Self-Guided Night Sky Tour: December 2020

Constellation Auriga

Constellation Auriga

Welcome to your December Night Sky Tour!

You can download your December Night Sky map – here.

There are four exciting events that occur in the evening sky this month.

  • The bright constellations and stars of winter have returned.
  • The red planet Mars shines brightly all night long, beginning high in the east and ending in the west at dawn. Its location is marked on the sky map, but basically, the bright red star overhead is Mars.
  • The Geminid meteor shower, the best meteor shower of the year, has recently peaked, so keep watch for shooting stars (and remember to make a wish!).
  • The planets Jupiter and Saturn have a very close conjunction in the early evening southwestern sky on the 21st—also the first day of winter. Let’s review how to spot this conjunction first.


Jupiter-Saturn Conjunction 

A planetary conjunction occurs when two or more planets appear very close to one another. This month Jupiter and Saturn appear the closest since 1226 (that’s right, the year 1226) at less than one degree of distance from each other.

To view this rare event, go to a site with very good views to the southwestern horizon such as Red River Beach about 30-45 minutes after sunset.  The two bright stars low in the southwest over Nantucket Sound are Jupiter and Saturn.  Jupiter is the brightest of the two.

The two planets will fit in the same field of view of an ordinary pair of binoculars and even a telescope at low power.

Since Cape Cod weather doesn’t always cooperate, keep an eye on the weather. The two planets appear close together throughout December, so you’ll be able to see them at some time over the month.


The Winter Constellations

The constellations of autumn are relatively faint and have few bright stars.  The winter sky, however, has many bright stars and the constellations are easy to recognize.  There are few experiences better than the winter constellations under a dark Cape Cod sky.


Ursa Major and North Star

Ursa Major and North Star

Finding the North Star and navigating the sky with the Sky Map

Be sure to use a dim red light to look at the map.  A bright white light from a flashlight or mobile phone will ruin your night vision and you won’t be able to see the stars as well.

If you’re on Cape and you know the direction Nantucket Sound is from where you are (it is always to the south), then turn so the Sound is to your back. Now you are facing north. If you are viewing the night sky from off-Cape, please orient yourself so that you are facing north. To begin to find the North Star, first turn the sky map so North is at the bottom of the map as you hold it in front of you. If you have a fairly clear view of the North, you should be able to see the ladle part of the Big Dipper rising in the northern sky.  The two end stars of the ladle point towards the North Star. These stars, “The Pointers”, point to the North Star.  Simply follow a line that extends between the two stars about five times the distance between the two Pointers and you will come to the North Star.  It’s the brightest star in this part of the sky and is always in the same location—it’s position never changes during the night or the seasons.



Constellation Cassiopeia


Think of the North Star as if it is the center of a clock.  If you follow a line upwards to the 12 o’clock position you will come to the bright constellation of Cassiopeia.  (Refer to the map if you need visual assistance.) At this season Cassiopeia looks like a distorted letter “M”. The constellation represents a woman sitting in a chair, with the left most stars representing her head and torso and the other stars representing the rest of her body, her legs and feet.  This bright constellation is one of the easiest to identify and recognize once you learn how to spot it.  Cassiopeia is in a particularly bright part of the winter Milky Way.



Constellation Perseus


Moving to the right from Cassiopeia we come to the constellation of Perseus, the hero.  Perseus isn’t as obvious as Cassiopeia, but it does have bright stars and some distinct parts that once you assemble them, it all comes together as a human figure.

Consulting your map as you move away from Cassiopeia, you will come to the bright white star of Mirfak, the brightest star in Perseus.  Mirfak is derived from Arabic and means “The Elbow”. From Mirfak four different strands of stars representing Perseus’ arms and legs radiate.

The first strand curves back towards Cassiopeia but then curves back over Mirfak.  This represents the upraised sword of Perseus.

Extending to the right from Mirfak a second strand of four stars represents the other arm and hand of Perseus that holds the head of the gorgon Medusa. The star that represents Medusa’s head is called Algol, from the Arabic al-ghoul, The Ghoul. It is a variable star, that is its brightness varies over a period of about two and half days.  This variability accounts for its evil reputation.

