This Week's Sky at a Glance, October 20 – 29

FRIDAY, OCTOBER 20

■ The Moon, a day short of first quarter, shines low in the south at the end of twilight. Before the Moon gets any lower, binoculars will help show that it's sitting almost smack on the four-star handle of the Sagittarius Teapot.

SATURDAY, OCTOBER 21

■ First-quarter Moon (exact at 11:29 p.m. EDT). Now the Moon, in the dim area between Sagittarius and Capricornus, is almost smack on top of Herman's Cross, a.k.a. The Dogs: the little four-star asterism described and mapped in the September Sky & Telescope, page 43. Its stars are all about magnitude 4½ , quite a bit fainter than the four of the Teapot's handle. Definitely use binoculars. Herman's Cross is 2° long from end to end.

■ The shadow of Jupiter's fast-moving moon Io crosses onto the eastern edge of Jupiter's face at 8:07 p.m. EDT, followed closely by Io itself 20 minutes later.

Io's shadow leaves Jupiter's face at 10:17 p.m. EDT, followed by Io 19 minutes later.

Jupiter's Red Spot should transit the planet's central meridian later in the night, around 12:30 a.m. EDT. Convert these times to your own time zone.

■ The Orionid meteor shower should be near its peak late tonight. The good Orionid-watching hours are from about 1 a.m. to the first light of dawn. The Moon will have set. This shower is a modest one; in a very dark sky you might see 8 or 10 meteors per hour. The shower's radiant is in the east at Orion's dim club, between Betelgeuse and the feet of Gemini. See Bob King's Orionid Meteors Fly; Two Moons Shadow Jupiter.

SUNDAY, OCTOBER 22

■ This is the time of year when the Big Dipper lies down horizontal low in the north-northwest after dark. The farther south you are, the lower. Seen from 40° north (New York, Denver) even the Dipper's bottom stars twinkle nearly ten degrees high. But at Miami (26° N) the entire Dipper skims along out of sight just below the northern horizon.

MONDAY, OCTOBER 23

■ The Moon, a little past first quarter, is at one of its most entrancing telescopic phases: the sunrise terminator crosses Mare Imbrium with its dramatic contrasts of flat plain and shadow-casting mountains and crater walls. And this evening and tomorrow evening the Moon accompanies Saturn with its dramatic rings and pinpoint telescopic moons.

TUESDAY, OCTOBER 24

■ Look for bright Capella sparkling low in the northeast these evenings. Look for the Pleiades cluster about three fists at arm's length to its right. These harbingers of the cold months rise higher as evening grows late. Watch for Aldebaran to come up below the Pleiades.

Upper right of Capella, and upper left of the Pleiades, the stars of Perseus lie astride the Milky Way.

WEDNESDAY, OCTOBER 25

■ The Ghost of Summer Suns. Halloween is approaching, and this means that Arcturus, the star sparkling low in the west-northwest in twilight, has become "the Ghost of Summer Suns." What does this mean? For several days centered on October 25th every year, Arcturus occupies a special place above your local landscape. It closely marks the spot where the Sun stood at the same time, by the clock, during hot June and July — in broad daylight, of course!

So at Halloween season every year, you can see Arcturus as the chilly ghost of the departed summer Sun.

■ With a telescope, watch Europa slowly disappear into Jupiter's shadow just off Jupiter's western edge tonight around 11:35 p.m. EDT; 8:35 p.m. PDT.

THURSDAY, OCTOBER 26

■ By mid-evening the Great Square of Pegasus is getting very high in the southeast — and it's tilting clockwise off of one corner to lie more level like a square. Autumn advances.

The Great Square's right-hand edge points far down toward Fomalhaut, by about four fists at arm's length. Its left edge points less far down, and less directly, to Beta Ceti (Diphda or Deneb Kaitos). Fomalhaut is 1st magnitude, Beta Ceti is 2nd mag.

FRIDAY, OCTOBER 27

■ The Summer Triangle Effect. Here it is nearly the end of October, but Deneb still shines near the zenith as the stars come out. And brighter Vega is still not far from the zenith, toward the west. And the third star of the "Summer" Triangle, Altair, remains very high in the southwest. They seem to have been there for a couple months! Why have they stalled out?

