This is the Dudley Observatory’s Skywatch Line for Monday and Tuesday July 18th and 19th, written by Joe Slomka.
Monday’s Sun sets at 8:30 PM; night falls at 10:35. Dawn breaks at 3:28 AM and ends with the Sun rising at 5:34.
Cetus, the Whale, houses the Moon on both nights. Monday’s 21-day-old Moon rises in the East at 11:45 PM, appears 31 arc-minutes in size, 66% illuminated and sets at 12:23 PM, Tuesday. Tuesday’s Moon does not rise, but the Moon does rise at 12:06 AM on Wednesday’s Last Quarter, 31 arc-minutes and 63% lit.
Saturn rises first in Capricornus 9:50 PM, shining with zero magnitude, 18 arc-seconds, highest at 2:56 AM 25° high at Civil Dawn. Minor Planet 4Vesta, is next, rising in Aquarius at 10:43 PM, 6th magnitude, a half arc-second, 98% lit, 19° to Saturn’s lower left and 29° highest at 3:50 AM. Neptune, in Pisces, is next rising at 11:03 PM, 8th magnitude, a tiny 2 arc-seconds and 41° high. Aries houses Jupiter and Uranus. Jupiter, in Cetus, rises at 11:33 PM, shimmers with minus 2nd magnitude, a large 43 arc-seconds and 48° high; note that Jupiter lies only 3° from the Moon. Using a telescope, it may be possible to see the Jovian moon Callisto, since it is the most distant from Jupiter. Mars is next to rise at 12:51 AM, glowing with zero magnitude, 7 arc-seconds, and 44° high. Mars lies 9° West of Uranus, which rises at 1:09 AM, glimmering with 5th magnitude, 3 arc-seconds and 41° high. Venus, in Orion, is the last to rise, but lowest due to gradual motion into the Sun’s glare. It rises at 3:40 AM, blazes with minus 4th magnitude, 11 arc-seconds, 85% lit but only 13° high. If the observer arises early enough, he may be able to see the open star cluster M35 only 1.5° from Venus; observers should take care to avowing accidently looking at the rising Sun.
At nightfall, the “W” shaped constellation Cassiopeia points to a home plate shaped Cepheus. Cepheus is home to a famous star, Delta Cephei. Some stars vary in brightness because they are eclipsed by another star, dust cloud, or planet. Delta Cephei belongs to a group of stars that vary because of changes within the star itself. Variable stars have been known since antiquity; Algol and Mira are examples. What makes Delta Cephei so special is that Henrietta Levitt discovered that its variations indicate its brightness; the period of variation is linked to the luminosity. If an observer sees a Cepheid variable, the observer knows how bright it truly is. Since he sees it dimmer, the astronomer can approximate how far away it is. Cepheids made it possible to discover the vast distances between our galaxy and other galaxies. Cepheids are among the tools astronomers use to gauge the size of the universe.