The third line of stars extends below Mirfak and curves around to the left in a short arc.  This represents a foreshortened view of Perseus’ right leg.

The fourth line of stars extends in a long arc of five stars and is Perseus’ extended left leg.  You will notice that this line of stars leads you right to the beautiful open star cluster of the Pleiades.

Put all four of these lines of stars together and you can visualize Perseus striding through the night sky with his sword held high.


Auriga - art impression

Centuries-old artistic impression of Auriga


Now turn your map so East is at the bottom.

Lying below Perseus towards the eastern horizon is the constellation Auriga that is said to represent the figure of a charioteer. The brightest star in Auriga is the star Capella, a yellowish star and the sixth brightest star in the night sky.

Using your map and beginning with Capella you can trace out the shape of the constellation as an irregular pentagon. The stars that represent the angles of the pentagon are easily seen and marked on the map.  Note the three stars below and to the right of Capella that forms a triangle. This little group is called “The Kids” and represents young goats held by the charioteer.



Constellation Taurus

Taurus the Bull

From Capella look south and you will see a bright orange-red star embedded in a group of stars that form a “V”.  The star’s name is Aldebaran from the Arabic for “The Follower” and it and its surrounding group of stars represent the head of Taurus the Bull. Taurus is a very ancient zodiacal constellation and has been identified with a bull from the remote past. 

The V-shaped group of stars around Aldebaran is an open star cluster called the Hyades. The name derives from Greek and alludes to the seasonal rains associated with their influence on the weather.  If you follow each line of stars of the V to the left, you will see two stars that represent the tips of the Bull’s horn. 

Moving up the sky from Aldebaran and the Hyades, you will see the tight cluster of white stars called the Pleiades.  The Pleiades’ name is derived from Greek and is a reference to sailing. You may have heard them referred to as The Seven Sisters.

The mythology surrounding these stars is too extensive to summarize, but they are significant in any culture that organized their beliefs about the stars. 

Two examples:

Canst thou bind together the brilliant Pleiades?

                                                                        —The Book of Job

Many a night I saw the Pleiads, rising thro’ the mellow shade,

Glitter like a swarm of fireflies tangled in a silver braid.

                                                                        —Tennyson, Locksley Hall



Constellation Gemini

Gemini and the Geminid Meteor Shower

Around mid-month the zodiacal constellation Gemini rises shortly after sunset.  If you wait till 8 o’clock it will have risen enough to identify it easily.

The most prominent features of the constellation are the two bright stars Castor and Pollux that represent the heads of two brothers standing side by side.  As the map illustrates, from each of these stars a line of four or five stars extends to the right.  These represent the body, legs and feet of the brothers.

As marked on the map, near the star Castor is the radiant of the Geminid meteor shower that occurs several days before and after the night of December 13th and 14th.  The radiant refers to the apparent location in the sky where the meteors seem to emerge. The Geminids are the most prolific meteor shower of the year and as long as the moon doesn’t interfere and you are in a reasonably dark location, you will see several dozen meteors an hour.

That’s the good news.  The challenging news is that this is December and it is cold.  Unless you dress very warmly you will soon find your teeth chattering and your patience disappearing. Here are some observation tips.

  • It’s best to use a chaise lounge lawn chair. This keeps your neck from getting stiff and orients you towards the sky.
  • Use a sleeping bag to keep yourself warm in addition to a down jacket, hat, gloves, and thermal underwear. Astronomer Michael Payne adds a blanket to the sleeping bag just to be sure.
  • Scan the sky in the east and overhead. As your eyes become sensitive to the dark you will soon begin to see the meteors.  After a bit of time your eyes will start picking up the movement of fainter meteors. The Geminids can have very long bright meteors with trails that take several seconds to dissipate.
  • If you have a good pair of binoculars, use this occasion to look at the Pleiades and Hyades. They are stunning with even the smallest optical aid.

We hope you enjoyed your winter Night Sky Tour. Have a healthy holiday season.