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

Late this week, the waning Moon passes Venus, Mars, and company in early dawn. (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. In the Far East: move it halfway.)

Sky & Telescope diagram

Friday, July 17

  • During dawn Saturday and Sunday, the Pleiades, Mars, Aldebaran, and dazzling Venus form a mirror-reversed letter "L" in the east, as shown at right. The waning crescent Moon comes passing through as well.

    Saturday, July 18

  • Asteroid occultation. Late tonight, the moderately large asteroid 790 Pretoria occults a 10th-magnitude star in Pegasus for observers in a wide swath of land from Florida through Minnesota and past Winnipeg. The International Occultation Timing Association is campaigning to have this event well covered by observers doing accurate timings, preferably by video recording. See Scotty Degenhardt's page on this event, and Steve Preston's map and finder charts.
  • On Sunday morning, the Moon adds itself to the bottom leg of the Pleiades-Mars-Aldebaran-Venus "mirror-L" shape before dawn, as shown above.

    Sunday, July 19

  • Before dawn Monday morning for the West Coast, Jupiter's moon Io eclipses Europa partially (dimming it by 0.4 magnitude) from 4:01 to 4:05 a.m. Pacific Daylight Time. See the article and complete list of such "mutual events" between Jupiter's satellites in the July Sky & Telescope, page 51.


    Monday, July 20

  • On this day in 1969, at 4:18 p.m. EDT, the first men landed on the Moon. You can follow a complete, 40-years-later realtime replay of all mission-control communications with Apollo 11 throughout the entire mission. NASA is also providing historical information about Apollo 11 and has set up a 40th anniversary website.

    Tuesday, July 21

  • On this date in 1969, the Apollo 11 crew lifted off from the Moon: at 1:54 p.m. EDT.
  • New Moon (exact at 10:35 p.m. EDT), coinciding with a total eclipse of the Sun on July 22nd local date in India, China, and the western Pacific.

    Wednesday, July 22

  • Total eclipse of the Sun for parts of India at dawn, China later in the morning, and the western Pacific Ocean during the day local time. The eclipse is partial over a much wider area: from Siberia and Kazakhstan through Southeast Asia and Indonesia. Details.

    Facing west after sunset. Binoculars help.

    Back in the evening sky, the Moon helps guide the way to Mercury, Regulus, and Saturn. The blue 10° scale is about the size of your fist held at arm's length.

    Sky & Telescope diagram

    Thursday, July 23

  • Look very low in the west after sunset for the thin crescent Moon, as shown above. Over the Moon twinkles Regulus, departing for the year. Saturn glows higher to the Moon's upper left. Look far to the right of the Moon for Mercury, very low. When twilight is this bright, binoculars really help.

    Friday, July 24

  • The crescent Moon is below Saturn, as shown above.
  • Got a big scope and a dark sky? Explore the Abell 2199 galaxy cluster, off the Keystone of Hercules, using the charts and travel guide in Ken Hewitt-White's "Going Deep" column in the July Sky & Telescope, page 61.

    Saturday, July 25

  • Look for Saturn off to the right of the Moon, and maybe a bit higher depending on your latitude. How soon after sunset can you see it?
  • This evening through Monday evening, Saturn is passing less than 1/3° north (upper right) of the 4th-magnitude star Sigma Leonis, Leo's hind foot.
  • The red long-period variable stars V Bootis, R Bootis, and S Hydrae should be at maximum light (7th or 8th magnitude) around this date. V Boo is easy to locate with binoculars less than 1° from 3rd-magnitude Gamma Boo; use the comparison-star chart in the July Sky & Telescope, page 54.

    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 each issue of Sky & Telescope, the essential magazine of astronomy. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).

    Sky Atlas 2000.0 (the color Deluxe Edition is shown here) plots 81,312 stars to magnitude 8.5. That includes most of the stars that you can see in a good finderscope, and typically one or two stars that will fall within a 50× telescope's field of view wherever you point. About 2,700 deep-sky objects to hunt are plotted among the stars.

    Alan MacRobert

    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 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 more detailed and descriptive Night Sky Observer's Guide by Kepple and Sanner, or the classic Burnham's Celestial Handbook). Read how to use them effectively.

    Can a computerized telescope take their place? I don't think so — not for beginners, anyway, and especially not on a telescope mount that is less than rigid and top-quality mechanically. 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 and a curious mind." Without these, "the sky never becomes a friendly place."

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

    This Week's Planet Roundup

    Jupiter's black impact scar on the night of July 20, 2009

    The new Jupiter impact scar is the dark mark in the planet's South Polar Region (near top). "Here is the spot recorded in marginal seeing," writes S&T's Sean Walker. "It appears to be spreading out as predicted." The shot was made with a 14.5-inch reflector and stacked video at 3:46 UT July 21, 2009.

    The mark is near System II longitude 210°. To find the times of its future central-meridian crossings, add 2 hours and 6 minutes to our listed times of the Great Red Spot's transits.

    S&T: Sean Walker

    Mercury is hidden deep in the glow of sunset.

    Venus and Mars (magnitudes –4.1 and +1.1, respectively) are in the east during dawn. Venus is a dazzler; Mars, to Venus's upper right, is 120 times fainter. They're moving farther apart: from 10° to 13° separation this week.

    Aldebaran, similar to Mars in both brightness and color, twinkles to the right of the line between the two planets. Above Mars are the Pleiades. Far left of the whole group shines bright Capella.

    Jupiter (magnitude –2.8, in Capricornus) rises by the end of twilight and shines highest in the south in the early-morning hours.

    Impact on Jupiter! A black dust scar like those of the Comet Shoemaker-Levy 9 impacts in 1994 has suddenly appeared in Jupiter's south polar region. See our article.

    Saturn (magnitude +1.1, in Leo) is getting low in the west after dusk. Look early! In a telescope Saturn's rings are narrowing, appearing only 2½° from edge on. And they're getting very dim. The rings will turn edge-on to the Sun and go black on August 10th. They'll turn edge-on to Earth on September 4th, but by then Saturn will be lost in the sunset.

    Uranus (magnitude 5.8, in Pisces), is high in the south before dawn.

    Jupiter and Neptune on July 9, 2009

    On the morning of July 9th, Bob Kimmel of Rockledge, Florida, took this image of Jupiter (overexposed) and Neptune when they were 1/2° apart. Why are they so different in brightness? Jupiter is a bigger planet, but mostly it's because Jupiter is closer to the Sun (so it gets lit more brightly) and closer to Earth.

    Kimmel used an 8-inch f/5 Newtonian reflector with a Canon D20a DSLR camera body on his back patio. This is an unguided 20-second exposure at ISO 3200.

    Bob Kimmel

    Neptune (magnitude 7.8, in Capricornus) remains only about 1° from Jupiter — but it's 17,000 times fainter, as shown here. See our finder charts for Uranus and Neptune, good for the rest of this year.

    Both of these outermost planets show a very pale blue-green tint in a medium-size to large telescope, if your eye is particularly sensitive to color. Otherwise, or in a smaller telescope, they're basically gray.

    Pluto (14th magnitude, in northwestern Sagittarius) is at its highest in the south during evening. If you've got a big scope and a dark sky, you can take on the Pluto challenge using the finder chart in the June Sky & Telescope, page 53.

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

    "Science is built up of facts, as a house is with stones. But a collection of facts is no more a science than a heap of stones is a house."
    — Henri Poincaré (1854–1912)

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