Astronomical conjunctions are events that bring two or more objects together into the same area of the sky, as seen from us on Earth. Conjunctions of 2 or 3 planets, 1 or 2 planets and the Moon, or a planet and a bright star can provide sky-watchers with memorable sights. Two fine planetary conjunctions will grace our December night skies this year.
The first involves Mars and Neptune. Mars is conspicuous at magnitude 0.0 among the dim stars of Aquarius in the southern sky. Neptune, on the other hand, is so far away and thus so dim, that we cannot see it at all without optical aid. But the two planets will appear so close that Mars will point the way to see Neptune —- provided we use binoculars.
On December 1st, Neptune’s dim bluish light may be seen just slightly above and left of Mars. Five days later on the 6th, Mars’s faster orbital speed will have closed the gap between them even more, so that Neptune will appear nearly on top of Mars! The next night, December 7th, the two will have switched, with Neptune then seen just below and right of Mars. It is rare to have the great guide, such as Mars is providing, to find something so faint as Neptune (magnitude +7.9). Indeed, it will not be until October 19, 2210 that Mars and Neptune will again appear this close together to us!
The second conjunction occurs in the pre-dawn southeastern sky between Jupiter and Mercury. Mercury reaches its greatest western elongation from the Sun on December 15th, which puts it 10 degrees above the southeastern horizon, 45 minutes before sunrise, at magnitude –0.4. Jupiter’s orbit is making it appear to rise higher each day in December, and after the 15th, Mercury will appear to descend toward the giant planet. On December 21st, Mercury and Jupiter will be seen less than a degree apart, about 8 degrees above the horizon and 45 minutes before sunup.
Meanwhile, Venus is rising around 4 am in the eastern sky and is at maximum brightness, –4.9 magnitude on December 1st; fading only to –4.6 by December 31st. Look at Venus especially on the morning of December 3rd, when the waning crescent Moon will hang just 5 degrees above it. (another beautiful conjunction!) Saturn is losing altitude in the southwest evening sky, but can be seen in twilight on December 8th, 7 degrees above the horizon, with the waxing crescent Moon sitting just above it.
Despite these wonderful conjunctions, perhaps December’s best event is the annual Geminid Meteor Shower, which peaks on the night of December 13/14. The Geminids are one of the three best meteor showers of the year and this year the Moon will have set when it is at its best for viewing —– between 1 am and 5 am. Up to 120 meteors per hour may be seen from this shower, which appears to come from the constellation Gemini. Gemini is nearly overhead by 1 am on December 13th, and over toward the western sky until 5 am. To spot the longest meteor trails, look 30 to 50 degrees away from Gemini itself —– and dress warmly. December nights are cold.
Finally the 32nd Annual Planetarium Xmas Program; this year titled —- “What Christmas Means to Me” —– will be presented by the Kent County High School Planetarium and Radio Station —– WKHS FM 90.5 ——– beginning at 7:00 pm on December 14th, and continuing on December 17th, 18th and December 20th and 21st. As usual our program blends a good dose of stars and astronomy with the rich traditions of Christmas. We welcome all 1st-time visitors and all sky-watchers, and others, who have made our show an annual event for themselves.