This Week's Sky at a Glance, September 19 – 28

FRIDAY, SEPTEMBER 19

■ Titan, Saturn's largest moon, casts its shadow onto Saturn's face tonight in the second-to-last of these events for another 15 years. In times when we see Saturn's rings nearly edge on, like now, Titan repeatedly crosses in front of Saturn from Earth's viewpoint — and, more visibly, casts its tiny black shadow onto the planet.

Tonight Titan's shadow skims just inside Saturn's northern limb from 5:09 to 7:34 UT September 20th (UT date). In EDT that's tonight from 1:09 a.m. to 3:34 a.m. In PDT it's tonight from 10:09 p.m. to 12:34 a.m.

Saturn is up all night now, though it is highest in the steadiest seeing around the middle of the night local time. So all of North and Central America again get a good chance at this event. See Bob King's Last Call for a Remarkable Titan Shadow Transit.

SATURDAY, SEPTEMBER 20

■ Tonight Saturn is at opposition. So the Seeliger effect on Saturn's rings should be at its most apparent.

For a few days around opposition, Saturn's rings become noticeably brighter than usual compared to Saturn's globe. This is caused by the solid ring particles backscattering sunlight to us when the Sun is almost directly behind us. The dusty surfaces of the Moon, Mars and asteroids do something similar too, but Saturn's clouds do not. In the case of Saturn's rings the effect is named for Hugo von Seeliger, who studied it in detail and published his findings in 1887.

This year the rings are turned almost edge-on to our line of sight. And they're also almost edge-on to the incoming sunlight. Will this mess up the Seeliger effect in some way?

The rings have been fairly dark, because they are also almost edge-on to the Sun (though that angle is now gradually increasing). Will the rings' current dimness mask the Seeliger effect, or enhance it? Study their brightness compared to the globe tonight, and then again in a few days, and again in a week.

SUNDAY, SEPTEMBER 21

■ By 9 or 10 p.m. two of the best-known deep-sky objects, the Double Cluster in Perseus and the Great Andromeda Galaxy M31, are in high view in the east. They're only 22° (two fists) apart. They're both cataloged as 4th magnitude but to the naked eye they look rather different from each other, the more so the darker your sky. See for yourself using the finder chart below.

Sky too bright? Use binoculars!

The two clusters of the Double Cluster (NGC 869 and NGC 884) are at very similar distances about 7,600 light-years away. M31, at 2.5 million light-years, is 330 times farther. Fainter M34, another, smaller open cluster (magnitude 5.5), is only 1,500 light-years 0ut.

■ New Moon (at 3:54 p.m. EDT). A partial solar eclipse crosses New Zealand and parts of the South Pacific and Antarctica.

MONDAY, SEPTEMBER 22

■ Fall begins today in the Northern Hemisphere at the moment of the equinox, 2:19 p.m. EDT (11:19 a.m. PDT). This is when the center of the Sun crosses Earth's equator, and equivalently the celestial equator, in its six-month journey south from solstice to solstice.

■ 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).

■ For evening observers the annual Algol eclipse season has begun. The prototype eclipsing variable star should be at minimum brightness, magnitude 3.4 instead of its usual 2.1, for about two hours centered on 11:01 p.m. EDT (8:01 p.m. PDT). Algol takes several additional hours to fade and to rebrighten.

Find it with the photo-realistic chart above. And for judging its brightness, use this comparison-star chart, which has north up. (In the sky, celestial north is always the direction toward Polaris. When you're outside at night using charts with north up, turn them around to match this direction.)

At any random time you glance up at Algol, you have a 1-in-30 chance of catching it at least 1 magnitude fainter than normal. See Bob King's When Algol Winks, Will You Wink Back?

TUESDAY, SEPTEMBER 23

■ Arcturus shines in the west these evenings as twilight fades out. Capella, equally bright, is barely 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 the same height 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 about the same height too — exactly so if you're at latitude 43° north (from Boston to Buffalo, Milwaukee, Boise, Eugene). Seen from south of that latitude, Fomalhaut will appear higher than Capella and Arcturus. Seen from north of there, it will be lower.

WEDNESDAY, SEPTEMBER 24

■ The asteroid 1 Ceres, the largest and first discovered, is nearing its October 2nd opposition. It's in binocular range at magnitude 7.6 this week and next, near Eta Ceti. Use the finder chart in the October Sky & Telescope, page 50. The tick marks on its path there are for 0:00 UT on the dates indicated, which falls on the evening of the previous date for the Americas.

THURSDAY, SEPTEMBER 25

■ Arcturus shines ever lower in the west-northwest after dark. The narrow kite shape of its constellation, Boötes, 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 Boötes, 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.

FRIDAY, SEPTEMBER 26

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

Face southwest and crane your head way, way up, and 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 southwest the Swan appears to be flying downward along the Milky Way.

