This is Dudley Observatory’s Skywatch Line for Friday, May 18 through Sunday, May 20, written by Sam Salem.
On Friday, the Sun rises at 5:30am and sets at 8:15pm; the waxing crescent Moon rises at 8:29am and sets at 11:47pm. The waxing crescent Moon is near Castor and Pollux, the two brightest stars in the constellation Gemini the Twins, on Friday and Saturday. The other bright star on the other side of the crescent Moon is Procyon, the brightest in the constellation Canis Minor the Lesser Dog.
Venus outshines all the bright stars at magnitude –3.9, beams close to the west horizon shortly after sunset. Catch Venus as soon as darkness falls as the dazzling planet will sink below the horizon by early evening. Jupiter, at magnitude –2.5, currently just past opposition offering planetary observers a detail-laden disk almost 45 arc seconds across. The best telescopic views are when Jupiter is near the meridian, which happens after midnight. Saturn shines at magnitude 0.3, rises a little before midnight. Mars clears the southeastern horizon a little before 1:00am.
Use the waxing crescent Moon to guide you to the location of the constellation Cancer the Crab on Saturday. Cancer is a constellation of the zodiac. However, Cancer, which is Latin for crab, is the dimmest of the 13 constellations of the Zodiac. Regulus, the brightest star in the constellation Leo the Lion, shines on one side of Cancer, while the Gemini stars, Castor and Pollux, shine on the other side. Then, on a dark night, when the Moon has dropped out of the evening sky, use Regulus, Castor and Pollux to locate Cancer the Crab. Cancer’s brightest star is magnitude 3.5. This means that none of Cancer’s stars can be seen from light-polluted cities or suburbs. If you locate Cancer on a moonless dark sky away from city lights you might be able to make out a patch of haze in the midst of Cancer’s stars with the naked eye. With binoculars, you’ll see a cluster of stars within Cancer, called Praesepe, Latin for “manger”. It is also known as the Beehive cluster, or the open star cluster Messier 44. It is one of the nearest open clusters to Earth. The Beehive cluster was among the first objects that Galileo studied with his telescope.
On May 19, 1910 the Earth passed through the tail of Halley’s Comet. It was the most intimate contact between the Earth and any comet in recorded history. A few years earlier, astronomers had found the poisonous gas cyanogen in a comet. It was then assumed that if Earth passed through the comet’s tail everyone would die. Astronomers explained that the gas molecules within the tail were so tenuous that absolutely no ill effects would be noticed. However, some people bought “Anti-Comet Pills”, special “comet umbrellas”, and gas masks to counter the effects of the cyanogen gas. Earth had passed through the tail without a single loss.