This Week’s Sky at a Glance, September 8 – 16

This Week’s Sky at a Glance, September 8 – 16


Venus, Mercury, Mars low at dawn, Sept. 9, 2017

Mercury pairs up with Regulus this morning and the next. The visibility of faint objects in bright twilight is exaggerated here.

Moon and Aldebaran Sept. 11 - 13, 2017, with occultation

On the morning of Tuesday the 12th, the last-quarter Moon occults Aldebaran before sunrise for parts of western North America, and after sunrise in eastern and central parts of the continent. (The Moon in these scenes is always drawn three times its actual apparent size.)

Venus, Regulus, Mercury, Mars in the dawn, Sept. 16, 2017

By the Saturday morning the 16th, brightening Mercury and still-faint Mars are almost kissing-close low in the east before dawn . . .

Venus, Regulus, Mercury, Mars in the dawn, Sept. 17, 2017

. . . and again the next morning. The Moon and Venus point the way!

Friday, September 8

• A dawn challenge: Very low in the east as dawn brightens on Saturday morning September 9th, Mercury, now a respectable magnitude 0, glows 1° to the right of Regulus, magnitude 1.3, under bright Venus as shown here. Mars, fainter at magnitude 1.8, is about 3° farther lower left. Bring binoculars.

Saturday, September 9

• The changes deep in the eastern dawn continue. On Sunday morning the 10th, Mercury glows 1° or less to the lower right of Regulus, while Mars remains below.

Sunday, September 10

• As dusk turns to night, Arcturus twinkles due west. It's getting lower every week. Off to its right in the northwest, the Big Dipper is turning more and more level.

Monday, September 11

• Tomorrow morning the 12th, the last-quarter Moon approaches Aldebaran and then occults it for much of North America: before or during dawn in the West, and after sunrise in a (hopefully) blue sky farther east. The star disappears on the Moon's bright limb (as shown here) and reappears from behind the dark limb. See the September Sky & Telescope, page 50. Map and timetables.

Tuesday, September 12

• The Great Square of Pegasus is high in the east after dark, balancing on one corner. From the Square's left corner extends a big line of three 2nd-magnitude stars running to the lower left. They mark the head, backbone and leg of the constellation Andromeda. (The line of three includes the Square's corner.) Upper left from the end of this line, you'll find W-shaped Cassiopeia tilting up.

Wednesday, September 13

• Vega now passes the zenith an hour after sunset, in late twilight, for those of us at mid-northern latitudes. Vega is bigger, hotter, and more luminous than our Sun, but at 25 light-years it's 1,600,000 times farther away.

Thursday, September 14

• Saturn is at eastern quadrature: 90° east of the Sun in the evening sky. So this month, telescope users see the shadow of Saturn's globe falling farthest eastward onto the rings behind it, enhancing Saturn's overall 3-D appearance.

Friday, September 15

• This evening Saturn's biggest and brightest moon, Titan, stands about four ring-lengths to Saturn's east. A 4-inch telescope will begin to show the orange color of its smoggy atmosphere.

• Before sunrise on Sunday morning the 16th, Mercury is only about 0.3° from dimmer Mars. Bring optical aid to scan very low in the east, lower left of Venus, as shown here.

Saturday, September 16

• On Sunday morning the 17th, The Moon and Venus point diagonally down toward Mercury and Mars, which appear nearly as close together as they did on Saturday morning.


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

Saturn on June 12, 2017, by Damian Peach and colleagues at Pic du Midi Observatory.

Saturn on June 12, 2017, captured by the crack June 2017 Pic du Midi planetary imaging team using the observatory's historic 1-meter f/17 reflector. Processed by Damian Peach. North is up.

Uranus from Pic du Midi June 12, 2017

The team also captured Uranus the same night, with white in its southern hemisphere. North is up.

Mercury is having a good apparition low in the dawn. Its brightness more than doubles this week, from magnitude +0.2 to –0.8. On the morning of September 10th Mercury is closely passing Regulus, which is much fainter at magnitude +1.4. On the 16th it's going even closer by Mars, magnitude +1.8. Look for this action down to the lower left of brilliant Venus. Bring binoculars.

Venus (magnitude –3.9, in Leo above Mercury and Mars) shines brightly in the east before and during dawn.

Jupiter (magnitude –1.7) is just above the west-southwest horizon after sunset. Look early with binoculars; it sets before twilight is over.

Saturn (magnitude +0.4, in Ophiuchus above Scorpius) glows in the south-southwest at dusk. Antares, less bright, twinkles 13° to Saturn's lower right.

Uranus (magnitude 5.7, in Pisces) and Neptune (magnitude 7.8, in Aquarius) are well up in the east and southeast, respectively, by late evening. It likely rains diamonds inside them. Use our finder charts.


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. Carbon dioxide warms the globe. Vaccines save lives. Bacteria evolve when challenged by antibiotics. Science and critical thinking are not a political conspiracy. They are how we discover reality. Civilization's survival depends on our ability, and willingness, to do so."
— Alan MacRobert, your Sky at a Glance editor


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

The post This Week’s Sky at a Glance, September 8 – 16 appeared first on Sky & Telescope.