Skywatch Line for Wednesday, October 6, and Thursday, October 7, 2021

This is Dudley Observatory’s Skywatch Line for Wednesday, October 6, and Thursday, October 7, written by Alan French.

The Sun sets at 6:28 P.M. on Wednesday and rises at 7:00 A.M. Thursday. Sunset Thursday is at 6:26 and the Sun will rise at 7:01 on Friday.

The Moon reached new at 7:06 A.M. Wednesday and is moving toward first quarter, which it will reach at 11:25 P.M. on Tuesday, October 12. The Moon sets soon after sunset and the night skies will be dark and moonless Wednesday and Thursday.

Have you noticed how Venus has been moving farther south in the evening sky, now shining brightly at magnitude -4.2 in the southwest during evening twilight? When Venus is in the evening sky it lies east of the Sun, and it will continue moving farther east through October 29, when it will be 47-degrees from the Sun.

Venus, traveling on its faster, inner orbit around the Sun, is now catching up with our Earth. Because it orbits between us and the Sun, it exhibits phases like our Moon. It is now just under 60% illuminated, appearing like a tiny waning gibbous Moon through a telescope. As it slowly catches up with Earth, we see less and less of its sunlight face. As our Moon moves from full to new, it grows fainter each night, yet Venus will continue to grow brighter until December 3, when it reaches magnitude -4.9.

The reason for this discrepancy is simple. The Moon, varying little in its distance from Earth, remains essentially the same size in our sky, but Venus, while catching up with us, is also moving closer, and its apparent size is increasing. For part of its journey, the increase in apparent size more than makes up for the smaller part of its visible face in sunlight.

At its current 60% illumination, Venus is 126 million km away and appears 19.8 arcseconds in diameter. On December 3, when it will be brightest, it will be 61.4 million kilometers away, and appear 25.5% in sunlight and 41.2 arcseconds in diameter. After December 3 the increase in apparent diameter will no longer quite make up for the smaller portion of the planet’s visible disk in sunlight, and the waning crescent Venus will slowly fade slightly, reaching magnitude -4.7 on December 21.

By 9:00 P.M. Jupiter and Saturn will be nicely placed in the south. Jupiter, at magnitude -2.7, will be 30-1/2 degrees high and a bit east of due south. Saturn, at magnitude +0.5, will be west of due south and 27-1/2 degrees high.

Even a modest telescope will show the four bright Galilean moons of Jupiter, appearing as stars to either side of the planet. At 9:00 P.M. Wednesday Europa will be to the east of Jupiter, and Io, Ganymede, and Callisto, will be stretched out to the west. At 9:00 P.M. on Thursday Io will be to the east, with Europa and Ganymede close together and not far from the planet’s limb, to the west, and Callisto, in its slower, outer orbit, still well to the west.

Jupiter now has 79 known moons, and no others are visible in modest amateur telescopes. If you spot what looks like an extra moon, it is a background star.

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