This Week’s Sky at a Glance, Sept. 24 – Oct. 2 – Sky & Telescope

Nova Cassiopeiae 2021 carries on, fluctuating unpredictably as it remains in binocular view six months after its March explosion. As of September 21st it was back up to magnitude 6.7, more than twice as bright as its original March eruption from its 15th-magnitude baseline. Charts and comparison stars.


■ Bright Jupiter and fainter Saturn continue to dominate the southern evening sky, 16° apart. All week Jupiter shines just above or upper right of 3rd-magnitude Delta Capricorni, Deneb Algedi (the name means “Tail of the Young Goat”). The scene is shown below for late twilight this evening.

Farther to the upper right of Saturn, look for Alpha and Beta Capricorni. Both are binocular double stars. Alpha is a wide pair of yellow-orange giants that the smallest binoculars easily resolve; so does our chart below. The components of Beta are half as far apart, much more unequal, and oriented roughly the same way as the Alpha pair.

As the stars come out this week, look for 3rd-magnitude Delta Capricorni just below or lower left of Jupiter. Tonight they’re 1.5° apart, about a finger’s width at arm’s length. Jupiter is 175 times brighter! How early (or more likely, late) in twilight can you first make out Delta Cap?

Delta Cap is an interesting star: a magnetic white subgiant, spectral class A7m, nine times brighter than the Sun and closely orbited by a small, dim orange dwarf almost exactly once a day. The dwarf passes in front of the bright star every orbit from our point of view, dimming it by 0.24 magnitude. That’s theoretically discernable by eye but without near-match comparison stars nearby, good luck. The system is only 39 light-years away.

■ Jupiter’s Great Red Spot should cross Jupiter’s central meridian around 9:04 p.m. Eastern Daylight Time. For full timetables of the Red Spot’s transits as well as the doings of Jupiter’s moons and their shadows, see the Celestial Calendar section of Sky & Telescope magazine.


■ 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.

The waning gibbous Moon doesn’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 just to the right of bright Altair high in the south; then along the shaft of the Northern Cross overhead, and straight down through Cassiopeia and northern Perseus to the east-northeast horizon.


■ Bright Arcturus, pale yellow-orange, shines ever lower in the west-northwest at nightfall. 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.

■ Tonight Jupiter’s Great Red Spot should cross the planet’s central meridian around 10:42 p.m. EDT (7:42 p.m. Pacific). It remains in good view for about an hour before and after that time.


■ Arcturus shines in the west in late twilight these evenings. Capella, equally bright, is just rising in the north-northeast (depending on your latitude; the farther north you are the higher it will be). They’re both magnitude 0.

Later in the evening, Arcturus and Capella shine at equal heights in their respective compass directions. When will this happen? That depends on both your latitude and longitude.

When it does, turn around and look low in the south-southeast. There will be 1st-magnitude Fomalhaut at the about same height too — exactly so if you’re at latitude 43° north (about the latitude of Boston, Buffalo, Milwaukee, Boise, Eugene). Seen from south of that latitude, Fomalhaut will appear higher than Capella and Arcturus are. Seen from north of there, it will be lower.


■ Last-quarter Moon tonight (exact at 9:57 p.m. Eastern Daylight Time). The Moon rises around 11 or midnight in Gemini, down below Capella and the rest of Auriga.

An hour later look off to the Moon’s right, and there’s Orion stepping up over the eastern horizon (for viewers at mid-northern latitudes).


■ The starry W of Cassiopeia stands high in the northeast after dark. The right-hand side of the W (the brightest side) is tilted up.

Look at the second segment of the W counting down from the top. Notice the dim naked-eye stars along that segment (not counting its two ends). The brightest of these, on the right, is Eta Cassiopeiae, magnitude 3.4. This is a remarkably Sun-like star just 19 light-years away, and it has a orange-dwarf companion, magnitude 7.3, separation 13 arcseconds a lovely binary in a telescope.

Left of it, and fainter, is a naked-eye pair in a dark sky: Upsilon1 and Upsilon2 Cassiopeiae, a good 0.3° apart. They’re yellow-orange giants unrelated to each other, 200 and 400 light-years distant from us. Upsilon2 is slightly the brighter of the pair. It’s also the closer one.

■ Night owl? Look east any time from 1 or 2 a.m. to early dawn Thursday morning, and you’ll find the waning Moon near Pollux in Gemini. Above Pollux is Castor.


■ The big asteroid 2 Pallas, almost three weeks past opposition, stands high in the southeast by 9 or 10 p.m. It’s still in small-telescope reach at magnitude 8.4. This week Pallas is 8° or 9° upper right of Neptune, magnitude 8.7, which also is past opposition. Read about both and hunt them down using the finder charts in Asteroid Pallas Makes a Point in Pisces.


■ Vega is the brightest star just west of the zenith after dark. Face west and look to Vega’s right by 14° (nearly a fist and a half at arm’s length) for Eltanin, the nose of Draco the Dragon. The rest of Draco’s fainter, lozenge-shaped head is a little farther behind. Draco always eyes Vega as they wheel around the sky.

The main stars of Vega’s own constellation, Lyra — faint by comparison — extend to its left (by 7°).

■ Before and during early dawn Saturday morning October 2nd, look below the crescent Moon by about a fist at arm’s length for Regulus, forefoot of Leo already making his early-apparition appearance, as shown below.

Leo announces spring when it’s on the rise in the early evening sky. But Leo’s arrival in the east at the beginning of dawn? That announces October.


■ During evening, look just above the northeast horizon — far below high Cassiopeia — for bright Capella on the rise. How soon Capella rises, and how high you’ll find it, depends on your latitude. The farther north you are, the sooner and higher.

■ Vega is the brightest star very high in the west, and Arcturus is getting low in the west-northwest. The brightest star in the vast expanse between them, about a third of the way from Arcturus up toward Vega, is Alphecca, magnitude 2.2 — the crown jewel of dim Corona Borealis. Alphecca is a 17-day eclipsing binary, but (like most variable stars!) its brightness dips are too slight for the eye to see reliably.

■ Before and during early dawn Sunday morning October 3rd, the waning crescent Moon forms a flat, almost isosceles triangle with Regulus and Algieba (Gamma Leonis) to Regulus’s left or upper left, as shown above (for North America).



This Week’s Planet Roundup

Mercury is hidden deep in the glare of the Sun.

Venus, brilliant at magnitude –4.2, shines low in the 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 –2.7 and +0.5, 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 soon after dark, and later they tilt the other way, with Saturn now the lower one. Saturn sets around 2 a.m. daylight-saving time, followed down by Jupiter about an hour later.

In the evening look for 1st-magnitude Fomalhaut 23° (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 videos of it taken by two 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.

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 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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