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GLENN ROBERTS: Autumnal equinox ushers in fall

Sky watchers in P.E.I. can catch a glimpse of a harvest moon on Wednesday, Sept. 7, 2017.
Sky watchers in P.E.I. can catch a glimpse of a harvest moon on Wednesday, Sept. 7, 2017.

After months of splendid, warm weather, summer officially draws to a close for the northern hemisphere later this month on Sept. 22, 5:02 p.m., when the autumnal equinox occurs.

On this date, the length of day matches night (thus the term "equinox" meaning "equal"). The nights slowly grow longer as we head towards the winter solstice in December, when the days begin to get longer, and we head back towards summer once again. Learn more about equinoxes by going to Google.

Jupiter, the fourth brightest object in the night sky after the sun, moon and Venus, is at its dimmest (mag. -1.7) and smallest for the year this month, though it’s still the brightest object in the evening sky. It starts the month approximately 10 degrees (hand's width at arm's length) above the WSW horizon about 45 minutes after sunset, but, by the end of the month, appears only half that height around the same time in the evening. Jupiter drops below the western horizon about 90 minutes after sunset as September begins, but about an hour after sunset by month's end. Look for a very dim, thin, waxing crescent "young" moon to the lower left (close to the horizon) of Jupiter on Sept. 21. On the evening of Sept. 22, the crescent moon will be to Jupiter's upper left.

 

Events (AST):
Sept. 6 - Full ("harvest") moon; 4:02 a.m.
Sept. 13 - Last quarter moon; 3:24 a.m.; moon at perigee
Sept. 15 - Cassini spacecraft plunges into Saturn's atmosphere
Sept. 20 - New moon; 2:29 a.m.
Sept. 22 - Autumnal equinox; 5:02 p.m.
Sept. 27 - First quarter moon; 11:53 p.m.; Moon at apogee

 

As darkness falls, Saturn (mag. +1.0) appears about 1/3 of the way up the SSW sky about an hour after sunset and will set during late evening. Saturn is the farthest planet that can be easily seen with the naked eye. It's magnificent ring system is tilted 27 degrees towards Earth this month, close to being its most open in the past 15 years. Look for Saturn just below the waxing crescent moon on the evening of Sept. 26.

Special note: NASA's 20-year-old Cassini mission to Saturn will end Sept. 15, when the spacecraft will make a planned plunge into the planet's atmosphere. As it slowly spirals around Saturn over the course of the next two weeks, the spacecraft will make a number of important studies of the planet's ring system and upper atmosphere, before plunging into that atmosphere and disintegrating

Venus is our "morning star" this month, rising about two and a half hours before sunrise at the beginning of September and about two hours before sunrise by month's end. Its phase increases to 90 per cent lit this month, which means it will be very bright.

On the morning of Sept. 12, Mercury (mag. 0.0) is at its greatest elongation (18 degrees west of the sun), making its best morning apparition of the year. On the morning of Sept. 16, about 70 minutes before sunrise, look to the east to see Venus with the crescent moon to its upper right. Just to the lower left of Venus sits Regulus, the brightest star in the constellation of Leo - the Lion. To the lower left of Regulus, closer to the horizon, you will spot Mercury (above) and Mars (below) very close together. By the morning of Sept. 18, the very thin, waning crescent moon will have slipped below Regulus, and Mercury and Mars will have switched positions, with dim Mars now above Mercury. You may need binoculars to see the latter two planets in the brightening twilight.

Until next month, clear skies.

 

Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. His column appears in The Guardian on the first Wednesday of each month. He welcomes comments from readers, and anyone who would like to do so is encouraged to email him at glennkroberts@gmail.com.

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