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Half-lit first quarter moon on January 16

Click here to see animation. As seen from the north side of the moon's orbital plane, the 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.

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.

Image top of post: Moon on January 15, 2016 by Patrick Casaert of La Lune The Moon

Tonight – January 16, 2016 – the moon reaches its half-illuminated first quarter phase. The exact phase is reached at 23:26 Universal Time on this date. Although this first 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 first quarter moon comes on January 16 at 6:26 p.m. EST, 5:26 p.m. CST, 4:26 p.m. MST and 3:26 p.m. PST.

From around the world tonight, people will see the moon in their evening sky. It’ll appear half-illuminated.

One half of the lunar disk is always illuminated by sunlight, while the nighttime half is submerged in the moon’s own shadow. At first 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 sunrise on the first quarter moon as it waxes toward full 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 evening twilight, when the sky isn’t quite dark yet, to eliminate glare from the moon itself.

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

As seen from the moon, the terminator on the last quarter Earth depicts sunset, as the last quarter Earth wanes toward its new phase.

Simulation of the last quarter Earth as viewed from the first quarter moon (2016 January 16 at 23:26 Universal Time). As seen from the moon, the Earth's terminator represents sunset, and the Earth is waning toward new phase. Image credit: Earth and Moon Viewer

Simulation of the last quarter Earth as viewed from the first quarter moon (2016 January 16 at 23:26 Universal Time). As seen from the moon, the Earth’s terminator represents sunset, and the Earth is waning toward new phase. Image credit: Earth and Moon Viewer

However, when the last quarter moon comes on February 1, the lunar terminator will show you where it’s sunset on the last quarter moon.

On that future date – February 1, 2016 – from the vantage point of the last quarter moon, the earthly terminator will show you where it’s sunrise on the first quarter Earth, as illustrated below.

Simulation of the first quarter Earth as viewed from the last quarter moon (2016 February 1 at 3:28 Universal Time). As seen from the moon, the Earth's terminator represents sunrise and the Earth is waxing toward full phase. Image credit: Earth and Moon Viewer

Simulation of the first quarter Earth as viewed from the last quarter moon (2016 February 1 at 3:28 Universal Time). As seen from the moon, the Earth’s terminator represents sunrise and the Earth is waxing toward full phase. Image credit: Earth and Moon Viewer

After the January 16, 2016, first quarter moon, the moon will wax toward full moon.

Meanwhile, as seen from the moon, the January 16 last quarter Earth will wane toward Earth at its new phase.

Approximately one week after today’s first quarter moon, it’ll be a full moon in Earth’s sky but a new Earth in the moon’s sky.

Bottom line: Enjoy the January 16, 2016 first quarter moon! And know that the Earth and moon are like mirrors to each other in that – when we see a first quarter moon in our sky – those on the moon would see a last quarter Earth.

Bruce McClure

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