Sky at a Glance | September 25th, 2010

Twilight view
In twilight on Friday the 24th, look for the Moon and Jupiter far apart in the east.

Friday, Sept. 24

  • As twilight descends this evening, the Moon and Jupiter come into view far apart low in the east, as shown here. The Moon is two days past full.

    Saturday, Sept. 25

  • 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 11:50 p.m. Eastern Daylight Time. The "red" spot appears very 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 our list for the rest of this observing season.

    Cetus rising
    Now at naked-eye visibility, Mira in Cetus climbs the eastern sky in late evening — in this case, over hills in Iran. Click image for larger view.
    Babak A. Tafreshi

    Sunday, Sept. 26

  • Mira, the prototype red long-period variable star in Cetus, is now visible to the unaided eye. It was magnitude 4.6 as of September 21st, on its way to a predicted maximum of about 3.7 in early October. Cetus is in good view in the east-southeast by about 11 p.m. daylight saving time. Estimate its brightness using the comparison-star chart in the September Sky & Telescope, page 58.

    Monday, Sept. 27

  • After the waning gibbous Moon rises in late evening, look for the Pleiades just to its left, by about 2° (depending on where you live).

  • Jupiter's Red Spot should transit around 1:28 a.m. Tuesday morning EDT; 10:28 p.m. Monday evening PDT.

    Tuesday, Sept. 28

  • This is the time of year when, after nightfall, the dim Little Dipper (you'll need a dark sky!) dumps water into the bowl of the Big Dipper far below it in the north-northwest.

    During the current dark-of-the-Moon period, Comet Hartley 2 passes south of Cassiopeia high in the evening sky. Click image for larger charts. The comet symbols are at 0:00 UT on the September and October dates indicated. Remember, 0:00 UT falls on the evening of the previous date in the times zones of the Americas.
    Sky & Telescope
  • Now that the Moon is gone from the evening sky, start keeping an eye on Comet Hartley 2. It's about 7th magnitude and excellently placed in western Cassiopeia. Hartley 2 should brighten to 5th magnitude in the next three weeks. See the article and finder chart in the October Sky & Telescope, page 56, or online.

  • Jupiter's Red Spot should transit around 9:19 p.m. EDT.

    Wednesday, Sept. 29

  • As evening grows late and Jupiter rises high in the southeast, look for Fomalhaut, the Autumn Star, sparkling far to its lower right in the south-southeast.

    Thursday, Sept. 30

  • Last-quarter Moon (exact at 11:52 p.m. EDT).

  • Jupiter's Red Spot should transit around 10:57 p.m. EDT.

  • The bright eclipsing variable star Algol should be in one of its periodic dimmings, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 1:39 a.m. Friday morning EDT; 10:39 p.m. Thursday evening PDT. Algol takes several additional hours to fade and to rebrighten. Use our comparison-star chart. (For all times of Algol's minima this month, good worldwide, see the October Sky & Telescope, page 59, or use our Algol predictor.)

    Friday, Oct. 1

  • Shortly after dark at this time of year, five constellations form a line descending from the zenith down to the west-northwest horizon. Near the zenith is the star Deneb: the head of the Northern Cross and the tail of Cygnus, the Swan. Next down is Lyra with bright Vega, then dim Hercules, then little Corona Borealis, and then big Bootes with bright Arcturus low in the west-northwest.

    Saturday, Oct. 2

  • Jupiter's Red Spot should transit around 12:35 a.m. Sunday morning EDT; 9:35 p.m. Saturday evening PDT.


    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 the center of 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 must have a detailed, large-scale sky atlas (set of charts). The standards are the
    Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger Sky Atlas 2000.0 (stars to magnitude 8.5); and the even larger and deeper Uranometria 2000.0 (stars to magnitude 9.75). And read how to use your charts effectively.

    You'll also want a good deep-sky guidebook, such as Sky Atlas 2000.0 Companion by Strong and Sinnott, or the more detailed and descriptive Night Sky Observer's Guide by Kepple and Sanner, or the classic if dated Burnham's Celestial Handbook.

    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. 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."


    This Week's Planet Roundup

    Mercury (bright at magnitude –1) is having a fine morning apparition. Look for it low in the east about 45 minutes before your time of sunrise. It sinks lower as the week advances. Look also for little Regulus increasingly far to its upper right.

    Venus, though brightest now at magnitude –4.8, is sinking very low in the southwest during bright twilight. It sets well before dark. In a telescope Venus is becoming an ever thinner, ever longer crescent.

    Mars, vastly dimmer at magnitude +1.5, remains 6½° to Venus's upper right all week. That's about one field-of-view width in typical binoculars. You'll need them. Good luck.

    Jupiter (magnitude –2.9, at the Pisces-Aquarius border) is just past opposition. As twilight fades, Jupiter grows very obvious low in the east. It shines high in the east-southeast by mid-to late evening — by far the brightest starlike point in the sky. It's highest in the south around midnight or 1.

    Jupiter and satellites
    Callisto, Io, Europa, and Ganymede (top left to lower right) pose with Jupiter for a wide-field view on the night of September 25th. "I shot the image with my 14-inch Meade SCT and Canon 20Da DSLR," writes Richard Tresch Fienberg, former Sky & Telescope editor in chief. "It's a composite: 1/10 second exposure to get the moons, 1/500 sec to get Jupiter."
    Richard Tresch Fienberg
    Jupiter is having an unusually close apparition; it continues to appear 49 arcseconds wide through mid-October. In fact this opposition was closer than any other of Jupiter from 1963 to 2022 (but only 1% or 2% closer than in any year when opposition occurs from mid-August through October, including last year and next. See our article.)

    Jupiter's Great Red Spot is near System II longitude 157°. Assuming it stays there, here's a list to print out of all the Great Red Spot's predicted transit times (times and dates in UT) for the rest of this observing season.

    Saturn is hidden behind the glare of the Sun.

    Uranus (magnitude 5.7) is only 1° to 1½° from Jupiter this week.

    Neptune (magnitude 7.8, at the Aquarius-Capricornus border) is well placed earlier in the evening. See our finder charts for Uranus and Neptune online or in the September Sky & Telescope, page 56. Can you see any color in Uranus and/or Neptune?

    Pluto (magnitude 14.0, in northwestern Sagittarius) is still fairly high in the south-southwest right after dark. The sky is free of moonlight then by the 27th or 28th. Use the large finder chart 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 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.


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