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The Sky This Week, 2016 May 3 - 10

Mercury in the solar spotlight
The Moon and Mercury, 2016 April 8,
as seen from the dome of the USNO 12-inch telescope

The Moon spends most of the week as a slender crescent, best seen during the twilight hours. New Moon occurs on the 6th at 3:30 pm Eastern Daylight Time. Luna returns to the evening sky by the week’s end, passing through the setting stars of the Great Winter Circle I the west.

The big attraction in the sky this week is the transit of Mercury across the face of the Sun on May 9th. Unlike transits of Venus, of which only seven have been observed since the invention of the telescope, it’s possible to see several transits of Mercury in one’s lifetime. Our last opportunity to see Mercury’s disc on the Sun was November 8, 2006, when the Sun set with Mercury in transit as seen from here in Washington. This year we’ll see the entire event, with Mercury entering the Sun’s disc at 7:14 am EDT. Mid-transit occurs at 10:58 am, and Mercury’s egress from the disc will occur at 2:41 pm. Our next chance to see a Mercury transit will come on November 11, 2019. If you miss either of these, you’ll have to wait until November 13, 2032, but that event won’t be visible from the U.S. Our next chance after 2019 won’t occur until May 7, 2049! Transits of Mercury, as you may have noticed, only occur in May or November, which are the months when Mercury’s orbital nodes cross the ecliptic. They also require extreme caution to observe, since the only way to see the event is to point a telescope at the Sun. There are only two safe ways to do this, and both require extreme care to set up. The safest method is to project the Sun’s light through the telescope onto a piece of white-backed cardboard set a foot or two behind a low-power eyepiece. If your telescope has an aperture that’s more than four inches use a cut-out mask that limits the aperture to less than three inches. This will keep your telescope from getting too hot from the concentrated sunlight and will save your eyepiece from possible permanent damage. The only other way to view the transit is directly using an aperture-covering solar filter. Do not use exposed film, smoked glass, or welder’s glass for this as they pass dangerous amounts of invisible infrared radiation that will cause severe eye damage. Modern safe solar filters use reflective metallic coatings to reflect all but one-millionth part of the Sun’s light and infrared before it enters the telescope. In both cases be sure you cover the telescope’s finder scope with an opaque mask. Mercury’s disc will look like a small sunspot, except it will slowly move with respect to real sunspots.

High overhead in the later evening sky you’ll find the seven stars of the "Big Dipper" asterism. As mentioned last week these seven stars are part of the constellation of Ursa Major, the sixth-largest constellation in the sky. From a dark sky site you can actually trace a reasonable bear-like outline if you ignore the preposterously-long "tail" formed by the "handle" stars of the Dipper. Five of the seven Dipper stars belong to a group of stars that share a common proper motion across the sky. In total there are several dozen stars in the Ursa Major Moving Group scattered over a wide arc of the sky, all moving toward a point in the constellation of Sagittarius. Ursa Major is a favorite among amateur astronomers with large-aperture telescopes; its bounds contain hundreds of far-flung galaxies that are associated with the Virgo Galaxy Cluster, of which our Milky Way is an outlying member.

Jupiter now crosses the meridian at around the end of evening twilight. You still have several hours to enjoy a view of him in the telescope while he’s well-placed in the south. Watch the Great Red Spot rotate across Old Jove’s face on the evening of the 4th. On the following night the moon Ganymede casts its large shadow on the planet’s disc. Both events should be easily visible in a 4-inch telescope.

Mars and Saturn are now visible low in the southeast by 11:00 pm. As the night turns to early morning they march along the southern horizon in close proximity to the ruddy star Antares in the constellation Scorpius. Mars is distinctively pink in color and outshines all other objects in the sky except for Jupiter. Modest telescopes will now show considerable detail on the planet’s dusty plains as he moves toward opposition on May 21st. Saturn shows his famous rings to their best advantage in any telescope. The ringed planet will reach opposition on June 2nd.

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