This Week's Sky at a Glance, September 10 – 18

The recurrent nova RS Ophiuchi has faded down past magnitude 9.0 as of September 10th. But Nova Cassiopeiae 2021 has jumped up yet again, to 6.6 on that date, a magnitude brighter than its original explosion last March. This is its seventh bump-up since March. It won't quit! It's now classed as a "very slow nova." Charts and comparison stars.

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FRIDAY, SEPTEMBER 10

■ We continue to have three bright evening planets this week: Venus low in the west-southwest at dusk, Jupiter higher in the southeast to south during evening, and dimmer Saturn nearly two fists to Jupiter's right. By late evening, faint Uranus and Neptune lurk high in the east.

■ The two brightest stars of September evenings are always Vega high overhead and Arcturus in the west, both magnitude zero. Draw a line from Vega down to Arcturus. A third of the way down you cross the dim Keystone of Hercules. Two thirds of the way you cross the delicate semicircle of Corona Borealis with its one modestly bright star: Alphecca, the gem of the crown.

SATURDAY, SEPTEMBER 11

■ Look left of the thick crescent Moon for the head stars of Scorpius lined up not quite vertically, and farther left for orange Antares. Halfway between the Moon and Antares (for the Americas) is the brightest of the head stars, the long-term eruptive variable Delta Scorpii. It's been holding fairly steady around magnitude 1.8 since 2010.

Delta Sco is one of the Gamma Cassiopeiae class of hot variable stars spinning at nearly breakup speed. Read about the original at Meet Gamma Cassiopeiae, the Classic Eruptive Variable.

SUNDAY, SEPTEMBER 12

■ Now the Moon shines upper right of Antares. as shown below. They're about 3° apart in early evening for the Americas.

MONDAY, SEPTEMBER 13

■ First-quarter Moon (exact at 4:39 p.m. EDT), shining in the feet of Ophiuchus. The Moon is about midway between Antares to its lower right and the Sagittarius Teapot to its left (for North America). Look soon after twilight ends, before they sink lower.

TUESDAY, SEPTEMBER 14

■ Now the Moon shines in the lid of the Sagittarius Teapot. The Teapot is tilting and pouring to the right, like always in September. Binoculars will help you pick it out from moonlight and light pollution. (Remember that the Teapot spans some 13°, about twice the width of a typical binocular's field of view.)

WEDNESDAY, SEPTEMBER 15

■ Jupiter, Saturn, and the waxing gibbous Moon form a gently curving line this evening, with the Moon on the lower right. Extend a line from Saturn through the Moon by the same amount onward, and you're in the Sagittarius Teapot.

THURSDAY, SEPTEMBER 16

■ Now the Moon shines about 5° under Saturn, as indicated below. (Actually, the "Harvest Moon effect" more often refers to the Moon continuing to light the early-evening landscape for several days after full near the September equinox. Before full the Moon lights the early evening every month, though it's not necessarily low.)

■ Polaris crosses a line. Roger Sinnott, Sky & Telescope's longtime master of all things celestial-mechanical, writes, "I discovered a neat curiosity when working on the Skygazer's Almanac for 2021. Owing to precession, on September 16th Polaris reaches right ascension 3h (equinox of date) for the first time in 26,000 years, after having crossed 2h in 1964 and 1h in 1828.

"Of course, this doesn't have any profound ramifications. But Polaris's changing R.A. is very noticeable when you look it up in star charts or catalogues for equinox 2000.0 vs. 1950.0. It also shows up easily when you compare the 2021 Skygazer's Almanac [our annual events-through-the-night chart, which comes with every January's S&T ] with those from 10, 20, or 40 years ago. The nearer Polaris gets to the celestial pole, the more rapidly its R.A. changes."

FRIDAY, SEPTEMBER 17

■ And now the gibbous Moon hangs lower right of the bright Jupiter at dusk, as shown above. Later in the night, as the sky turns, The Moon becomes directly below Jupiter. That's how you'll find them around midnight (daylight saving time).

SATURDAY, SEPTEMBER 18

■ You can see in the stars that the season is changing: We've reached the time of year when, just after nightfall, cold-weather Cassiopeia has already climbed a little higher in the northeast than the warm-weather Big Dipper has sunk in the northwest. Cas bedecks the high northern sky in early evening during the fall-winter half of the year. The Big Dipper takes over for the milder evenings of spring and summer.

Almost midway between them stands Polaris. It's currently a little above the midpoint between the two.

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This Week's Planet Roundup

Mercury (about magnitude +0.2) is very deep down in the sunset, 16° lower right of Venus this week. You might have a chance at it with binoculars or a wide-field scope. About 15 or 20 minutes after sunset, scan for it just above your horizon due west. Good luck.

Between Venus and Mercury is fainter Spica, magnitude +1.0.

Venus, brilliant at magnitude –4.1, shines in the west-southwest during twilight. It still sets around twilight's end.

Jupiter and Saturn shine in the southeast to south these evenings. They're magnitudes –2.8 and +0.4, respectively, on opposite sides of dim Capricornus.

Jupiter starts the evening as slightly the lower of the two. Saturn glows 17° (almost two fists) to Jupiter's upper right. They level out around 9 or 10 p.m. daylight-saving time. By then they're about at their highest in the south at their telescopic best. After that they start to tilt the other way, with Saturn now the lower one.

Look for 1st-magnitude Fomalhaut about two fists (22°) to Jupiter's lower left. And this week, Jupiter is passing 1½° north of 3rd-magnitude Delta Capricorni.

Uranus (magnitude 5.7, in southern Aries) gets high in the east after midnight.

Neptune (magnitude 7.8, at the Aquarius-Pisces border) is high in the southeast by 10 p.m.

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All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.

Eastern Daylight Time, EDT, is Universal Time minus 4 hours. Universal Time is also known as UT, UTC, GMT, or Z time. To become more expert about time systems than 99% of the people you'll ever meet, see our compact article Time and the Amateur Astronomer.

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Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential magazine of astronomy.

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or Jumbo Edition), which shows stars to magnitude 7.6.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many. The next up, once you know your way around, are the even larger Interstellarum atlas (stars to magnitude 9.5) or Uranometria 2000.0 (stars to magnitude 9.75). And be sure to read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as the big Night Sky Observer's Guide by Kepple and Sanner.

Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically, meaning heavy and expensive. And as Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."

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Audio sky tour. Out under the evening sky with your
earbuds in place, listen to Kelly Beatty's monthly
podcast tour of the heavens above. It's free.

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"The dangers of not thinking clearly are much greater now than ever before. It's not that there's something new in our way of thinking, it's that credulous and confused thinking can be much more lethal in ways it was never before."
            — Carl Sagan, 1996

"Facts are stubborn things."
            — John Adams, 1770

 

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