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This Week's Sky at a Glance

by Alan M. MacRobert

Some daily events in the changing sky for July 10 – 18.

Friday, July 10

  • Jupiter shines to the right of the bright Moon late this evening, and lower right of the Moon when they're high before dawn Saturday morning.

    Saturday, July 11

    Now that we're far into summer, the Summer Triangle is very high in the east after dark, with the Moon-washed Milky Way running horizontally through it. Bright Vega is the Summer Triangle's highest corner. Lower left of Vega, by two or three fist-widths at arm's length, is Deneb. Farther to the lower right of Vega shines the triangle's third corner, Altair.

  • Mars is 4° south (lower right) of the Pleiades before dawn Sunday morning.

    Sunday, July 12

  • The red long-period variable stars SS Virginis and T Sagittarii should be at maximum light (7th or 8th magnitude) this week.

    Monday, July 13

  • A small telescope will usually show Titan, Saturn's largest moon. Tonight Titan is four ring-lengths to Saturn's east.

    Tuesday, July 14

  • The Milky Way is high in July, and that means lots of planetary nebulae await your telescope. Hunt out some lesser-known ones shortly after dark (before moonrise) using Sue French's "Deep-Sky Wonders" column and charts in the July Sky & Telescope, page 57.

    Wednesday, July 15

  • Last-quarter Moon (exact at 5:53 a.m. Eastern Daylight Time).

  • 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 2:25 a.m. Thursday morning Eastern Daylight Time. The "red" spot is actually pale orange-tan. It should be visible for about an hour 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 all of the Red Spot's central-meridian crossing times, good worldwide, use our Red Spot calculator or print out our list for 2009.

    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
    Thursday, July 16

  • On this day in 1969, Apollo 11 took off for the Moon. The first humans landed on the Moon four days later. You can follow a complete real-time replay of the mission-control communications with Apollo 11 throughout the mission. NASA is also providing historical information about Apollo 11 and has set up a 40th anniversary website.

  • On this day in 1994, the first piece of Comet Shoemaker-Levy 9 hit Jupiter — creating the first of a series of dark blemishes on the planet that were visible in telescopes worldwide, bringing the reality of planetary impacts to widespread public attention.

    Friday, July 17

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

    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 get this event well covered by observers doing accurate timings, preferably by video recording. See Scott Degenhardt's page on this event, and Steve Preston's finder charts.

  • On Sunday morning the Moon adds itself to the bottom leg of the Pleiades-Mars-Aldebaran-Venus "mirror-L," as shown above.




    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).

    Pocket Sky Atlas
    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     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 mounts that are less than top-quality mechanically: accurate and wobble-free. 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 on July 2, 2009
    Jupiter's Great Red Spot (rather pale this year) was crossing the planet's central meridian when Fabio Carvalho of Sao Carlos, Brazil, took this image on July 2nd. Also note the sharply defined Red Spot Hollow, and the very active, turbulent North Equatorial Belt. South is up. The central meridian longitude at the time of the image was 136° (System II). Carvalho used a 10-inch f/6 Newtonian reflector and a SkyNyx 2-OM camera.

    For all of the Red Spot's central-meridian crossing times, good worldwide, use our Red Spot calculator or print out our list for 2009.

    Fabio Carvalho
    Mercury is hidden in conjunction with the Sun.

    Venus and Mars (magnitudes –4.2 and +1.1, respectively) are due east during dawn. Venus is a dazzler; Mars, to Venus's upper right, is 130 times fainter. They're moving farther apart: from 7° to 10° separation this week. Twinkly Aldebaran, similar to Mars in both brightness and color, is below Venus early in the week, and to Venus's right late in the week. Look above or upper left of Mars for the Pleiades.

    Four reasons combine to create the great brightness disparity between Venus and Mars. Mars is farther from the Sun so it's illuminated less brightly; Mars is a smaller planet; its surface is darker and less reflective than Venus's white clouds; and Mars is currently farther from Earth.

    In a telescope, gibbous Venus appears about 17 arcseconds in diameter; Mars is a tiny 5 arcseconds.

    Jupiter (magnitude –2.7, in Capricornus) rises around 10 p.m. daylight saving time and shines highest in the south before dawn. In a telescope it's a big 47 arcseconds wide.

    Dim-ringed Saturn on June 16, 2009
    On June 16th Saturn's rings were still tipped 3.7° to our line of sight, but they were tipped a mere 0.8° to the incoming sunlight. All year the rings' tilt to the Sun has been steadily decreasing, and accordingly, the rings have been getting darker and darker. Keep watch! Saturn is becoming harder to observe as it moves lower in the west each evening.

    S&T's Sean Walker took this stacked-video image using a 14-inch Schmidt-Cassegrain scope at f/22, a DMK21AU04.AS camera, and Custom Scientific RGB filters.

    S&T: Sean Walker
    Saturn (magnitude +1.1, in Leo) is getting lower in the west after dusk. Look early! In a telescope Saturn's rings are narrowing, appearing only 3° from edge on. And see how they've dimmed! 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 glow of sunset.

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

    Neptune (magnitude 7.8, in Capricornus) remains only about 3/4° from Jupiter, but it's 16,000 times fainter. See our finder charts for Uranus and Neptune.

    Pluto (14th magnitude, in northwestern Sagittarius) is at its highest in the south around 11 p.m. See 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.

    "The universe is full of magical things patiently waiting for our wits to grow sharper."
    — Eden Phillpotts, "A Shadow Passes," 1918

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