December 2020 — Night Sky Guide & Key Dates

Geminids
Image Credit: NASA

December 1st marks the start of the meteorological winter, which by definition, lasts from December to February. By contrast, the astronomical winter stretches from 21 December to 20 March 2021.

Fascinating celestial events to look forward over the coming month includes several occultations, which is when an object is hidden by another object that transits between itself and the observer. In addition, there are also a couple of meteor showers to enjoy in the night sky, namely the the Geminids and the Ursids.

So, without further ado, let’s gets straight to it!

December 4: Io crosses Jupiter

On this night, it is possible to see Io cross Jupiter with right optical equipment. Both the bright disk of the moon and the black shadow that it casts are relatively easy to spot through a telescope. The eclipse starts at 16:23 GMT and lasts nearly 19 hours, so you should be able to see part of it in any time zone, as long as Jupiter is in your night sky.

December 7: Lunar occultation of 46 Leonis

On this day,  the Moon will cover the star 46 Leonis in the constellation Leo. Scientifically these occultations are interesting as they can be used to learn about the Moon’s topography. For amateurs, they are also a great chance to determine the size of the Moon on your own or just to see a star disappear behind the Moon’s black disk. The occultation takes just under 1.5 hours and starts at 2:48 GMT. The effect can be seen with the naked eye.

December 12: Lunar occultation of Venus

If you want to see a more exciting occultation then you are in luck (maybe) as the Moon will cover Venus on this day. However, the Moon is only visible in the sky on the west coast of the US and Alaska. For all those outside of this area, you can still see the Moon and Venus close together early the next morning.

December 13: Geminid meteor shower

If you want to amaze your close ones with a meteor shower, the 13th Dec is your best chance as this is the peak of the Geminid meteor shower. The shower is named after the constellation Gemini, because the meteors will seem to come from that location. Don’t worry too much though, as the meteors are generally visible all over the sky. The Geminids are a very active meteor shower, with up to 120 meteors per hour. That’s 2 per minute!

Remember that this is just the peak of the shower, so you can try days around this date if the weather is not cooperative. The visible meteor activity, however, will go down!

December 14: Lunar occultation of Mercury

Another chance to see a planetary occultation on this day, we promise that these are usually quite rare! Like with Venus, the timing is not great and it happens during the day/morning in the GMT timezone. However, since Mercury is quite bright, you will be able to see it with some luck and a good unobstructed skyline.

December 17: Europa and Ganymede align
On this day two of the Jovian moons will be within 4 degrees of each other at 17:28 GMT. You’ll need a telescope to see Europa and Ganymede, though.

December 21: Winter solstice and beginning of astronomical winter and the great conjugation of Jupiter and Saturn

On this day we enter astronomical winter, which lasts until March 20th. This is also the shortest day, which means that we are headed towards spring from now on!

Additionally, we get a rare treat this night as Jupiter and Saturn make their closest approach on the sky. This conjugation happens only once every 19.6 years, but typically the planets are not this close. With only 0.1 degree separation, it is possible to capture both planets in the same field of view of a telescope. The last time that this was possible was in 1623!

December 22: Ursid meteor shower

You get another chance to see some shooting stars on this day, this time from the constellation Ursa Minor, from the area around Polaris. But be warned, the Ursids are less much active then that Orionids, on average with only around 12 shooting stars per hour.

December 27: Double transit on Jupiter

Today both Europa and Io are crossing Jupiter at very similar times. This is a rare opportunity to see both moons and both shadows as a series of four alternating black and white dots on the disk of Jupiter. This happens between 16:36 and 17:11 GMT.

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