SATURDAY, SEPTEMBER 27

■ At nightfall, the waxing crescent Moon shines about 3° or 4° left of orange Antares in upper Scorpius as shown below. The whole stick-figure Scorpion is tilting and lying down on its way out for the season.

SUNDAY, SEPTEMBER 28

■ Now the Moon shines above the Cat's Eyes pair of stars in the tail of Scorpius, shown above. The pair is a little less than a fist at arm's length below the Moon.

This Week's Planet Roundup

Mercury remains hidden deep in the glow of sunset.

Venus rises about a half hour before the very beginning of dawn. Watch for it to come up a little to the left of due east, far below bright Jupiter and somewhat to the left.

After Venus is up but before the sky gets too bright, you'll spot Regulus, much fainter, several degrees above it. They get farther apart all week.

Mars, a weak magnitude 1.6 in Virgo, still may be catchable with binoculars very low in the west-northwest as twilight fades. Try about 30 or 40 minutes after sunset. Good luck.

Mars is very slowly nearing the end of its long 2024-2025 apparition. It won't actually reach conjunction with the Sun until the start of 2026!

Jupiter, magnitude –2.1 in Gemini, rises around 1 a.m. daylight-saving time and dominates the east as the morning hours advance. Castor and Pollux shine to Jupiter's left or upper left. By the beginning of dawn the three stand very high in the east, with Orion off to their right and brighter Venus coming into view far below them and a bit left.

Saturn was at opposition on September 2oth. This week it still rises around sunset and looms low in the east-southeast as the stars come out, glowing at magnitude +0.7. It's on the border of Pisces and Aquarius, lower right of the Great Square of Pegasus (which is standing on one corner).

Saturn climbs higher through the evening. The best time to observe it with a telescope is late evening through midnight when it's good and high toward the southeast and south.

Uranus (magnitude 5.7, in Taurus near the Pleiades) rises around 9 or 10 p.m. and gets high by midnight. At high power in a telescope it's a tiny but definitely non-stellar dot, 3.6 arcseconds wide.

Neptune is at opposition this week, but it's so far from Earth that this makes no difference. Neptune is still a telescopic "star" of magnitude 7.8 and at very high power it's still just 2.4 arcseconds wide. Use the finder chart for Neptune with respect to Saturn in the September Sky & Telescope, page 49. With a pencil, put a dot on the path of each of the two planets for your date.

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 and graphics that also depend on longitude (mainly Moon positions) are for North America. Eastern Daylight Time (EDT) is Universal Time minus 4 hours. UT is also known as UTC, GMT, or Z time.

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 a more detailed 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.

For the attitude every amateur astronomer needs, read Jennifer Willis's Modest Expectations Give Rise to Delight.

Once you get a telescope, to put it to good use you'll need a much more detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas, in either the original or Jumbo Edition. Both show all 30,000 stars to magnitude 7.6, and 1,500 deep-sky targets — star clusters, nebulae, and galaxies — to search out among them.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many, as well as many more deep-sky objects. It's currently out of print, but maybe you can find one used.

The next up, once you know your way around well, are the even larger Interstellarum atlas (with 201,000+ stars to magnitude 9.5 and 14,000 deep-sky objects selected to be detectable by eye in large amateur telescopes), and Uranometria 2000.0 (332,000 stars to mag 9.75, and 10,300 deep-sky objects). And read How to Use a Star Chart with a Telescope. It applies just as much to charts on your phone or tablet (which many observers find more versatile) as to charts on paper.

You'll also want a good deep-sky guidebook. A beloved old classic is the three-volume Burnham's Celestial Handbook. An impressive more modern one is the big Night Sky Observer's Guide set (2+ volumes) by Kepple and Sanner. The pinnacle for total astro-geeks is the new Annals of the Deep Sky series, currently at 11 volumes as it works its way forward through the constellations alphabetically. So far it's up to H.

Can computerized telescopes replace charts? Not for beginners I don't think, and not for scopes on mounts and tripods that are less than top-quality mechanically. Unless, that is, you prefer spending your time getting finicky technology to work rather than learning how to explore the sky. 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 and a curious mind." Without these, "the sky never becomes a friendly place."

If you do get a computerized scope, make sure that its drives can be disengaged so you can swing it around and point it readily by hand when you want to, rather than only slowly by the electric motors (which eat batteries).

However, finding faint telescopic objects the old-fashioned way with charts isn't simple either. Do learn the essential tricks at How to Use a Star Chart with a Telescope.

Audio sky tour. Out under the evening sky with your
earbuds in place, listen to Kelly Beatty's monthly
podcast tour of the naked-eye 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; and whatever may be our wishes, our inclinations, or the dictates of our passion, they cannot alter the state of facts and evidence."
            — John Adams, 1770