2022, December 18: Moon-Spica Conjunction, Five Planet Display

December 18, 2022: Before sunrise, the crescent moon is with Spica.  After sundown, the five bright planet display of Venus, Mercury, Saturn, Jupiter, and Mars is beginning to appear.

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by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 7:13 a.m. CST; Sunset, 4:22 p.m. CST.  Check local sources for sunrise and sunset times for your location.

Jupiter’s Great Red Spot’s transit times, when it is in the center of the planet in the southern hemisphere: 8:37 UT, 18:33 UT; Dec. 19, 4:28 UT. Convert the time to your time zone. In the US, subtract five hours for EST, six hours for CST, and so on.  Use a telescope to see the spot.  Times are from Sky & Telescope magazine.

This is the 50^th anniversary of the last Apollo lunar mission – Apollo 17.  On December 18, 1972, the astronauts were in the “Transearth Phase” – crossing the gap from the moon to Earth. The NASA summary for the day reads, “During the remainder of transearth flight, the crew performed another light-flash experiment, and operated the infrared radiometer and ultraviolet spectrometer.”

Here is today’s planet forecast:

Morning Sky

The crescent moon – showing earthshine in the lunar night – is less than halfway up in the south-southeast, 3.9° to the upper left of Spica, Virgo’s brightest star.

Spica – meaning “the ear of corn” – is a blue-white star, the tenth brightest star visible from the mid-northern latitudes. Shining from a distance of 250 light years, it is hot, over 40,000° F, and over twice the sun’s size.  The star emits a tremendous amount of energy and it is not large.  Hotter stars can be larger than the sun, but they are not enormous.

In contrast, topaz Arcturus – meaning “the bear-guard” – is over 50° above the east-southeast horizon, to Spica’s upper left. It is the second brightest star visible from northern lands.  The rosy color indicates a temperature cooler than the sun, around 7,000° F compared to the sun’s 10,000° F.  Arcturus shines with the intensity of over 100 suns from a distance of about 40 light years.  It is about 20 times the sun’s diameter and nearly 10 times Spica’s size.

A bright reddish star is large while the hot blue-white stars, like Spica, Vega, and Regulus are larger than the sun, but smaller than the red giants and red super giants, like Arcturus, Aldebaran, Antares, and Betelgeuse.

The stars that we see in the night sky are unusually bright, like lighthouse beacons.  Most of the sun’s neighbors are reddish stars, but not giants.  For example, Barnard’s star is only six light years away from the sun.  It is one-two thousandth the sun’s brightness with a temperature of over 5,000° F – distinctly reddish-orange like Betelgeuse and Antares, but the star is less than one-tenth the sun’s diameter.    Even nearby, the star is not visible without the optical assist of a telescope.

A planet’s temperature cannot be determined by its color.  Rather the planets shine from reflected sunlight.  Mars is red-orange because is surface has large regions of iron-oxide, rust, dust.  Planet temperatures can be determined by the heat or infrared energy they release.  Telescopes on mountaintops or in space, such as the Webb Space Telescope, collect the infrared or heat energy of celestial objects.

Evening Sky

The early winter, five-planet display is taking shape.  The main issue now is seeing Mercury and Venus when Saturn is visible.  While the Ringed Wonder is brighter than most of the stars in the sky tonight, the view to it is washed out by the sun’s brightness.  Try to find it to the upper left of Mercury at 30 minutes after sunset.

Use a binocular to locate brilliant Venus and Mercury low in the southwestern sky at 30 minutes after sunset.  Venus is to the right of the southwest direction and nearly 4° up in the sky.  Even though it is low, Venus can be seen without optical help, although a binocular may be necessary to initially find it.

Mercury is 5.8° to the upper left of Venus and in the same binocular field of view with the Evening Star.  Can you see Mercury without the binocular?

Sky watchers at more southerly latitudes have an easier view of Venus and Mercury.  For them, the planets are higher and can be seen later into twilight.

Saturn, about 30° up in the south-southwest, is nearly 35° to the upper left of Mercury.  It is not likely visible without optical help at this level of twilight.  If the sky is exceptionally clear, it may be visible.

Certainly, bright Jupiter and Mars are visible because they are farther away from the bright western twilight.  Look carefully for them.

Fifteen minutes later, 45 minutes after sundown, Mercury is very low in the southwest.  Saturn is visible to the upper left in the south-southwest.  Bright Jupiter is about halfway up, 45°, in the south-southeast.

Mars is in the east-northeast.  Likely, you can find the stars Aldebaran and Capella as well.  The three make a diagonal line compared to the horizon.

The best time to see the three bright outer planets is during the next two hours, before Saturn is too low in the western sky.  They dot the plane of the solar system, the ecliptic.

Beginning on the 24^th, the five planets and the crescent moon can be seen together for a few evenings, before Mercury dims 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° up in the southwest, likely too low in the sky for a clear view.  For sky watchers farther westward, the planet is higher in the sky and in clearer air.

Tomorrow morning, the crescent moon is between Spica and the Scorpion’s pincers.

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