Some daily events in the changing sky for July 18 – 26.

If you're in Europe or Asia, get ready for the eclipse of the Sun coming up on August 1st! It's at least partial over most of both continents. Skywatchers in easternmost Canada also have a partial view right at sunrise. See our article.

Facing west in early twilight

As summer progresses, Mars, Saturn and Regulus draw apart and sink lower in the twilight. Meanwhile, Venus is gradually creeping up into view just above the sunset horizon.

Sky & Telescope diagram

Friday, July 18

  • Some doorstep astronomy: This is the time of year when Arcturus shines in the west, and the Big Dipper poses in the northwest, equally high after dark (for skywatchers at mid-northern latitudes). If you look for Alcor, the tiny companion of Mizar (the middle star of the Big Dipper's handle), remember that a line from Mizar through Alcor points to bright Vega, the brightest star nearly overhead toward the east.

    Saturday, July 19

  • 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 10:02 p.m. Eastern Daylight Time. The "red" spot appears very pale orange-tan. It should be visible for at least 50 minutes before and after in a good 4- or 6-inch telescope if the atmospheric seeing is sharp and steady. A light blue or green filter helps. The Red Spot transits about every 10 hours 56 minutes, with slight changes as Earth's viewing angle changes. For all its transit times, good worldwide, see our listing or applet online.

    Sunday, July 20

  • Stay up to about 11 p.m. this week, and you'll find the Great Square of Pegasus looming up in the east — an early foreshadowing of fall. It's a bit bigger than your fist at arm's length, and it's balancing on one corner.

    Monday, July 21

  • Jupiter's Great Red Spot transits around 11:40 p.m. Eastern Daylight Time.

    Tuesday, July 22

  • Jupiter's moon Io disappears behind Jupiter's western limb at 10:05 p.m. EDT. Then it reappears out of eclipse from Jupiter's shadow, just east of the planet, around 12:41 a.m. Wednesday morning EDT.

    For lots more about observing Jupiter's moons, and a listing of all events happening among them this month, see the July Sky & Telescope, page 62.

    Wednesday, July 23

  • Jupiter's Great Red Spot transits around 10:18 p.m. Pacific Daylight Time; 1:18 a.m. Thursday morning Eastern Daylight Time.

    Thursday, July 24

  • The modest but long-lasting Delta Aquarid meteor shower is strongest this week. It's best seen from southerly latitudes and before the first light of dawn.

    Friday, July 25

  • Last-quarter Moon (exact at 2:42 p.m. Eastern Daylight Time).

    Saturday, July 26

  • Jupiter's Great Red Spot transits around 10:48 p.m. Eastern Daylight Time.

    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 standards are Sky Atlas 2000.0 or the smaller Pocket Sky Atlas) and good deep-sky guidebooks (such as Sky Atlas 2000.0 Companion by Strong and Sinnott, the even more detailed Night Sky Observer's Guide by Kepple and Sanner, or the classic Burnham's Celestial Handbook). Read how to use them effectively.

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

    This Week's Planet Roundup

    Mercury (about magnitude –1.5) sinking deep into the glow of dawn. Early in the week, use binoculars to look for it above the east-northeast horizon 20 or 30 minutes before sunrise.

    (To find your local sunrise time, and much else, make sure you've put your location and time zone into our online almanac. If you're on daylight saving time like most of North America, make sure the Daylight Saving Time box is checked.)

    Venus (magnitude –3.9) is still very deep in the glow of sunset. Using binoculars, look for it just above the west-northwest horizon 20 to 40 minutes after sundown, as shown at the top of this page. Venus is making its way up very slowly toward a grand "Evening Star" showing in late fall and winter.

    Mars and Saturn (magnitudes +1.7 and +0.8) are sinking lower in evening twilight, as shown at the top of this page. They're moving apart now; on July 18th they're separated by 4°, but on the 26th by 8°. Twinkly little Regulus (magnitude +1.4) remains 7° to Saturn's lower right. Look early before they all sink too low and set!

    Lots is happening on Jupiter! In this stacked-video image taken July 23rd, Christopher Go caught the pair of small, eye-like dark spots in the north edge of the North Equatorial Belt (NEB, the dark red-brown belt just below center). Note the busy diagonal detail in the South Equatorial Belt, just above center. The Great Red Spot was on the far side of the planet at the time of this picture; it's currently near System II longitude 127°. South is up, to match the south-up view in many telescopes.

    Christopher Go

    Jupiter (magnitude –2.7, in Sagittarius) shines with a steady glare in the southeast at nightfall. It's left of the Sagittarius Teapot and just below the bowl of the smaller, dimmer Teaspoon. Jupiter now gets its highest in the south by midnight daylight saving time. For high-resolution scopes, Jupiter's Great Red Spot and its little red companions have been putting on quite an interesting performance; see press release from the Hubble Space Telescope site.

    Uranus and Neptune (magnitudes 6 and 8, respectively, in Aquarius and Capricornus) are high in the south before the first light of dawn. Use our article and finder charts.

    Pluto (magnitude 14.0, in the northwestern corner of Sagittarius) is highest in the south in late evening. If you've got a big scope and a dark sky, use our article and finder chart. (Moonlight will be gone by the end of this week.)

    All descriptions that relate to your horizon or zenith — 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 (UT, UTC, or GMT) minus 4 hours.

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    • About Alan MacRobert

    Alan M. MacRobert became an avid Sky & Telescope subscriber in 1966 at age 14, joined the editorial staff in 1982, and is now a senior contributing editor, semi-retired. He played a role in practically every part of the magazine and the company's other products for more than a generation, both on the amateur-observing side and the science-reporting side. In 1994 a book collection of his observing how-tos and telescopic sky tours was published as Star Hopping for Backyard Astronomers. He has produced This Week's Sky at a Glance online every week since 1989.

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