This Week’s Sky at a Glance, October 13 – 21


Moon, Regulus, Mars and Venus at dawn, Oct. 15-18, 2017

The waning crescent Moon occults Regulus for most of the US on the morning of the 15th (see the entry for the 14th at left). Then the Moon descends day by day toward Mars and Venus. These scenes are drawn exact for the middle of North America. (In Europe, move each Moon symbol a quarter of the way toward the one for the previous date.) For clarity, the Moon is always shown three times its actual apparent size.

Friday, October 13

• Now that we're in mid-October, Deneb has replaced Vega as the zenith star soon after nightfall (for skywatchers at mid-northern latitudes). So, accordingly, Capricornus has replaced Sagittarius low in the south.

Saturday, October 14

• Early on Sunday morning the 15th, the bright limb of the waning crescent Moon occults Regulus for telescope users in much of North America. The disappearance happens during dawn for the East and earlier in darkness farther west. The West Coast misses the disappearance; the Moon won't have risen yet.

But then, up to an hour or more later, Regulus reappears from behind the Moon's dark limb. This will be a naked-eye event for much of the USA and southeastern Canada! The reappearance will be "arguably the best lunar occultation of the year for the US, with the Moon only 20% sunlit," writes David Dunham of the International Occultation Timing Association (IOTA). "And for those with telescopes of about 10 inches or larger, it might provide a fleeting opportunity to see Regulus’s elusive, 12th-magnitude white-dwarf companion, discovered in 2005, just before the blazing primary emerges. The only other time that the companion was imaged was during an occultation of Regulus by the asteroid 268 Adorea that Joan and I recorded with a 10-inch 'suitcase' telescope from Papua New Guinea on Oct. 13, 2016."

Here are a map and detailed timetables of the disappearance and reappearance for many locations, including the altitudes of the Moon and Sun at those times. (The page consists of three long tables, not very clearly divided: first the disappearance, then the reappearance, then the locations of cities.)

To record the grazing occultation along the northern limit, writes Dunham, "I believe the best place will be Bemidji, MN, but other places, such as Billings, MT, and Bismarck, ND, will be good too, as well as rural parts of Ontario and Quebec." Details of the graze.

Sunday, October 15

• Vega is the brightest star in the west these evenings. Less high in the southwest is Altair, not quite as bright. Just upper right of Altair, by a finger-width at arm's length, is little orange Tarazed (Gamma Aquilae). Straight down from Tarazed runs the stick-figure backbone of Aquila, the Eagle.

• Algol is at minimum brightness in eclipse, magnitude 3.4 instead of its usual 2.1, for about two hours tonight centered on 1:25 a.m. EDT (10:25 p.m. PDT). Algol takes several additional hours to fade and to rebrighten.

Monday, October 16

• The Great Square of Pegasus is now high in the east-southeast after dark, still, for now, balanced on one corner for the world's mid-northern latitudes.

• As dawn brightens on Tuesday morning the 17th, a super-thin waning crescent Moon hangs above Venus and left or lower left of faint Mars, as shown above. Look very low in the east. Binoculars will help.

Tuesday, October 17

• Draw a line from Altair, the brightest star high in the southwest after dark, to the right to Vega, high in the west and even brighter. Continue the line half as far onward, and you hit the Lozenge: the pointy-nosed head of Draco, the Dragon, with orange Eltanin as the tip of his nose.

Wednesday, October 18

• Algol is at minimum light this evening for a couple hours centered on 10:14 p.m. EDT.

Thursday, October 19

• The modest Orionid meteor shower is active for the next few nights in the early-morning hours. The shower's radiant is near Orion's Club, low in the east after midnight and high in the south by the beginning of dawn. The sky will be free of moonlight.

• New Moon (exact at 3:12 p.m. EDT).

Friday, October 20

• Look for Capella sparkling low in the northeast this week. Look for the Pleiades cluster about three fists at arm's length to its right. These harbingers of the cold months to come rise higher as evening grows late.

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

Saturday, October 21

• This is the time of year when the Big Dipper lies down horizontal low in the north-northwest after dark. How low? The farther south you are, the lower. Seen from 40° north (New York, Denver) its 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.


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 guide to astronomy.

Pocket Sky Atlas, 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 above is the Jumbo Edition for easier reading in the night. Larger view. Sample chart.

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, is the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, or the bigger 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."

This Week's Planet Roundup

Mercury and Jupiter are buried deep in the sunset.

Venus (magnitude –3.9) rises around the beginning of dawn and shines very low due east as dawn brightens. Above it is faint Mars, magnitude +1.8, only 1/200 as bright. They're drawing farther apart every day; on the morning of Saturday the 14th they're 5° apart, and by the 21st they're separated by 10°. Venus is getting lower, Mars higher.

Saturn (magnitude +0.5, in Ophiuchus to the right of the Sagittarius Teapot) glows low in the southwest at dusk. It soon sinks away and sets.

Uranus (magnitude 5.7, in Pisces) and Neptune (magnitude 7.8, in Aquarius) are up after dark in the east and southeast, respectively. Use our finder charts online or in the October Sky & Telescope, page 50.


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 (UT, UTC, GMT, or Z time) minus 4 hours.


"This adventure is made possible by generations of searchers strictly adhering to a simple set of rules. Test ideas by experiments and observations. Build on those ideas that pass the test. Reject the ones that fail. Follow the evidence wherever it leads, and question everything. Accept these terms, and the cosmos is yours."
— Neil deGrasse Tyson, 2014


"Objective reality exists. Facts are often determinable. Vaccines save lives. Carbon dioxide warms the globe. Bacteria evolve to thwart antibiotics, because evolution. Science and reason are not fake, and not a political conspiracy. They are how we discover reality. Civilization's survival depends on our ability, and willingness, to use them."
— Alan MacRobert, your Sky at a Glance editor


"Facts are stubborn things."
— John Adams, 1770


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