December 18, 2022: Before sunrise, the crescent moon is with Spica. After sunset, the display of the five bright planets of Venus, Mercury, Saturn, Jupiter and Mars begins to appear.
by Jeffrey L. Hunt
Chicago, IL: Sunrise, 7:13 a.m. CST; Sunset, 4:22 p.m. CST. Check local sources for sunrise and sunset times for your location.
Transit time of Jupiter’s Great Red Spot, when it is at the center of the planet in the southern hemisphere: 8:37 UT, 6:33 UT; December 19, 4:28 GMT. Convert the time to your time zone. In the US, subtract five hours for EST, six hours for CST, etc. Use a telescope to see the place. The hours are from Sky & Telescope magazine.
It’s the 50e anniversary of the last Apollo lunar mission – Apollo 17. On December 18, 1972, the astronauts were in the “transterrestrial phase” – crossing the gap between the moon and the Earth. NASA’s summary for the day reads: “During the remainder of the transterrestrial flight, the crew performed another light flash experiment and used the infrared radiometer and ultraviolet spectrometer.”
Here is today’s planetary forecast:
The crescent moon – showing a burst of land in the lunar night – is less than halfway up south-southeast, 3.9° top left of Spica, the brightest star in the Virgin.
Spica – meaning “cob of corn” – is a blue-white star, the tenth brightest star visible from mid-northern latitudes. Shining from a distance of 250 light years, it’s hot, over 40,000°F and more than twice the size of the sun. The star emits a huge amount of energy and it is not large. Hotter stars may be larger than the sun, but they are not huge.
In contrast, Topaz Arcturus – which means “the bear guard” – is more than 50° above the east-southeast horizon, to the upper left of Spica. It is the second brightest star visible from the northern lands. The pink color indicates a temperature cooler than the sun, about 7,000°F compared to the sun’s 10,000°F. Arcturus shines with the intensity of more than 100 suns at a distance of about 40 light years. It is about 20 times the diameter of the sun and nearly 10 times the size of Spica.
A bright reddish star is large while hot blue-white stars, like Spica, Vega, and Regulus are larger than the sun, but smaller than red giants and red supergiants, like Arcturus, Aldebaran, Antares, and Betelgeuse.
The stars we see in the night sky are exceptionally bright, like headlight beacons. Most of the Sun’s neighbors are reddish stars, but not giants. For example, Bernard’s Star is only six light years from the sun. It’s one two-thousandth the brightness of the sun with a temperature of over 5,000°F – distinctly red-orange like Betelgeuse and Antares, but the star is less than a tenth the diameter of the sun. Even close by, the star is not visible without the optical assistance of a telescope.
A planet’s temperature cannot be determined by its color. On the contrary, the planets shine with reflected sunlight. Mars is red-orange because its surface has large regions of iron oxide, rust, dust. The temperatures of the planets can be determined by the heat or infrared energy they release. Telescopes on top of mountains or in space, such as the Webb Space Telescope, collect infrared or energy from celestial objects.
The display of the five planets of the beginning of winter is taking shape. The main problem now is to see Mercury and Venus when Saturn is visible. While the ringed wonder is brighter than most stars in the sky tonight, the view of it is washed out by the brightness of the sun. Try to find it on the top left of Mercury at 30 minutes after sunset.
Use binoculars to locate bright Venus and Mercury low in the southwestern sky 30 minutes after sunset. Venus is to the right of the southwest direction and nearly 4° in the sky. Although low, Venus can be seen without optical aids, although a binocular may be needed to find it initially.
Mercury is 5.8° upper left of Venus and in the same field of binocular vision as the Evening Star. Can you see Mercury without binoculars?
Skywatchers at more southern latitudes have an easier view of Venus and Mercury. For them, the planets are higher and can be seen later in the twilight.
Saturn, about 30° south-southwest, is nearly 35° upper left of Mercury. It is probably not visible without optical aid at this level of twilight. If the sky is exceptionally clear, it may be visible.
Admittedly, the bright Jupiter and Mars are visible because they are further away from the bright western twilight. Look for them carefully.
Fifteen minutes later, 45 minutes after sunset, Mercury is very low in the southwest. Saturn is visible in the upper left in the south-southwest. Bright Jupiter is halfway up, 45°, south-southeast.
Mars is to the east-northeast. Probably, you can also find Aldebaran and Capella stars. The three form a diagonal line with respect to the horizon.
The best time to see the three bright outer planets is during the next two hours, before Saturn gets too low in the western sky. They dot the plane of the solar system, the ecliptic.
From 24ethe five planets and the crescent moon can be seen together for a few evenings, before Mercury darkens considerably.
At 10:28 p.m. CST, Jupiter’s Great Red Spot is visible through a telescope. The atmospheric disturbance is at the center of the planet in the southern hemisphere. From Chicago, the planet is about 15° southwest, probably too low in the sky for a clear view from Chicago. For skywatchers farther west, the planet is higher in the sky and in clearer air.
Tomorrow morning, the crescent moon is between Spica and the Scorpion claw.
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