Skywatcher's Guide: December 2020 and January 2021

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Stars and Constellations

In December, the summer constellations are still visible for the first few hours of the night towards the west.  The Summer Triangle is still fairly high, with Vega heading to the west-northwest, Deneb closest to the zenith, and Altair in the west-southwest.  The center of the Milky Way is now below the horizon, but there is still a good portion of our galaxy we can see streaking high across the sky. The fall sky is now very prominent, getting high in the eastern sky.  The "W" of Cassiopeia is very high in the north-northeast. In the absence of the Big Dipper (part of our spring sky) Cassiopeia can be used to locate the north star: The top (open side) of the "W" faces to the north, so in that direction look for a star about the same brightness as the main stars in Casssiopeia, and that will most likely be Polaris.  Next, the Great Square of Pegasus is nearly in the middle of the sky.  Andromeda is nearby in the northeast, with Perseus just below.  There is also a fairly bright star called Fomalhaut in the south, though its constellation Piscis Austrinus is not easy to distinguish.  Finally, the winter sky is beginning to come up along the eastern horizon.  Taurus the bull is now up in the east with the bright star Aldebaran and the Pleiades (aka the seven sisters or Subaru) star cluster.  Auriga the charioteer is also up in the northeast, with the bright star Capella.

In January, the summer constellations are now very low in the west.  Deneb is the highest point of the Summer Triangle in the northwest, with Vega below near the horizon, and Altair more towards the west.  The Milky Way continues to streak across the sky, though the summer portion is now giving way to the winter portion.  The fall constellations are now right in the middle of the sky.  More of the winter constellations are now up, most notably Orion the hunter in the east below Taurus.  Gemini the twins are also up in the east-northeast just below Auriga.

Interesting Stars Visible in December and January (during observatory hours)

Name / Designation Apparent Magnitude
(lower = brighter)
Distance
(light-years)
Notes
Sirius -1.44 8.6  
Vega 0.03 25  
Capella 0.08 42  
Rigel 0.18 770  
Procyon 0.4 11  
Betelgeuse 0.45 427  
Altair 0.76 17  
Aldeberan 0.87 65  
Pollux 1.16 38  
Markab 1.25 140  
Deneb 1.25 3230  
Regulus 1.36 77 means "Little King"
Castor 1.58 52  
Polaris 1.97 431  
Alpheratz or Sirrah 2.07 97  
Mirach 2.07 199  
Algol 2.09 93 variable star
Enif 2.38 670  
Almak 2.1 / 5.0 & 6.3 355  triple star system w/ 64 yr orbit
Albireo 3.2 / 5.8 & 5.1 390 / 380  possibly a triple star system
Eta Cassiopeiae 3.5 / 7.4 19 480 yr orbit

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Solar System

Mercury will be passing behind the Sun in December, but may be visible in the evening sky during the second half of January.

Venus is prominent in the eastern sky every morning, getting closer to the Sun each day.

Mars is visible high in the evening sky, moving through Pisces and Aries.

Jupiter and Saturn are visible right after sunset throughout December and the beginning of January, but will pass behind the Sun by the end of January.

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Calendar of Night Sky Events

Date Event
12/07/20 Last Quarter Moon.
12/13/20 Peak of Geminids meteor shower.
12/14/20 New Moon and total solar eclipse — Not visible from Tucson..
12/19/20 Mercury at superior conjunction. — Passing behind the Sun.
12/21/20 Southern Solstice. — Official beginning of our winter.
12/21/20 First Quarter Moon.
12/21/20 Appulse of Jupiter and Saturn. — Separated by 0.1°
12/29/20 Full Moon
01/02/21 Earth at perihelion. — Our closest approach to the Sun for the year.
01/03/21 Peak of Quadrantids meteor shower.
01/06/21 Last Quarter Moon.
01/09/21 Appulse of Mercury and Saturn. — Separated by 1.7°
01/09/21 Appulse of Mercury and Jupiter. — Separated by 1.5°
01/12/21 New Moon.
01/14/21 Pluto at conjunction. — Passing behind the Sun.
01/20/21 First Quarter Moon.
01/21/21 Appulse of Mars and Uranus. — Separated by 1.7°
01/23/21 Mercury at greatest eastern elongation. — Visible in the evening sky.
01/23/21 Saturn at conjunction. — Passing behind the Sun.
01/28/21 Full Moon.
01/28/21 Appulse of Venus and Pluto. — Separated by 0.7°
01/28/21 Jupiter at conjunction. — Passing behind the Sun.

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Deep Sky

The summer Milky Way is now partially below the horizon at the beginning of the night, but the winter Milky Way is getting more and more prominent.  There are many open star clusters that can be seen with only binoculars scanning this part of the sky.  For example we have the asterism of the Coathanger between Aquila and Cygnus in the fainter constellation of Vulpecula.  The Pleiades (M45) is visible naked-eye in the east just above the face of Taurus which itself is another cluster called the Hyades.  We also have the Double Cluster in Perseus high in the north-northeast.  Finally, the constellation of Auriga in the northeast contains M36, M37, and M38 in close proximity.

