September 24, 2010

Happy Birthday Neptune!

103989main_neptune1.jpgNeptune turns 164 on September 23rd! Actually the fourth largest planet in our solar system is billions of years old but it was discovered in 1846. Neptune is about 4 times Earth’s diameter and 30 times further from the Sun than Earth. Uranus and Neptune are the two planets in our solar system which are not visible to the naked eye. British astronomer and mathematician John C. Adams began calculating Neptune’s position in 1843. He submitted his extremely accurate prediction of the planet’s position to the Astronomer Royal of England who didn’t act on Adams’ prediction apparently because he lacked confidence it Adams’ calculations.

Image: Planet Neptune’s atmosphere is primarily methane. In this image Neptune’s moon Triton appears in the foreground. Image courtesy NASA/JPL Click to enlarge

At about the same time a French mathematician named Urbain J. J. Leverrier began working on the project. In mid-1846 he sent his calculations to the Urania Observatory in Berlin, Germany. The director of the observatory, Johann G. Galle, had just charted the area of the sky where both Adam’s and Leverrier’s calculations predicted the planet would be. On Sept. 23, 1846, Galle and his assistant, Heinrich L. d’Arrest, found Neptune near the position predicted by Leverrier. Both Adams and Leverrier are credited with the discovery.

moon-jupiter-uranus-neptune-23-sept-10.gifNeptune orbits the Sun once every 165 years so it’s roughly in same part of the sky now as it was at the time of it’s discovery 164 years ago in the constellation Aquarius. You’ll need a telescope and fairly dark skies to spot it. The chart shows the position of the Moon, Jupiter, Uranus and Neptune on September 23rd. The view is to the southeast around 11 pm. The Moon’s position from night to night will move about 15 degrees to the east. Because of their much greater distances the planets positions will change very little from night to night. Click chart to enlarge

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September 17, 2010

Mercury in the Morning

mercury-19-sep-10.gifSpeedy little Mercury will make his best morning appearance of 2010 this week. Early risers will want to check out the eastern horizon about 5:45 am. Tiny reddish Mercury will appear just east of (below) the star Regulus in constellation Leo.

 Chart at left was prepared using John Walker’s Your Sky program.  It shows the eastern horizon at 5:45 am EDT September 19th

Don’t miss it, Mercury will only be visible until September 25th or so. By mid October the tiny planet will be at superior conjunction (on the far side) with the Sun. In early December he’ll be back in the evening sky very briefly before an inferior conjunction (on this side) with the Sun just before Christmas. The little planet is like a tailgater in morning rush hour traffic…always in a hurry!

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Jove, the King of Planets, Rules the Night

jup1.jpgTuesday, September 21st that big old gas ball Jupiter reaches “opposition” which means the planet appears opposite the Sun in our view. This means that as the Sun sets in the west Jupiter is rising to the east. Opposition marks the middle of the best time of year to see the planet. You shouldn’t have any problem spotting Jupiter. At magnitude -2.9 it will be the brightest object in the east, except for the Moon. Go out 30-45 minutes after sunset or after darkness falls and look to the east. For the best views wait until around midnight when Jove will be high above your southern horizon.

Binoculars will reveal the 4 “Galilean” moons, Io, Europa, Ganymede and Callisto. If you have a pair of “image stabilized” binos observing the Jovian moons should be easy. With “standard” binoculars steady them against a post, porch post or the corner of a building for a stable view.

The 2010 and 2011 oppositions of Jupiter will be especially good because the planet will reach perihelion, the point nearest the Sun in it’s nearly 12 year orbit, in March of ‘11. When it’s nearest the Sun it’s also nearest us and for this opposition the planet’s disc will be 50″ wide.

This opposition will be all the more interesting because the Jovian moon Europa will transit (pass in front of) Jupiter’s disc Tuesday evening followed by an occultation (disappearance behind the planet) of Io early Wednesday morning. Binoculars or a telescope will be required to observe these disappearances and reappearences. Furthermore, Uranus reaches opposition a few hours after Jupiter, the two planets will be at conjunction (appearing less than a degree apart) on the 24th and a very nearly full Moon will pass within about 5 degrees of the planets around midnight Wednesday night/Thursday morning.

jupiter-21-sep-10-ys.gifIn the unlikely event that you have difficulty spotting Jupiter use the Moon as a “finder” Wednesday night. Unfortunately the presence of a very bright “Harvest Moon” will wash out any telescopic observation of the planets.

This chart shows the view to the southeast at 10pm EDT on Tuesday, September 21st. Click to enlarge

jupiter-21-sep-10-f.JPGThe transit of Jupiter’s moon Europa will begin at approximately 02 UT (10pm EDT) Tuesday evening, September 21. About 1 minute later at 02:01 UT Europa’s shadow will begin transiting Jupiter’s disc. Europa’s transit will end at 04:42 UT (12:42 am EDT Wednesday), Europa’s shadow will exit Jupiter’s disc at 04:46 UT. While Europa is transiting Jupiter watch as Io approaches from the opposite direction. Io will disappear behind Jupiter’s disc at 06:00 UT (2:00 am EDT). These timings are based on estimates published in Ottewell’s “Astronomical Calendar, 2010″ for more accurate timings contact me. A telescope will be required to observe the transit of Europa and it’s shadow. Use averted vision at the eyepiece, the moon will be very difficult to detect. Europa’s shadow more distinct.

You may be able to observe Europa’s “disappearence” in front of Jupiter’s disc and Io’s behind it with binoculars. The latter phases of this transit and occulation will be more easily observed because Jupiter and it’s moons will have risen further above the horizon with less atmospheric turbulence.

During this period Jupiter and Uranus will appear less than a degree apart. The much dimmer (magnitude 5.7) Uranus is about a degree NNW (above and slightly to the right) of the much brighter disc of Jupiter.

I prepared the chart using the Stellarium planetarium freeware. The positions of the Jovian moons are designated by the letters G (Ganymede), E (Europa), I (Io) and C (Callisto). The chart shows the positions of the moons at 01:30 UT (9:30 PM EDT September 21st). The red arrows indicate the apparent motion of Europa and Io between 01:30 and 06:00 UT. Click chart to enlarge

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