Moon, Antares and Venus on September 12

A chart showing Venus near the sunset point, and the moon and Antares above it.
The moon pairs up with the star Antares at dusk/nightfall September 12, 2021. The illuminated side of the moon points at Venus near the horizon.

Moon and Antares

We’re less than two weeks away from the equinox. Summer is nearly gone on our northern part of the globe. And the red star Antares – a summer star for the Northern Hemisphere – is shifting noticeably westward at each new sunset. Soon, it’ll be gone behind the sun. You can still see this star on September 12, 2021, though. It’ll be near a fat waxing crescent moon.

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Antares is the brightest star to light up the constellation Scorpius the Scorpion. Our sky chart at top depicts the view as seen from mid-northern North American latitudes. But, from most parts of the globe, the moon will be in the neighborhood of Antares in your sky, too, around September 12. Be sure to look for Venus, too! You can’t miss it as the brightest planet.

Who won’t see Antares near the moon around this date? Only those regions of the world near and north of the Arctic Circle. Antares is a far-southern star. So it never climbs above the horizon from Earth’s most northerly latitudes.

Antares, the far-southern star

Meanwhile, as seen from near and south of the Antarctic Circle, Antares always stays above the horizon. It’s circumpolar for far-southern latitudes. This star, like the midnight sun of summer, stays above the horizon in this part of the world all year round, instead of just seasonally.

There will, of course, be a moment this month when the moon and Antares have the same right ascension (corresponding to earthly longitude) on the sky’s dome. The moon will pass some 4 degrees north of Antares on September 13, at about 00:00 (midnight) UTC.

Month as measured by the stars

The moon travels full circle in front of the constellations of the zodiac every month. In fact, next month – in October 2021 – the moon will lap Antares again on October 10, at about 06:30 UTC.

The month, as measured by the moon’s successive return to the same star, is called the sidereal month. The sidereal month has a mean duration of about 27.32 days.

In contrast, the month as measured by the moon’s return to the same lunar phase is called the lunar month. It has a mean duration of 29.53 days, thus averaging 2.21 days longer than the sidereal month.

First quarter moon on September 13

By the way, the moon will reach its half-illuminated first quarter phase on September 13 at 20:39 UTC. That’s roughly 21 hours after the moon’s conjunction with Antares.

Next month, in October 2021, the span of time between the moon-Antares conjunction and the first quarter phase will increase by some two days. The conjunction occurs on October 10, at about 06:30 UTC, and first quarter moon on October 13, at 03:25 UTC. Why is that happening? It goes back to Antares’ westward shift with respect to the sunset horizon, mentioned above. And that shift of Antares is due to Earth’s own motion in orbit around the sun.

Bottom line: On September 12, 2021, watch for the wide crescent moon to be in the vicinity of the red star Antares. The moon will swing to the north of Antares on September 13 at about 00:00 (midnight) UTC, and then reach first quarter phase at 20:39 UTC.

Phases of the moon for 2021

September 12, 2021

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Bruce McClure

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