Skywatch Line for Wednesday, April 5th, and Thursday, April 6th, 2017

This is the Dudley Observatory Skywatch Line for Wednesday, April 5th, and Thursday, April 6th, written by Louis Suarato.

The 71% illuminated, waxing gibbous Moon rises at 1:51 p.m. Wednesday. By nightfall, you’ll find it high above the southern horizon, in the constellation Cancer, to the lower left of M44, the Beehive Cluster. Thursday, Leo’s brightest star, Regulus, will shine less than a degree to the left of the Moon. Look for Mars and Mercury low in the west after sunset. Later in the evening, you’ll notice the Pleiades star cluster above Mars. Mercury, the lower of the two, will be 10 degrees above the horizon at 8 p.m., while Mars will be 23 degrees above the horizon to its upper left. Mercury will return as a morning star by month’s end. Jupiter rises at 7:29 p.m. in Virgo. Jupiter is approaching opposition, its point in the sky directly opposite the Sun, illuminating the entire face of the planet. Opposition occurs at 10 p.m. on Friday. Saturn rises at 1:14 a.m. Thursday. Saturn becomes stationary at 5 a.m. Thursday, and begins its retrograde motion thereafter. This illusion is caused by a faster Earth in orbit passing slower Saturn in its orbit. Instead of approaching Saturn, Earth passes it, and Saturn begins to move in the opposite direction in our sky.

The axial tilt of Earth not only affects our seasons, it also affectshow we view the circumpolar constellations. When looking up at Polaris, the North Star, we notice five constellations circling the star. In the northern hemisphere, these five constellations are Ursa Major, Ursa Minor, Draco, Cepheus, and Cassiopeia. From our latitude, these constellations never rise or set, and are viewable throughout the night. As Earth orbits the Sun, and our seasons change, the axial tilt provides us with a different angle of view toward these constellations in relation to the North Star. An example of this celestial seasonal clock can be explained using the Big Dipper asterism within the circumpolar constellation, Ursa Major. One can determine the season, by the Big Dipper’s position in relation to the North Star at nightfall. During the winter months, when the northern hemisphere is tilted away from the Sun, the Big Dipper stands straight up on its handle to the right of Polaris. During the summer, when the northern hemisphere is tilted toward the Sun, the Big Dipper appears on the opposite side of Polaris, vertical, with its bowl lower than its handle. During the Fall, when Earth is neither tilted toward, nor away from the Sun, the Big Dipper appearsbelow Polaris, horizontal along the horizon, with its bowl facing up, in the early to late evening. Likewise, during Spring, when the northern hemisphere is not tilted toward, or away from the Sun, but on the opposite side of the Sun than in the Fall, the Big Dipper lies horizontal, but above Polaris, with its bowl facing down.

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