This is the Saint Louis Science Center’s NIGHT SKY UPDATE for the week of Tuesday, December 20.  All times are given as local St. Louis time (Central Standard Time).  For definitions of terminology used in the night sky update, click the highlighted text.

Information updated weekly or as needed.

Join us for our next star party, Friday, January 6, 2017 held in association with the St. Louis Astronomical Society. For details, see the information at the bottom of this page.

The Sun and the Moon

Sunrise is at 7:15 a.m. on Tuesday, December 20 and sunset is at 4:43 p.m. providing us with roughly 9.5 hours of daylight.  Even after sunset, the light from the Sun will still dimly illuminate our sky for about 1.5 hours.  This period of time is called twilight, which ends around 6:18 p.m. this week.  For those with a sun dial, solar transit or local noon occurs around 11:59 a.m. this week.

The first day of winter will occur this week on December 21. This day is called the Winter Solstice which has been celebrated by ancient and modern cultures around the world. The winter solstice marks the day when one end of Earth’s axis is pointed away from the Sun due to Earth’s tilt. For northern hemisphere observers this occurs in December and for those in the southern hemisphere it occurs in June.

It might come as a surprise that our winters in the northern hemisphere coincide with Earth’s nearest point to the Sun. This point is called perihelion which occurs on January 4, 2017. At perihelion the Earth is roughly 91 million miles from the Sun. To learn more about the solstice and Earth’s seasons visit




 20 Dec

7:15 a.m.

4:43 p.m.

 21 Dec

7:15 a.m.

4:43 p.m.

 22 Dec

7:16 a.m.

4:44 p.m.

 23 Dec

7:16 a.m.

4:45 p.m.

 24 Dec

7:17 a.m.

4:45 p.m.

 25 Dec

7:17 a.m.

4:46 p.m.

 26 Dec

7:18 a.m.

4:46 p.m.

 27 Dec

7:18 a.m.

4:47 p.m.

 28 Dec

7:18 a.m.

4:48 p.m.


Moonrise for Tuesday, December 20 occurs at 11:59 a.m. on the preceding day and moonset will occur at 11:58 a.m.  On Tuesday, December 20 the Moon will be exhibiting its last quarter phase with about 50% of the lunar disk illuminated.

International Space Station (ISS) Observing

Visible passes of ISS from St. Louis for the week of December 20 occur in the morning and evening hours. The best of these occurs on the evening of December 20. Use the table below for information about this and other visible passes of ISS this week.

Catch ISS flying over St. Louis starting Tuesday, December 20



Max. altitude


Time Alt. Az. Time Alt. Az. Time Alt. Az.
20 Dec -0.6 18:06:04 10 W 18:08:41 22 SW 18:11:17 10 S
21 Dec -1.9 17:13:38 10 WNW 17:16:47 43 SW 17:19:56 10 SSE
28 Dec -0.2 06:17:30 10 S 06:19:50 19 SE 06:22:11 10 E

Magnitude (Mag): The Measure of brightness for a celestial object.  The lower the value is, the brighter the object will be.

Altitude (Alt):  The angle of a celestial object measured upwards from the observer’s horizon.

Azimuth (Az):  The direction of a celestial object, measured clockwise from an observer’s location with north being 0°, east being 90°, south being 180° and west being 270°.

For information about ISS flyovers and other visible satellites, visit

Detailed information regarding all unmanned exploration of our universe, missions past, present, and planned, can be found at Jet Propulsion Laboratories:

The Planets Visible Without a Telescope


Venus can be seen low in the west about 20 minutes after sunset in the southwestern sky.  Venus sets by 8:13 p.m.


The Red planet is currently found in Aquarius rising before the Sun and can be easily seen 30 minutes after sunset in the southwestern sky.  Mars sets by 9:35 p.m.


The largest planet in our solar system has started another apparition and can be found rising in the east at 2:01 a.m.  It is currently found in the constellation Virgo and is best seen in the eastern sky between the hours of 3:00 a.m. and 6:00 a.m.

Uranus and Neptune 2016

Both of the outer gas/ice giants are once again nicely placed in our evening skies.  Both have also reached opposition and as such will rise before the Sun sets.  Both planets will require binoculars to see and as such it will be best to wait until the end of twilight to start your search.  Uranus can be found in the constellation Piscis and Neptune is found in Aquarius.

