This is Dudley Observatory’s Skywatch Line for Friday, December 19, through Sunday, December 21, written by Alan French. With the Moon reaching new late on Sunday, this weekend’s night sky will be dark and moonless. Saturday morning will be the last chance for predawn sky watchers to easily spot a slender old Moon just before sunrise. By 6:15 am Saturday morning the crescent Moon will be almost six degrees above the east southeastern horizon. Look for Saturn to the upper right of the Moon, 9 degrees away. (A fist held at arm’s length spans 10 degrees across the knuckles.) Winter officially begins here in the northern hemisphere at the Winter Solstice at 6:03 pm on Sunday. At our Winter Solstice the Sun is farthest south and takes its lowest path across the sky. For those in the southern hemisphere it marks the beginning of summer, with the Sun riding highest in the sky. Jupiter rises in the east northeast just after 8:30 pm and transits and is due south just after 3:30 am when it will be sixty-two degrees above the horizon. By dawn, bright Jupiter, outshining all the stars, will be in the west southwest. While you might not want to get up at 3:30 am to see Jupiter at its highest and best, it will be 20 degrees above the eastern horizon by 10:30 pm and steadily held binoculars or any modest telescope will show its four brightest moons. Jupiter’s four largest and brightest Moons were discovered by Galileo in 1610. All are bright enough to spot with even modest optical aid, although the glare of Jupiter may hide them from a binocular view when they are close to the planet. If they were not close to brilliant Jupiter, they would be visible to the unaided eye. You can see the current positions of the four Gallilean moons at this web site: shallowsky/jupiter/ . You can also use the site to see their positions at dates and times of your choice.Binocular viewers should look for times when the moons are well away from Jupiter. Telescope observers are not so restricted. The web site, and most planetarium programs andany dedicated Jupiter satellite app, will also show satellite events: transits of moons and their shadows across the face of Jupiter, and occultations and eclipses of them by Jupiter and its shadow. These events are fascinating to watch through a telescope and are visible through even a very modest instrument. Shadow transits are quite obvious, especially when the shadow is near the planet’s limb. The most challenging event is a moon’s transit across the face of the planet. The moon’s visibility depends greatly on the contrast between it and the Jovian features it crosses. Most of Jupiter’s confirmed 67 moons are far too faint to be seen visually through any telescope. Himalia, however, is within reach of larger amateur telescopes. At magnitude 14.6 at best, it is a challenge to spot among the faint stars, but has been successfully observed by a few amateurs.
Posted on: Thu, 18 Dec 2014 20:48:42 +0000
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