This is Dudley Observatory’s Skywatch Line for Friday, October 11, through Sunday, October 13, written by Sam Salem.
On Friday, Sun rises at 7:04am and sets at 6:20pm; the waxing gibbous Moon rises at 5:52pm and sets at 4:37am. Full Moon occurs on Sunday at 5:08pm. October’s Full Moon is the “Hunter’s Moon”. In early autumn, the Full Moon rises about half an hour later each night compared with a normal lag close to fifty minutes. The added early evening illumination helps hunters track down their prey.
Look above the nearly full Moon, on Saturday evening, for the Great Square of Pegasus through the moonlight. The Square is balancing on one corner. Your fist at arm’s length fits inside it. Watch when the Square’s top corner is exactly above its bottom corner. This will probably by sometime soon after the end of twilight.
Soon after dark, you’ll find zero-magnitude Arcturus, the brightest star in the constellation Boötes, low in the west-northwest at the same height as zero-magnitude Capella, the brightest star in the constellation Auriga, in the northeast. Turn to the south-southeast, and there will be 1st-magnitude Fomalhaut at the same height. In the southwest at that time you’ll find bright Jupiter about as high as Fomalhaut. Jupiter continues to dominate the early evening sky in southern constellation Ophiuchus, the Serpent-bearer. The giant planet shines at magnitude –2.0 and stands some 15 degrees above the southwestern horizon as twilight fades to darkness. When viewed through a telescope, you also should see the four Galilean moons, Io, Europa, Ganymede, and Callisto, as bright points of light arrayed around the planet.
Although autumn began and the stars of winter’s Orion now rule the morning sky, the Summer Triangle remains prominent on early October evenings. The constellation Orion the Hunter appears on its side as it rises, with ruddy Betelgeuse to the left of the three-star belt and blue-white Rigel to the belt’s right. As Orion climbs higher before dawn, the constellation rotates and Betelgeuse lies at the upper left and Rigel at the lower right of the constellation pattern. Look high in the west after darkness falls for brilliant star Vega in the constellation Lyra the Harp. Vega, at magnitude 0.0, is the brightest member of the Summer Triangle. The second-brightest star, magnitude 0.8 Altair in constellation Aquila the Eagle, lies some 35 degrees southeast of Vega. The triangle’s dimmest member, magnitude 1.3 Deneb in constellation Cygnus the Swan, stands about 25 degrees east-northeast of Vega. Deneb passes through the zenith about an hour after the last traces of twilight disappear.
Try watching for Earth’s shadow in both the evening and morning sky,. It is the blue-grey darkness in the direction opposite the Sun, darker than the twilight sky. Earth’s shadow can be seen ascending in the eastern sky at the same rate that the Sun sets below the western horizon. The shadow is curved, just as the shadow of any round object is curved. Earth’s shadow is best seen when the horizon is low, such as over the sea, and when the sky conditions are clear. In addition, the higher the observer’s elevation is to view the horizon, the sharper the shadow appears. The pink band above the shadow in the east after sunset, or west before dawn, is called the Belt of Venus. The arch’s light pink color is due to the backscatter of reddened light from the rising or setting Sun. A very similar effect can be seen during a total lunar eclipse. The zodiacal light, which is caused by the diffuse reflection of sunlight from the interplanetary dust in the Solar System, is also a similar phenomenon.