The brighter of the two planets is Uranus shining with an apparent magnitude of 5.8.  Neptune’s apparent magnitude of 7.9 is within reach of binoculars but light pollution will make it difficult.  For up to date maps of the location of Uranus and Neptune follow the link below or use the free planetarium software Stellarium.

Deep Sky Object of the Week

A few years ago the night sky update included a section that highlighted one constellation a month and a few objects of interest inside of it.  Unfortunately of the 88 constellations there are only about 60 we can see in St. Louis and of these there are only so many that have enough objects to reference that would interest both beginners and advanced observers.

In 2015 we changed things a bit for this section and instead of highlighting one constellation for each month we highlighted one Messier object a week using the Astronomical League’s Binocular Messier program as our guide.  We will continue this into 2016 but instead of using the Astronomical League’s binocular Messier program we will use their Binocular Deep Sky Program.

The Astronomical League is an amateur astronomy society that is composed of over 240 local amateur astronomy societies across the United States and includes members at large and other supporting members.  Their goal is to promote the science of astronomy through education, incentive and communication.

One of the many ways the Astronomical League has assisted amateur astronomers around the world is by creating various observing programs that highlight different aspects of astronomy and how an amateur astronomer can observe the sky and learn more about astronomy in doing so.  Some of these programs are introductory and are targeted at those beginning to learn about observational astronomy and some are extremely advanced, require specialized equipment and require a large amount of time to complete.  These observing programs cover most any type of object or way to observe the sky so everyone should be able to find one that matches their interests and abilities.

It is important to note that these programs are not part of any class or lecture series but are rather lists of objects that highlight types of objects or observing methods relevant to astronomy.  Taking part in these programs is done under one’s own choice.  To officially complete each program you do have to be a member of the Astronomical League but you do not have to join to use them as observing guides or education tools.  I would urge anyone interested in astronomy to look at these programs as they will help organize observing sessions and will help refine observing skills you already have.  You can find out more information about the Astronomical League’s observing programs here

The observing program we will use to help guide us through the 2016 observing year is the Binocular Deep Sky Program.  This is a program that will appeal to both beginning and advance observers.  Too often it is thought that you need to have a telescope for astronomical observations.  Telescopes make great observing tools and yes they can show you more than binoculars but they do have their limitations.  Cost, size, weight and complexity will often be a surprise to people when they first learn about telescopes.  These factors can keep people from using telescopes they own or from buying one at all.  The best advice to follow is a good observing tool is one you will use.

The binocular deep sky program is an introduction to deep sky objects beyond the Messier catalog.  It will introduced viewers to the New General Catalog (NGC) and some of the lesser known deep sky catalogs such as the Stock, Collinder and Melotte.  Each week we will highlight a deep sky object that is part of the Astronomical League’s binocular deep sky program.  This program is tougher than Messier binocular program as the objects are not always as obvious and they will require larger binoculars than the Messier program.  50mm binoculars are recommended but many of the targets can be seen through smaller aperture instruments.

I would urge each observer to fulfill the requirements of the program even if you do not intend to join the League for completion.  The requirements involve logging observing data that can help refine observing skills that will be useful later down the road.  If you have an interest in astronomy and learning more about observational astronomy I would also recommend checking out one of the two excellent astronomy societies near St. Louis.  Both are members of the Astronomical League and both do numerous public observing nights around town.  These clubs are the

St. Louis Astronomical Society and

Astronomical Society of Eastern Missouri

If you do not live in the St. Louis, Missouri area chances are you have similar astronomical societies where you live.

The deep sky object for the week of November 29 is the open star cluster Trumpler 2 (Tr 2).  This open star cluster is part of the Trumpler catalog of star clusters studied by astronomer Robert Julius Trumpler.  He is famous for studying the magnitudes of distant star clusters and noting that they appeared redder than expected.  This was found to be caused by interstellar dust that absorbed some of the light from these more distant stars.  Trumpler is also the astronomer that developed the Trumpler classification system used to categorize open star clusters.  This catalog remains in use today and is a common reference for the night sky update.

Tr 2 is a group of 20 stars estimated to be 78 million years old.  At a distance of 2,000 light years Tr 2 shines with an apparent magnitude of 5.9 making it an easy target for binoculars.  Its Trumpler classification is III, 2, p meaning it is detached from background stars with no central concentration, it has a moderate range of stellar magnitudes and it has a poor population with less than 50 members.

