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.

Watch the Moon step eastward below Jupiter on its way to full on the 20th.


■ 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.

As the stars come out, look for 3rd-magnitude Delta Capricorni just below Jupiter. 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?

It's 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 a near-match comparison star right nearby, good luck. The system is only 39 light-years away.


■ 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.


This Week's Planet Roundup

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.

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



Image of New Jersey Eclipse Fan

New Jersey Eclipse Fan

September 17, 2021 at 6:10 pm

Whither Mars in "Planet Roundup"?

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mary beth

September 18, 2021 at 11:09 am

The Twilight Zone!!

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Anthony Barreiro

September 18, 2021 at 5:08 pm

Mars has disappeared on the far side of the Sun. Mars will be exactly on the opposite side of the Sun on October 8 (Universal Time). Mars will return to view in morning twilight by next February, near Venus.

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New Jersey Eclipse Fan

September 18, 2021 at 10:49 pm

Whew! I feel better now and will celebrate by eating a Mars bar. Thanks!

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September 20, 2021 at 5:20 pm

By December, Mars will rise about 1h 30m before the Sun at mid-northern latitudes. Binoculars should catch it easily. Late in the month the similarly colored Antares will be nearby.

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Anthony Barreiro

September 21, 2021 at 4:11 pm

Thanks. I'll look for them.

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mary beth

September 18, 2021 at 2:28 pm

Rod, Anthony, etc….see the White Flash on Jupiter on Monday? Just saw a news story about it. Thoughts?

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Anthony Barreiro

September 18, 2021 at 5:17 pm

Hi Mary Beth. I've only seen the same still images and videos everyone else has seen. I'm not aware of any reported observations by anybody who was looking at Jupiter through a telescope eyepiece. I certainly could not resolve such a small detail with my little refractor.

It is wonderful that a few very diligent and well-equipped observers were able to record this impact and share it with the world. Chance favors the prepared mind with a big telescope and a high frame rate video camera.

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September 18, 2021 at 11:23 pm

FYI all. I was out tonight looking at Jupiter at 129x using my 90-mm refractor, much detail visible but no dark spot or speck from the likely impact or any impacts seen my me 🙂 Here is my stargazing report from this evening. [Observed 2130-2230 EDT/0130-0230 UT. Full Moon 20-Sep-2021 2354/2355 UT. Bright, waxing gibbous Moon tonight. I viewed using 90-mm refractor, 14-mm eyepiece, and 1.8x Barlow lens for 129x views. True FOV ~ 33.6 arcminute. The Moon's angular size tonight ~ 31.45 arcminute according to Virtual Moon Atlas. I used Starry Night Pro Plus 8, and Stellarium 0.21.1. I also used #12 Yellow filter throughout observations. Aristarchus crater distinct on the Moon, Tycho crater, Copernicus, Archimedes, Aristillus, Autolycus, and many others visible along the terminator line. The waxing gibbous Moon was bright, even using the #12 Yellow filter. Saturn was lovely view along with Titan moon distinct. Cassini division visible, some cloud bands on Saturn, and small slice of Saturn's shadow on the rings distinct. At Jupiter, many cloud bands visible, 3 Galilean moons on one side (Io, Europa, Ganymede), one Galilean moon (Callisto) on the other side. The star 45 Cap < 10 arcminute angular separation from Jupiter's position tonight, looked almost like another moon . Stellarium 0.21.1 records the apparent magnitude +5.95, and spectral type A7 IV/V, a bit more than 176 light years distance with stellar parallax of 18.52 mas. Jupiter is about 4.142 au while I viewed tonight with a good angular size near 48 arcsecond. At 129x, the telescope view resolved to about 2.3 arcsecond so plenty of detail visible at Jupiter and Saturn, as well as the Moon. The Moon transit my location tonight at 2345 EDT/0345 UT. Jupiter transit at 2300 EDT/0300 UT, and Saturn transit at 2154 EDT/0154 UT. Jupiter and Saturn retrograde in Capricornus. The Moon in Aquarius and does not retrograde .]

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