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Half-lit last quarter moon on May 29

Image of last quarter moon above via US Naval Observatory

Tonight – May 28, 2016 – if you’re looking for the moon in the evening sky, you won’t find it. The moon reaches last quarter early in the morning on May 29. A last quarter moon rises in the middle of the night and shines in the morning sky.

The moon reaches its half-illuminated last quarter phase on May 29 at 12:12 Universal Time. Although this last quarter moon happens at the same instant worldwide, it occurs at different times by the clock, depending on one’s time zone. Here, in the mainland United States, the last quarter moon comes on May 29 at 8:12 a.m. EST, 7:12 a.m. CST, 6:12 a.m. MST and 5:12 a.m. PST.

From around the world, people will see the moon in the wee hours after midnight on May 29. No matter where you are on Earth, the moon will be highest up in your sky at roughly 6 a.m. local time (7 a.m. Daylight Saving Time). It’ll appear half-illuminated. Although half-lit, it’s called a last quarter or third quarter moon because the moon is three-quarters of the way in its journey from new moon to new moon.

Click here to download animation. | As seen from the north side of the moon's orbital plane, Earth rotates counterclockwise on its rotational axis, and the moon revolves counterclockwise around Earth. The terminators of the Earth and moon align at first and last quarter moon. This illustration shows a first quarter moon.[/caption]

Click here to see animation. | As seen from the north side of the moon’s orbital plane, Earth rotates counterclockwise on its rotational axis, and the moon revolves counterclockwise around Earth. The terminators of the Earth and moon align at first and last quarter moon. This illustration shows a first quarter moon.

One half of the lunar disk is always illuminated by sunlight, while the nighttime half is submerged in the moon’s own shadow. At last quarter moon, we see half the moon’s day side, and half its night side.

The lunar terminator – the shadow line dividing day from night – shows you where it’s sunset on the last quarter moon as it wanes toward new moon. It’s along the terminator that you have your best three-dimensional views of the lunar terrain through binoculars or the telescope. Try looking in the morning twilight, when the sky isn’t quite so dark, to eliminate glare from the moon itself.

If you were on the moon at its last quarter phase, and looking back at Earth, you’d see the Earth at its first quarter phase, as displayed on the image below.

As seen from the moon, the terminator on the first quarter Earth depicts sunrise, as the first quarter Earth waxes toward its full phase.

Simulation of the first quarter Earth as viewed from the last quarter moon (2016 May 29 at 12:12 Universal Time). Image credit: Earth and Moon Viewer

Simulation of the first quarter Earth as viewed from the last quarter moon (2016 May 29 at 12:12 Universal Time). Image credit: Earth and Moon Viewer

Approximately one week after the May 29, 2016 last quarter moon, it’ll be a new moon in Earth’s sky but a full Earth in the moon’s sky, as shown on the simulation below.

Simulation of the full Earth as viewed from the new moon (2016 June 5 at 3:00 Universal Time).

Simulation of the full Earth as viewed from the new moon (2016 June 5 at 3:00 Universal Time).

Bottom line: Enjoy the January 29, 2016 last quarter moon! And know that the Earth and moon are like mirrors to each other in that – when we see a last quarter moon in our sky – those on the moon would see a first quarter Earth.

Bruce McClure

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