still. 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.

A great thing about this observation is that it can be done from anywhere. Ben Orlove wrote from New York City: “I was sitting in the roof garden of my building, and there was the moon, right in front of me. You were right, this is a perfect time to visualize … the Earth’s motion.”

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 July 23, 2017 at 09:46 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.

Zefri Besar in Brunei Darussalam caught the last quarter moon on the morning of June 17, 2017, just after midnight.

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