Skywatch Line for Wednesday, September 28th and Thursday, September 29th, 2016

This is the Dudley Observatory Skywatch Line for Wednesday, September 28th and Thursday, September 29th written by Louis Suarato.

After hiding behind the Sun, and in its glow for weeks, Mercury has emerged to be seen again before sunrise. Wednesday, Mercury reaches perihelion, its closest distance to the Sun, and is at its greatest western elongation, at 18 degrees. Wednesdaymorning, look about 10 degrees below the 6% illuminated, waning crescent Moon for Mercury. Thursday morning, Mercury will be 2 degrees above the 2% illuminated, thin crescent Moon. With a diameter of 3,030 miles, Mercury is the smallest planet in our solar system, about the size of the continental United States, and two fifths the size of Earth. Jupiter’s largest moon, Ganymede, and Saturn’s largest moon, Titan, are both larger than Mercury. Mercury orbits the Sun in 88 days, and rotates three times during every two orbits. The length of Mercury’s day is 4,222.6 hours. The mean temperature on Mercury is 333°F, second only to Venus’ mean temperature of 867°F.

While you’re up and out early admiring the old Moon and Mercury, you may notice a conical shaped glow stretching up from the eastern horizon. This glow is known as the Zodiacal Light. This light, also known as the false dawn, is the reflection of sunlight on particles of dust circling the Sun in the inner solar system. 

Look for Venus over the west-southwestern horizon after sunset. Venus will set at 7:45 p.m., 52 minutes after sunset. Saturn remains visible over Antares in the constellation Scorpius, and Mars has moved into Sagittarius. Look for the Trifid Nebula 3 degrees above Mars. The Trifid Nebula, also known as M20, was discovered by Charles Messier on June 5, 1764. This nebula is a star-forming region located in the Scutum spiral arm of the Milky Way, and is a combination of an open star cluster of approximately 3,100 young stars, an emission nebula in its lower red area, a reflection nebula at its upper blue section, and a dark nebula which separates its lobes. This nebula shines at magnitude 6.30, and is 5,200 light-years away. 

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