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The low autumn evening crescent

Tonight – October 26, 2017 – the moon is now a fat waxing crescent in the evening sky, as seen from around the globe. First quarter moon will come on October 27, at 22:22 UTC (6:22 p.m. EDT, 5:22 p.m. CDT, 4:22 p.m. MDT or 3:22 p.m. PDT). Diana from Potsdam, New York, noticed an interesting phenomenon that occurs with the first quarter moon every autumn. She asked:

Why are the evening crescent and the first quarter moon always so low in the autumn evening sky?

Today’s featured sky chart at top is meant to show the fat waxing crescent moon as viewed from mid-northern latitudes. As seen from our Northern Hemisphere, it’ll be quite low in the southern sky as the sun is setting. Are you in the Southern Hemisphere? Then take everything we’re about to say and apply it to your autumn moons in the months of March and April!

No matter where you are on Earth, your waxing moon in autumn tends to move sideways from night to night, staying rather close to the horizon, rather than moving directly upward away from the sunset point on the horizon from night to night. That’s always the case for waxing moons in autumn – say, around September or October if you’re in the Northern Hemisphere – March or April if you’re in the Southern Hemisphere.

It happens because the ecliptic, or pathway of the moon and planets, appears low in the sky on autumn evenings.

Our view from the Northern Hemisphere now is in contrast to that seen in the Southern Hemisphere, where it’s spring now and the ecliptic swings high overhead. In springtime, from either hemisphere, the evening crescent moon travels almost straight up from the sunset point from day to day.

Also in springtime, the first quarter moon shines close to overhead at dusk and nightfall.

Time of moonset in your sky

The moon appears much larger on this chart than it does in the real sky. The ecliptic – the moon’s approximate monthly path in front of the constellations of the zodiac – swings to the south of the Summer Triangle and low in the Northern Hemisphere’s autumn sky. From the Southern Hemisphere, the moon goes above an “upside-down” Summer Triangle and way high in their springtime sky.

From northerly latitudes at nightfall, you’ll see the moon residing beneath the Summer Triangle and low in the southern sky over the next several days. As dusk/nightfall comes to the south of the equator, the moon appears high in the sky and above the “upside-down” Summer Triangle, which appears in the Southern Hemisphere’s north to northwest sky on October evenings.

Bottom line: The fat lunar crescent of October 26, 2017, is waxing toward first quarter phase. This post talks about why the first quarter moon in October appears so low in the sky as seen from northerly latitudes.

Bruce McClure