Some daily events in the changing sky for August 17 – 25.

Looking south at dusk

Watch the waxing Moon march eastward against Scorpius and Sagittarius from night to night this week. (These scenes are drawn for the middle of North America. European observers: move each Moon symbol a quarter of the way toward the one for the previous date.)

Sky & Telescope diagram

Friday, August 17

  • Look a little above the crescent Moon in late twilight to spot Spica. Much, much higher above them, and perhaps a bit to the right, shines brighter Arcturus. Both are "spring stars" (spring in the Northern Hemisphere) that are on their way to their seasonal exits from the evening sky. Spica goes first.

    Saturday, August 18

  • Venus is in inferior conjunction today, passing 8° south of the Sun.

  • Jupiter's Great Red Spot should cross Jupiter's central meridian (the imaginary line down the center of the planet's disk from pole to pole) around 11:30 p.m. Eastern Daylight Time. The "red" spot (pale orange) should be visible for at least 50 minutes before and after in a good 4- or 6-inch telescope if Jupiter is still high and the atmospheric seeing is sharp and steady. A light blue or green filter helps. (For the times of all Red Spot transits this month, visible worldwide, see the August Sky & Telescope, page 44.)

    Sunday, August 19

  • Jupiter's biggest moon, Ganymede, casts its little black shadow onto Jupiter's face this evening from 7:38 to 10:02 p.m. Eastern Daylight Time.

    Monday, August 20

  • First-quarter Moon (exact at 7:54 p.m. EDT). The Moon shines lower right of Jupiter and Antares, as shown above during twilight.

  • Jupiter's moon Io casts its tiny black shadow (tinier than Ganymede's yesterday) onto Jupiter's face from 6:53 to 9:04 p.m. EDT.

    Tuesday, August 21

  • Look for Antares near the Moon this evening with Jupiter above them, as shown above.

  • The red long-period variable stars R Ophiuchi and R and T Ursae Majoris should be at maximum light (7th or 8th magnitude) this week.

    Wednesday, August 22

  • The waxing gibbous Moon shines well left of Jupiter and Antares, as shown above.

    Thursday, August 23

  • The Moon is in the Teapot of Sagittarius this evening (evening for the Americas).

    Friday, August 24

  • Ceres, the first-discovered asteroid (magnitude 8.7), is passing 0.4° south of Xi Tauri late tonight.

    Saturday, August 25

  • Jupiter's smallest major moon, Europa, casts its very tiny black shadow onto Jupiter's face from 6:43 to 9:19 p.m. Eastern Daylight Time.

  • Make your plans now for the total eclipse of the Moon that will be visible throughout most of the Americas before or during dawn on Tuesday, August 28th. 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 foldout map in each issue of Sky & Telescope, the essential magazine of astronomy. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).

    Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of maps; the standard is Sky Atlas 2000.0) and good deep-sky guidebooks (such as Sky Atlas 2000.0 Companion or the enchanting though somewhat dated Burnham's Celestial Handbook). Read here how to use them effectively.

    More beginners' tips: "How to Start Right in Astronomy".

    This Week's Planet Roundup

    Mercury and Venus are deep in the glare of the Sun.

    Mars (magnitude +0.4, in Taurus) rises around midnight or 1 a.m. daylight saving time and shines very high in the east before dawn. It's near twinkly Aldebaran, which is similarly colored but less bright. Look above them for the Pleiades. In a telescope Mars is 8 arcseconds in diameter, half the size it will be around its Christmas-season opposition.

    Incidentally: if friends or family tell you they've read that Mars will become as big and bright as the full Moon in late August, point them to our article about the regular-as-clockwork August Mars Hoax.

    Jupiter on Aug. 2, 2007

    Jupiter is shrinking a bit as Earth pulls away from it, making its features a little harder to resolve. S&T's Sean Walker took this image on Thursday evening, August 2nd (at 1:08 Aug. 3 UT). "Seeing was better than average, but transparency was low," he says. Notice how, as Jupiter moves far from opposition, its celestial east limb is slightly shaded while sunlight strikes the western limb a little more directly.

    S&T: Sean Walker

    Jupiter (magnitude –2.3, in southern Ophiuchus) glares in the south-southwest during twilight and lower in the southwest later after dark. Get your telescope on it as early in the evening as you can. Antares, less bright, sparkles redly 5° below Jupiter; the two have been companions all summer. Other stars of Scorpius shine below them and to the right.

    Saturn is hidden in the glare of the Sun.

    Uranus (magnitude 5.7, in Aquarius) and Neptune (magnitude 7.8, in Capricornus) are well up in the southeast by late evening. Uranus is currently in the same telescopic field of view with Phi Aquarii, magnitude 4.2.

    Pluto (magnitude 13.9, in the northwestern corner of Sagittarius) is highest in the south as soon as it gets dark, about 18° east-northeast of Jupiter. Finder charts for Uranus, Neptune, and Pluto are in the July Sky & Telescope, page 60.

    All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's midnorthern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America. Eastern Daylight Time (EDT) equals Universal Time (UT, UTC, or GMT) minus 4 hours.

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