There aren't as many globular clusters we can see this time of year, but there are still a few.  M15 is visible near the head of Pegasus in the west-southwest.  The next brightest one is M2 in Aquarius to the southwest.

For nebulae, we have several in the plane of the galaxy, one of which is the North America Nebula (C20) to the west-northwest in Cygnus.  The spectacular Orion Nebula (M42) is now just rising at the beginning of the night in the east. For planetary nebulae we have the Ring Nebula (M57) in Lyra, the Dumbell Nebula (M27) in Vulpecula, and the Blue Snowball (C22) in Andromeda.

And now the galaxies:  Our neighbor the Andromeda Galaxy (M31) is now very high in the middle of the sky and is visible on dark nights with the naked eye.  Also nearby is the Triangulum Galaxy (M33), visible with binoculars.

Interesting Deep Sky Objects to Observe during December and January (during observatory hours)

Designation Name Apparent Magnitude Apparent Size Distance
(light-years)
Type
Messier 45 Pleiades 1.6 110' 440 open cluster
Messier 31 Andromeda Galaxy 3.4 3° x 1° 2,900,000 spiral galaxy
Messier 44 Beehive Cluster 3.7 95' 577 open cluster
Messier 42 Orion Nebula 4 85' x 60' 1400-1600 diffuse nebula
Messier 33 Triangulum Galaxy 5.7 67' x 42' 3,000,000 spiral galaxy
NGC 7293 Helix Nebula 7.3 16' 450 planetary nebula
Messier 27 Dumbbell Nebula 7.4 8' × 6' 1,250 planetary nebula
NGC 7009 Saturn Nebula 8 36" 2,400 planetary nebula
Messier 81 Bode's Galaxy 8.5 21' 1,200,000 spiral galaxy
Messier 57 Ring Nebula 8.8 1' 2,300 planetary nebula
Messier 82 Cigar Galaxy 9.5 14' 1,200,000 galaxy

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Frequently Asked Questions

What's so great about the Great Conjunction?

If you haven't seen it for yourself, you've probably heard about the Great Conjunction of Jupiter and Saturn.  Conjunctions happen all the time, so why is this one special?  First off, conjunctions are when two objects appear to come close together in the sky.  Since our solar system is very flat, the planets pass each other on a regular basis as they progress in their orbits around the Sun.  But the farther out they are the slower they appear to move, so Jupiter and Saturn being the farthest naked-eye planets pass each other the least often.  They only pass each other once every 20 years or so.  (Occasionally they pass each other three times in a single year, but this is a special case I'll get to later.)  So in general, conjunctions of Jupiter and Saturn are called "great" because they are the most rare of naked-eye planetary conjunctions.

But there's more to it.  Not every conjunction is equal in closeness.  I said the solar system is very flat, but it is not perfectly flat.  Each planet has a small inclination that takes them above and below each other's orbital plane, so the closeness of a conjunction depends on how close it happens to be to the point where the planets' orbit planes cross.  This time Jupiter and Saturn are very close to this point, so the conjunction is one of the best we've seen for a long time.  In fact, the last time they were this close together was 1623.  However, that conjunction also happened to be close to the Sun and may not have been observable, meaning the last time we could actually see Jupiter and Saturn this close together was in 1226!  Luckily, however, we only need to wait till 2080 to see them pass this close again.

You may have also heard reference to this event being called the "Christmas Star" or "Bethlehem Star".  There was a great conjunction in 7 BC, which is one of the explanations for the famous phenomenon, but in reality we don't really know for sure what it was.  There was also a particularly close conjunction of Venus and Jupiter in 2 BC that has been proposed as a candidate, and other possible events as well, but the evidence is just not conclusive.  Still, since it happens to fall near Christmas this year, it is easy to make the association.

Ok, I said I'd explain how Jupiter and Saturn might pass each other three times within a year, when there are normally 20 years in between these events.  The reason is retrograde motion.  Because the Earth is moving too as we are watching the other planets go around the Sun, sometimes their motion appears to go in reverse because of our difference in speeds.  If a conjunction happens just before this period starts, they will likely pass each other again as they appear to change directions, and then pass a third time when it is over.  The last time this happened was 1981, and the next time will be 2239.

If you have any questions you'd like me to answer in the next issue of SWG, please let me know.  I'm also happy to take suggestions or comments, and also pictures if you'd like to send them.  Happy viewing!

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Bibliography

  • Cornelius, Geoffrey. The Starlore Handbook: an Essential Guide to the Night Sky. San Francisco, CA: Chronicle, 1997. Print.
  • Ottewell, Guy. Astronomical Calendar 2012. Raynham, Mass: Universal Workshop, 2011. Print.
  • Ottewell, Guy. Astronomical Calendar 2013. Raynham, Mass: Universal Workshop, 2012. Print.
  • Ottewell, Guy. The Astronomical Companion. 2nd ed. Raynham, Mass: Universal Workshop, 2010. Print.
  • Astronomy Magazine. February 2013. Volume 41, No 2.
  • Astronomy Magazine. February 2013. Volume 41, No 3.
  • Sky & Telescope. February 2013. Volume 125, No 2.
  • Sky & Telescope. March 2013. Volume 125, No 3.

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Date of publication:
2020