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

The recurrent nova RS Ophiuchi has pale down previous magnitude 9.0 as of September tenth. But Nova Cassiopeiae 2021 has jumped up but once more, to six.6 on that date, a magnitude brighter than its authentic explosion final March. This is its seventh bump-up since March. It will not give up! It’s now classed as a “very slow nova.” Charts and comparison stars.


■ We proceed to have three vibrant night planets this week: Venus low within the west-southwest at nightfall, Jupiter larger within the southeast to south throughout night, and dimmer Saturn almost two fists to Jupiter’s proper. By late night, faint Uranus and Neptune lurk excessive within the east.

■ The two brightest stars of September evenings are at all times Vega excessive overhead and Arcturus within the west, each magnitude zero. Draw a line from Vega all the way down to Arcturus. A 3rd of the way down you cross the dim Keystone of Hercules. Two thirds of the way you cross the fragile semicircle of Corona Borealis with its one modestly vibrant star: Alphecca, the gem of the crown.


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

Delta Sco is among the Gamma Cassiopeiae class of sizzling variable stars spinning at almost breakup speed. Read in regards to the authentic at Meet Gamma Cassiopeiae, the Classic Eruptive Variable.


■ Now the Moon shines higher proper of Antares. as proven under. They’re about 3° aside in early night for the Americas.

Moon over Antares and Scorpius, Sept. 12-13, 2021
Sunday night the Moon shines over Antares. Monday night within the Americas finds the Moon simply a few hours previous its precise first-quarter section.


■ First-quarter Moon (precise at 4:39 p.m. EDT), shining within the ft of Ophiuchus. The Moon is about halfway between Antares to its decrease proper and the Sagittarius Teapot to its left (for North America). Look quickly after twilight ends, earlier than they sink decrease.


■ Now the Moon shines within the lid of the Sagittarius Teapot. The Teapot is tilting and pouring to the proper, like at all times in September. Binoculars will aid you decide it out from moonlight and lightweight air pollution. (Remember that the Teapot spans some 13°, about twice the width of a typical binocular’s subject of view.)


■ Jupiter, Saturn, and the waxing gibbous Moon type a gently curving line this night, with the Moon on the decrease proper. Extend a line from Saturn via the Moon by the identical quantity onward, and also you’re within the Sagittarius Teapot.


■ Now the Moon shines about 5° beneath Saturn, as indicated under. (Actually, the “Harvest Moon effect” extra typically refers back to the Moon persevering with to gentle the early-evening panorama for a number of days after full close to the September equinox. Before full the Moon lights the early night each month, although it isn’t essentially low.)

Waxing gibbous Moon passing under Saturn and Jupiter, Sept. 16-17, 2021.

Polaris crosses a line. Roger Sinnott, Sky & Telescope‘s longtime grasp of all issues 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 does not have any profound ramifications. But Polaris’s altering R.A. may be very noticeable whenever you look it up in star charts or catalogues for equinox 2000.0 vs. 1950.0. It additionally reveals up simply whenever you evaluate the 2021 Skygazer’s Almanac [our annual events-through-the-night chart, which comes with every January’s S&T ] with these from 10, 20, or 40 years in the past. The nearer Polaris will get to the celestial pole, the extra quickly its R.A. adjustments.”


■ 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).


■ 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.



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.

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.

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 authentic or Jumbo Edition), which reveals stars to magnitude 7.6.

Pocket Sky Atlas cover, Jumbo edition
The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6, and hundreds of telescopic galaxies, star clusters, and nebulae among them. Shown here is the Jumbo Edition, which is in hard covers and enlarged for easier reading outdoors by red flashlight. Sample charts. More about the current editions.

Next up is the bigger 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 you should definitely learn how to use sky charts with a telescope.

You’ll additionally need a good deep-sky guidebook, comparable to the massive Backyard Astronomer’s Guide, “A full appreciation of the universe can not come with out creating the abilities to search out issues within the sky and understanding how the sky works. This data comes solely by spending time beneath the stars with star maps in hand.”

Audio sky tour. Out beneath the night sky along with your
earbuds in place, hearken to Kelly Beatty’s month-to-month
podcast tour of the heavens above. It’s free.

“The risks of not considering clearly are a lot better now than ever earlier than. It’s not that there is one thing new in our way of considering, it is that credulous and confused considering could be way more deadly in methods it was by no means earlier than.”
            — Carl Sagan, 1996

“Facts are cussed issues.”
            — John Adams, 1770

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