Skywatch Line for Wednesday, October 20, and Thursday, October 21, 2021

This is Dudley Observatory’s Skywatch Line for Wednesday, October 20, and Thursday, October 21, written by Alan French.

The Sun sets at 6:05 P.M. on Wednesday and rises at 7:17 A.M. Thursday. Sunset Thursday is at 6:6:03 and the Sun will rise at 7:18 on Friday. This Thursday has 20 minutes less daylight than last Thursday.

The Moon reaches full Wednesday morning, so a waning gibbous Moon, appearing nearly full, will rise early Wednesday and Thursday evening. Moon rise on Wednesday is at 6:23 P.M. in the east. On Thursday it rises at 6:46 in the east northeast. On Wednesday night it will be more than 99 percent illuminated and Thursday it will be 98 percent illuminated, so both nights will feature a bright, nearly full Moon.

From the Northern Hemisphere planetary observers like to point their telescopes at planets when they are due south and highest, and we view them through the thinnest possible layer of atmosphere. The two largest planets, the gas giants Saturn and Jupiter, are now due south in the early evening sky, making it convenient to catch them at their best. Saturn transits – passes due south – at 7:36 P.M., while Jupiter transits at 8:38 P.M. Unfortunately, they do not rise very high from our northern seats, with Saturn reaching an altitude of 28 degrees and Jupiter reaching 32 degrees. Still, we’ll get the best views possible from here when they transit.

Any telescope magnifying 50 to 60 times will reveal Saturn’s rings. Look for the Cassini Division, a dark gap which separates the narrower, outer A ring from the inner B ring. The visibility of the Cassini Division depends on your telescope, your eyes, and the steadiness of the atmosphere.

Any modest telescope will show the four Galilean moons of Jupiter, appearing as stars, roughly in a line, to the east and west of the planet. Around 8:38 P.M. Wednesday you’ll only see two moons, Europa to the east and Ganymede to the west and farther away. (Some telescopes will reverse the positions of the moons.) The other two moons, Io and Callisto, will be passing in front of the planet and very difficult to spot – a good challenge for larger telescopes and an experienced eye. Even then, they may elude detection, their visibility depending on the contrast between the moon and the Jovian background’s color and brightness.

At 8:41 P.M. Wednesday the shadow cast by Io will move onto the east side of the planet. The dark, black shadows are far easier to spot than the moons themselves. If you look and don’t spot it at first, give it some time to move farther west and more “onto” the planet’s disk. If you’re still watching, or look again, at 9:45 P.M., Io will move off the western limb (edge) of Jupiter, first appearing as a small bump. At 10:59 Io’s shadow will move off the planet.

On Thursday night around 8:38 P.M. all four moons will be visible. Europa and Io will be to the east, close to the planet’s limb, and Ganymede and Callisto to the west, farther away than the eastern duo.

Take some time to carefully look at Jupiter. Look for two dark bands, the North and South Equatorial belts, crossing the planet from east to west. Depending on the telescope and steadiness of the atmosphere, or seeing, you may see other details, so look carefully. The steadiness of the view often varies from minute to minute, so patience may be rewarded.

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