This Week’s Sky at a Glance

All week, watch the changing triangle of Saturn, Spica, and Mars low in late twilight.
Alan MacRobert

Friday, August 3

  • Watch as the Saturn-Spica-Mars triangle, low in the west-southwest in twilight, morphs from one evening to the next.

  • As summer nears its second half (the dividing point comes this Monday), the big Summer Triangle approaches its greatest height in the evening: Face east after nightfall and look almost straight up; the brightest star there is Vega. Northeast from Vega (by two or three fist-widths at arm's length) is Deneb. Farther southeast of Vega is Altair.

    Saturday, August 4

  • The waning gibbous Moon rises in the east as twilight fades away. Look well to the Moon's left or upper left for the Great Square of Pegasus, balancing on one corner.

    Sunday, August 5

  • Vega is near the zenith soon after nightfall (as seen from mid-northern latitudes), and this tells you that the rich Milky Way in Sagittarius and the tail of Scorpius is now at its best low in the south. The region is loaded with telescopic star clusters and nebulae. Catch it while you can.

    Monday, August 6

  • By mid-evening this week, W-shaped Cassiopeia rises as high in the north-northeast as the bowl of the Big Dipper has sunk in the north-northwest.

    Tuesday, August 7

  • This evening the changing triangle of Saturn, Spica, and Mars, low in the west-southwest as twilight fades, is almost equilateral: about 4½° on a side.

    Wednesday, August 8

  • The two brightest stars of summer are Vega, nearly overhead after dark (for skywatchers at mid-northern latitudes), and Arcturus in the west. Look a third of the way from Arcturus to Vega for the dim semicircle of Corona Borealis, the Northern Crown. Look two thirds of the way for the dim Keystone of Hercules, about the same size.

    Mars is closing in on Saturn and Spica....

    Thursday, August 9

  • Last-quarter Moon (exact at 2:55 p.m. EDT). The lopsided-looking Moon rises around midnight with the Pleiades to its left.

    Friday, August 10

  • The waning Moon forms a tight triangle with Jupiter and fainter Aldebaran after they rise about 1 or 2 a.m. Saturday morning. Bright Venus rises far to their lower left around 3 a.m. (depending on where you live).

    Saturday, August 11

  • The Perseid meteor shower should be at its best late tonight. Find a dark spot with a wide-open view of the sky overhead, bundle up against the late-night chill, lie back in a lounge chair, watch the sky and be patient. After 11 or midnight you may see a meteor a minute on average; fewer earlier.

    The thick waning crescent Moon rises by 1 or 2 a.m. (with Jupiter above it). But its modest light, notes the International Meteor Organization, "should be considered more of a nuisance than a deterrent."

    You're also likely to see occasional Perseids for many nights before and after. See the August Sky & Telescope, page 50.


    Want to become a better amateur astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

    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. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).

    The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but that's less than one star in an entire telescopic field of view, on average. By comparison, Sky Atlas 2000.0 plots 81,312 stars to magnitude 8.5, typically one or two stars per telescopic field. Both atlases include many hundreds of deep-sky targets — galaxies, star clusters, and nebulae — to hunt among the stars.
    Sky & Telescope

    Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The standards are the little Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger and deeper Sky Atlas 2000.0 (stars to magnitude 8.5); and the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts effectively.

    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, the bigger Night Sky Observer's Guide by Kepple and Sanner, or the classic if dated Burnham's Celestial Handbook.

    Can a computerized telescope replace charts? I don't think so — not for beginners, anyway, and especially not on mounts and tripods that are less than top-quality mechanically (able to point with better than 0.2° repeatability). As Terence Dickinson and Alan Dyer say in their invaluable 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

    Jupiter on August 8th. The brownish North Equatorial Belt (below center) remains extremely broad. Really, what we're seeing is a combination of the NEB and the North Temperate Belt. Above center, the South Equatorial Belt is disturbed by the whitish wake of the Great Red Spot. The GRS itself now appears paler orange than Oval BA ("Red Spot Junior") just to its upper right here. Oval BA is catching up to the GRS and should pass it in coming weeks, writes imager Christopher Go, perhaps with interesting interactions.

    Mercury is still deep in the glow of sunrise.

    Venus and Jupiter (magnitudes –4.5 and –2.2) shine dramatically in the east before and during dawn. They've widened to about 18° apart now; Jupiter is the higher one. Look for orange Aldebaran, much fainter, 5° lower right of Jupiter. Also in Jupiter's area are the Hyades, and above are the Pleiades.

    The asteroids Ceres and Vesta, magnitudes 9.1 and 8.4, are in Jupiter's vicinity too! See article and finder charts: Ceres and Vesta, July 2012 – April 2013.

    Mars and Saturn (magnitudes +1.1 and +0.8, respectively) are low in the west-southwest at dusk, forming a triangle with similarly bright (but twinklier) Spica. Saturn and Spica are 4½° apart, with Saturn on top. Mars, to Spica's right, is approaching closer to them every day. It will pass between them on August 13th and 14th.

    Uranus (magnitude 5.8, at the Pisces-Cetus border) and Neptune (magnitude 7.8, in Aquarius) reach good heights in the southeast by midnight daylight-saving time. Finder charts for Uranus and Neptune.


    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) equals Universal Time (also known as UT, UTC, or GMT) minus 4 hours.


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