What you're seeing is the result of sunset and darkness arriving earlier and earlier during autumn. Which means if you go out and starwatch soon after dark, you're looking earlier and earlier by the clock. This counteracts the seasonal westward turning of the constellations.

Of course the "Summer Triangle effect" applies to the entire celestial sphere, not just the Summer Triangle. But the apparent stalling of that bright landmark inspired Sky & Telescope to give the effect that name many years ago, and it stuck.

Of course, as always in celestial mechanics, a deficit somewhere gets made up elsewhere. The opposite effect makes the seasonal advance of the constellations seem to speed up in early spring. The spring-sky landmarks of Virgo and Corvus seem to dash away westward from week to week almost before you know it, due to darkness falling later and later. Let's call this the "Corvus effect."

SATURDAY, OCTOBER 28

■ Full Moon (exact at 4:24 p.m. EDT). The Moon rises at sunset. As twilight deepens and the Moon climbs higher, there will be Jupiter shining just 3° or so below or lower left of it (for North America). Jupiter is currently 1,500 times farther away than the Moon — and it's 40 times larger in diameter!

Watch them appear to draw closer together through the evening. Jupiter will pass 2° below the brilliant Moon in the early morning hours.

It's no surprise that they appear in the same spot of the sky. Jupiter and the full Moon are both very close to opposition: in the opposite direction from the Sun as seen from Earth's point of view.

■ A very slight partial lunar eclipse occurs while the Moon is visible from Europe, Africa, and Asia. The partial eclipse will be deepest at 20:14 Universal Time October 28, when just 13% of the Moon's diameter will be shaded in the umbra of Earth's shadow. The entire partial eclipse runs from 19:34 to 20:54 UT. Slight penumbral shading will be visible on the Moon's celestial southeast to south side for roughly 45 minutes before and after partial eclipse.

SUNDAY, OCTOBER 29

■ Around the time when twilight fades out, zero-magnitude Capella has risen exactly as high in the northeast as zero-magnitude Vega has sunk in the west-northwest. How accurately can you time this event? Astrolabe not required. . . but it might help.

■ Algol in Perseus should be at its minimum brightness late tonight, magnitude 3.4 instead of its usual 2.1, for about two hours centered on 1:23 a.m. Eastern Daylight Time, 10:23 p.m. Pacific. Algol takes several additional hours to fade and to rebrighten.

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This Week's Planet Roundup

Mercury is out of sight in conjunction with the Sun.

Venus, brilliant at magnitude –4.5 in Leo, shines high in the east before and during dawn. It rises nearly 2½ hours before dawn's first light — a weird late-night light appearing on the eastern horizon.

In a telescope Venus, shrinking, is barely beginning to turn gibbous.

Mars is out of sight behind the glare of the Sun.

Jupiter (magnitude –2.9, in Aries) is nearly at its November 2nd opposition. It rises in twilight and dominates the east during the evening. It's highest in the south around 1 a.m. daylight-saving time. Dawn finds it low in the west.

Jupiter is now at its brightest and telescopically largest: 49 or 50 arcseconds wide!

Saturn (magnitude +0.7, in dim Aquarius) glows a steady pale yellowish in the southeast as twilight fades out. Fomalhaut twinkles nearly two fists at arm's length below it. Saturn is high in fine telescopic viewing position through the evening.

Uranus, magnitude 5.6 in Aries, is 10° east of Jupiter.

Neptune, magnitude 7.8 at the Aquarius-Pisces border, is 25° east of Saturn.

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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.

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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.

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), which shows all 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 mag 9.75). And be sure to read How to Use a Star Chart with a Telescope. It applies just as much to charts on your phone or tablet 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.

Can computerized telescopes replace charts? Not for beginners I don't think, especially not 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 the sky. 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."

But finding a faint telescopic object the old-fashioned way with charts isn't simple either. Learn the tricks at How to Use a Star Chart with a Telescope. 

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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.

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"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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