Some daily events in the changing sky for July 20 – 28.

Looking south in evening twilight

Watch the bright Moon pass Jupiter and Scorpius around midweek. (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. For clarity, the Moon is shown three times actual size.)

Sky & Telescope diagram

Friday, July 20

  • Mercury is at greatest elongation, 20° west of the Sun in the dawn.

  • Jupiter's moon Io reappears out of eclipse from Jupiter's shadow around 9:36 p.m. EDT. A small telescope will show it gradually swelling into view just off the planet's eastern limb.

  • 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 12:27 a.m. Saturday morning Eastern Daylight Time (9:27 p.m. Friday evening Pacific Daylight Time). It should be visible for at least 60 minutes before and after in a good 4-inch telescope if 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 July Sky & Telescope, page 52.)

  • Have you ever monitored an eclipsing binary star all the way through eclipse? Tonight, if you're in North America, you can track SZ Herculis high overhead dipping from magnitude 10.5 to 12 and back. Make a timing! See the article, chart, and timetable in the July Sky & Telescope, page 58.

    Saturday, July 21

  • It's the height of summer (for us in the Northern Hemisphere), so the big Summer Triangle shines very high in the east at nightfall. Its brightest star is Vega, nearly overhead. Its others are Deneb, the brightest star two or three fist-widths at arm's length to Vega's lower left when you face east — and Altair, a greater distance to Vega's lower right.

  • First-quarter Moon tonight (exact at 2:29 a.m. Sunday morning EDT).

  • Jupiter's Red Spot transits around 8:19 p.m. EDT.

    Sunday, July 22

  • By 11 p.m. this week the Great Square of Pegasus is up in the east, a forewarning of the approach of fall.

    Monday, July 23

  • The weak Southern Delta Aquarid meteor shower is most active for about a week starting around now.

  • Jupiter's Red Spot transits around 9:57 p.m. EDT.

    Tuesday, July 24

  • The Moon shines in the head of Scorpius near Jupiter and Antares, as shown below.

  • Tonight you may have another shot at timing an eclipse of SZ Herculis. See July 20 above (last item).

    Wednesday, July 25

  • This evening the Moon is on the other side of Jupiter and Antares, as shown below.

    Thursday, July 26

  • The bright Moon shines just off the spout of the Sagittarius Teapot this evening for skywatchers in the longitudes of the Americas. That means the Moon is near M8, the Lagoon Nebula in Sagittarius, which is normally an easy sight in even small binoculars. How much, if any, of the nebula can you still see through the moonlight?

    Friday, July 27

  • Jupiter's moon Io reappears out of eclipse from Jupiter's shadow around 11:31 p.m. Eastern Daylight Time. With a small telescope, you can watch it swelling into view just off the planet's eastern limb.

  • Around 10:30 p.m. Pacific Daylight Time, the bright asteroid 4 Vesta (still 6th magnitude) passes about 1/2 arcminute from the star HIP 78968, magnitude 7.8. The two can be found less than 2° north of Beta Scorpii, the most northern of the three bright stars forming the head of Scorpius. Use our Vesta finder chart. (The date ticks on the chart are for 0:00 Universal Time, which is on the evening of the previous date in the Americas). How soon can you see that Vesta is moving with respect to the star?

    Saturday, July 28

  • Jupiter's Red Spot transits around 9:06 p.m. EDT.

    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 dated Burnham's Celestial Handbook). Read here how to use them most effectively.

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

    This Week's Planet Roundup

    Mercury is having a fine apparition low in the glow of dawn, now that it has brightened to about magnitude 0. And it's still brightening. Look for it above the east-northeast horizon about 60 minutes before sunrise. It's far lower right of Capella, and far lower left of Aldebaran. See article.

    To find your local sunrise time (and much else), make sure you've put your location 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 on July 16th

    Thinner and thinner.... Late on the afternoon of July 16th, S&T's Sean Walker used a 12.5-inch reflector at f/25 to shoot waning Venus through red, green, and blue filters.

    S&T: Sean Walker

    Venus is bright at magnitude –4.6, but it's getting quite low in the west after sunset. A telescope — or even steadily mounted binoculars — shows Venus to be a thinning crescent.

    Mars (magnitude +0.6, between Aries and Taurus) rises after about 1 a.m. daylight saving time is and high in the east by dawn. Look for the Pleiades to its left or lower left, and Mars-like Aldebaran below the Pleiades. In a telescope, Mars is only 7 arcseconds in diameter. It's on its way to opposition this coming Christmas season, when it will reach 16 arcseconds wide.

    Jupiter (magnitude –2.5, in southern Ophiuchus) glares in the south in early evening and lower in the southwest later at night. Antares, less bright, sparkles redly 5° below it; the two are evening companions all this summer. Other stars of Scorpius shine below them and to their right.

    Saturn is about 9° to the right of Venus and is getting lost in the glow of sunset. So is Regulus, between the two planets and somewhat above. Use binoculars.

    Uranus (magnitude 5.8, in Aquarius) and Neptune (magnitude 7.8, in Capricornus) are well up in the southeast and south during early morning hours.

    Pluto (magnitude 13.9, in the northwestern corner of Sagittarius) is highest in early evening, about 17° east 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.

    To always get the up-to-date Sky at a Glance, bookmark this URL:
    http://SkyTonight.com/observing/ataglance .

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