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Go young moon hunting

Robert Pettengill of Austin, Texas caught a young moon with earthshine on the last evening of fall – December 20, 2017 – from Austin, Texas

Tonight – January 18, 2018 – given clear skies, most of us around the world should be able to spot the slender young moon in the southwest sky at evening twilight. It’ll set shortly after the sun. If you miss the moon on January 18, try again on January 19.

Day by day, the moon will move farther east of the setting sun, to appear higher up after the sun goes down.

Each evening after sunset, the young moon will appear higher in the evening twilight sky.

If you do see the moon, look for earthshine on the darkened portion of the moon. A moon with earthshine is not a moon in eclipse; no shadow or world in space is obscuring any other. Instead, you’re simply seeing night on the moon, and, as it happens, the lunar night is flooded with earthlight around now.

Why? Consider that when the moon appears as a slender crescent in Earth’s sky, the Earth appears as an almost-full waning gibbous Earth in the moon’s sky.

Earthshine is twice-reflected sunlight – sunlight reflected from the Earth to the moon, and then from the moon back to Earth.

View larger. | Eliot Herman in Tucson, Arizona caught a one-day-old crescent moon with earthshine on November 19, 2017.

On the average, the moon moves some 13o eastward per day in front of the backdrop stars of the zodiac. Meanwhile, as seen from Earth, the sun appears to move one degree eastward per day along the ecliptic. In other words, the moon moves an average 12o east of the sun daily. For reference, the angular diameter of the sun and moon span about one-half degree of sky. This movement of the moon on our sky’s dome is, of course, due to the moon’s motion in orbit around Earth.

The January new moon happened on January 17 at 2:17 Universal Time. For us in the United States, that translated to January 16 at 21:17 (9:17 p.m.) EST, 20:17 (8:17 p.m.) CST, 19:17 (7:17 p.m.) MST and 18:17 (6:17 p.m.) PST.

So, after sunset on February 18, the moon will be nearly two days old for us in the Americas.

At comparable latitudes in the world’s Eastern Hemisphere – Europe, Africa and Asia – the January 18 young moon will exhibit a thinner crescent hovering closer to the horizon. Click here for recommended almanacs; they can give you the time of moonset in your sky.

Hector Barrios caught the young moon on March 28, 2017 from Hermosillo, Mexico.

Bottom line: Given an unobstructed horizon in the direction of sunset, and a clear sky, the thin waxing crescent moon should appear low in the sky and close to the horizon around one hour (or less) after sunset on January 18, 2018.

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Bruce McClure