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For evening skywatchers, there is a last look at Saturn, during the first half of December, before it slips out of sight into the glare of the setting sun. That will leave only Mars in the evening sky, continuing to slowly fade as it recedes from the Earth. Meanwhile, the morning sky sees Venus dazzle well up in the east at the break of dawn. Little Mercury puts on a very good show of its own, albeit much lower in the east-southeast, for much of the month. Finally, Jupiter climbs out of obscurity at midmonth and has a rather close rendezvous with Mercury on the 21st; the biggest and smallest of the planets, vying for attention and standing side-by-side low in the dawn twilight.
In our schedule, remember that when measuring the angular separation between two celestial objects, your clenched fist held at arm's length measures roughly 10-degrees. Here, we present a schedule below which provides some of the best planet viewing times as well directing you as to where to look to see them. [Night Sky Guide for December 2018]
At about 6 a.m. local time on Friday, December 21, the planets Mercury and Jupiter will rise together from the east-southeastern horizon. Brighter Jupiter will sit only one degree to the lower right of Mercury, allowing both planets to appear within the same field of view of a telescope at low magnification (orange circle). Both objects should remain visible within the morning twilight beyond 7 a.m. local time.
At about 6 a.m. local time on Friday, December 21, the planets Mercury and Jupiter will rise together from the east-southeastern horizon. Brighter Jupiter will sit only one degree to the lower right of Mercury, allowing both planets to appear within the same field of view of a telescope at low magnification (orange circle). Both objects should remain visible within the morning twilight beyond 7 a.m. local time.
Credit: Starry Night software
Mercury — will have a very good morning apparition in December. This rocky little world passed between us and the sun on November 27th and is now shooting up rapidly into our eastern dawn sky. By the 9th it rises before morning twilight begins, and from the 10th through the end of the year, it is a zero magnitude object 25 to 30 degrees below and to the lower left of Venus. On December 15th is Mercury's greatest elongation when it's 21 degrees west of the sun. Mercury will also have a rather close encounter with Jupiter on the 21st (see Jupiter). On the morning of December 5th, a very slender crescent moon can be found low in the southeast sky about 45 minutes before sunrise and situated about 5 degrees below it will be Mercury.
In the eastern pre-dawn sky on Monday, December 3, the old crescent moon will occupy a position less than 6 degrees (a palm's width) above Venus. The following morning, the moon's eastward orbital motion (green line) will carry it 8 degrees to Venus' lower left.
In the eastern pre-dawn sky on Monday, December 3, the old crescent moon will occupy a position less than 6 degrees (a palm's width) above Venus. The following morning, the moon's eastward orbital motion (green line) will carry it 8 degrees to Venus' lower left.
Credit: Starry Night software
Venus — shines like a modern day Christmas Star – a "Star in the East" – before sunrise. It left the evening sky in late October, passing between the Earth and sun, and now is climbing to glory in the southeast before dawn. It's always bright, but this December its brilliance and altitude are exceptional. It's at its absolute brightest (magnitude -4.9) as the month begins, rising almost 3½ hours before the sun. If there's snow on the ground, check for shadows made by Venus light. Can you follow Venus past sunrise with your unaided eyes? The goddess of love reaches greatest elongation from the sun in January, but December is when she appears highest at dawn for viewers around 40 degrees north latitude. By mid-month Venus shines more than 30 degrees above the southeastern horizon at sunrise. In a telescope, the phase of this planet thickens in December from 26 to 47-percent illuminated while its angular diameter decreases noticeably. On the morning of December 3rd, a lovely waning crescent moon, less than four days from new, forms one side of a nearly equilateral triangle with Venus and the bluish 1st-magnitude star Spica this morning. Venus appears about 7 degrees below the moon; much fainter Spica appears a similar distance to the right of the moon.