This Week's Sky At A Glance, September 17 – 25 - Sky & Telescope

The Essential Guide to Astronomy

This Week's Sky At a Glance

By: Alan MacRobert September 17, 2021 9

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Nova Cas won't quit. Last week Nova Cassiopeiae 2021 jumped up yet again, to magnitude 6.6 — its seventh bump-up between fades since its original explosion to mag 7.7 last March. As of September 16th it was back down to 7.5. Charts and comparison stars.

■ The gibbous Moon hangs lower right of bright Jupiter at dusk this evening, as shown above. To their right, Saturn glows dimmer in the background. Later in the night, as the sky turns, The Moon will become directly below Jupiter and Saturn will turn to be lower than both of them. That's how you'll find them oriented 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. Cassiopeia 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.

■ The bright moon this evening forms a long, gently curving arc with Jupiter and then Saturn to its upper right. That's their order not only along the arc but in terms of brightness, and in terms of distance. The Moon is 1.3 light-seconds from Earth, Jupiter is currently 36 light-minutes away, and Saturn is 77 light-minutes away.

■ Sign of the changing season: The closing days of summer (the fall equinox comes on Wednesday the 22nd) always find the Sagittarius Teapot moving west of south during evening and tipping increasingly far over to the right, as if pouring out the last of summer through its spout.

■ Full Moon (exact at 7:55 p.m. Eastern Daylight Time). Look a couple of fists to its upper left for the Great Square of Pegasus, emblem of fall, ever higher now. By midnight, when the Moon shines nearly at its highest, the Great Square stands above it.

■ Jupiter's moon Europa reappears out of eclipse from Jupiter's shadow around 9:58 p.m. EDT. A small telescope will show it gradually swelling into view just off Jupiter's eastern limb. Then the Great Red Spot should cross Jupiter's central meridian around 11:34 p.m. EDT (8:34 p.m. PDT). For timetables of all such goings-on at Jupiter this month, see the September Sky & Telescope, pages 50-51.

■ Fall begins in the Northern Hemisphere, and spring in the Southern Hemisphere, at the equinox: 3:21 p.m. Eastern Daylight Time (19:21 UT). This is when the Sun crosses the equator — both Earth's equator and, equivalently, the celestial equator —heading south for the season.

■ Coincidentally, as if to mark this transition every year, Deneb is taking over from brighter Vega as the zenith star after nightfall (for skywatchers at mid-northern latitudes).

■ The big asteroid 2 Pallas, a little past opposition, stands high in the southeast by 10 p.m., in small-telescope reach at magnitude 8.4. This week it's only 7° or 8° upper right of Neptune, magnitude 8.7. Which is also, necessarily, a little past opposition. Read about both and hunt them down using the finder charts in Asteroid Pallas Makes a Point in Pisces.

■ Bright Arcturus, pale yellow-orange, shines ever lower in the west-northwest after dark. The narrow kite shape of its constellation, Bootes, extends two fists at arm's length to Arcturus's upper right. Arcturus is where the kite's downward-hanging tail is tied on.

To the right of the top of the kite, the Big Dipper is turning more level.

And this is the time of year when, during the evening, the dim Little Dipper "dumps water" into the bowl of the Big Dipper way down below. The Big Dipper will dump it back in the evenings of spring.

■ Cygnus the Swan floats just about straight overhead these evenings. Its brightest stars form the big Northern Cross.

When you face southwest and crane your head way, way up, the cross appears to stand upright. It's about two fists at arm's length tall, with Deneb as its top. Or to put it another way, when you face that direction the Swan appears to be diving straight down (something real swans never do).

The Moon won't rise now until about an hour after dark. So take this opportunity to look for the Milky Way running straight up from the west-southwest horizon, along the backbone of Aquila and to the just right of bright Altair high in the south, along the shaft of the Northern Cross overhead, and straight down through Cassiopeia and northern Perseus to the east-northeast horizon.

Mercury is hidden deep in the glow of sunset.

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

Jupiter and Saturn continue to shine in the southeast to south during evening. They're magnitudes –2.8 and +0.4, respectively, 16° apart on opposite sides of dim Capricornus. During twilight bright Jupiter, on the left, is slightly the lower of the two. They level out not long after dark, and later they tilt the other way, with Saturn now the lower one. Saturn sets around 2 or 3 a.m. daylight-saving time, followed down by Jupiter about an hour later. In the evening look for 1st-magnitude Fomalhaut some 22° (two fists) lower left of Jupiter. And less than 2° below or lower left of Jupiter is 3rd-magnitude Delta Capricorni, described in the caption above. Also, see Amateurs Spot New Impact Flash at Jupiter. With two videos of it taken by diligent, tireless amateur Jupiter-impact monitors!

Uranus (magnitude 5.7, in southern Aries) climbs high in the east by midnight.

Neptune (magnitude 7.8, at the Aquarius-Pisces border) is high in the southeast by 9 or 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 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 a Star Chart at the 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."

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.

"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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