Tr 2 is found in the constellation Perseus which is easily found high in the north northeast by 6:00 p.m.  It is easiest to find our target by locating the constellation Cassiopeia first.  Cassiopeia appears as a bright W-shape of stars and can be found in the northeast by 6:00 p.m. Next step is to identify the stars Gamma and Delta Cassiopeiae and start hop from them to the southwest. Following this path for roughly 15° will bring you to a bright star called Eta Persei.  As you followed this path you should have seen the Perseus Double Cluster (NGC 869 and NGC 884).  If you can find these Tr 2 is about halfway between them and Eta Persei.

Tr 2 will appear as a loose group of stars that covers a 19’ (arc minute) area of sky.  For comparison that is a little more than half of the Moon’s apparent size in the sky.  Due to its large scattered appearance it will be helpful to use a planetarium software to help track down the exact location.  For a start use the maps linked below.

The deep sky object for the week of December 6 is the open star cluster Trumpler 3 (Tr 3).  Like last week’s star cluster Tr 3 is part of the Trumpler catalog of open star clusters.  It is a rather obscure object that will be difficult to see not due to its brightness but rather due to its scattered appearance.  Tr 3 is made up of 30 stars that are thought to be roughly 70 million years old.  At a distance of 2,250 ly Tr 3 shines with an apparent magnitude of 7.0.  The Trumpler classification for Tr 3 is III, 3, p which means it is detached from background stars with no concentration, It has a wide range of stellar magnitudes and it has a poor stellar population having less than 50 stars.

Tr 3 is located in the constellation Cassiopeia. To begin your search look north around 6:00 p.m. and you will find the W-shape of Cassiopeia.  Next use the attached map to identify the stars Gamma and Epsilon Cassiopeiae.  Once you have identified these stars use those to star hop to the west and you will find the Tr 6 about 6° from Epsilon Cassiopeiae.  This will be a difficult cluster to identify due to its scattered nature.  It covers about 22 arc minutes of sky which about 2/3 of the full moon’s angular appearance.  In addition to maps I will link an image of Tr 3 for a visual reference.  It is also recommended that you use a desktop planetarium software to help locate Tr 3.

The deep sky object for the week of December 13 is the open star cluster Melotte 20 (Mel 20).  Another name for Mel 20 is the Alpha Persei Moving Group.  Moving groups of stars are the remnants of stellar associations the drift through the galaxy.  The namesake for this moving group is the bright star Mirfak located in the constellation Perseus.  At a distance of 601 light years Mel 20 shines with an apparent magnitude of 1.2.  This puts it well within naked eye visibility even in the city.  Visually it will appear as a bright scattering of stars that has a pine tree shape.  To see the pine tree shape you will need to use your binoculars.  The bright star Mirfak is near the top of the tree shape.

To find Mel 20 you need to locate the constellation Perseus.  This is easily done by first looking for the W-shape of Cassiopeia high in the north northeastern sky by 6:00 p.m.  Once you have found Cassiopeia follow the stars Gamma and Delta Cassiopeiae to the southwest.  Not long after you do this you will encounter a long bright chain of stars.  This is the constellation Perseus.  The first bright star you will find in Perseus is Eta Persei followed by Gamma and then Alpha or what will be labeled as Mirfak.  Once here you will find a number of bright stars that are well detached from background stars.  This is Mel 20.  Use the maps and image below to help identify this bright open star cluster.

The deep sky object for the week of December 20 is the open star cluster NGC 1342.  This star cluster is found in the constellation Perseus at a distance of 1,800 light years.  There are 40 stars known to be part of this star cluster which formed about 300 million years ago. With a distance of 1,800 light years NGC 1342 shines with an apparent magnitude of 6.7.  Its Trumpler classification is III, 3, p meaning it is well detached from background stars with no concentration, it has a wide variance of stellar magnitudes and it has a poor stellar population having less than 50 stars.

To find NGC 1342 you will have to find the constellation Perseus.  The easiest way to do this is first find the constellation Cassiopeia.  She can be found high in the northern sky by 6:00 p.m. and is distinguished by a bright W-shape of stars.  Once here look for a long bright chain of stars that runs from her west side down towards the southeast.  This bright chain of stars is Perseus.  It appears to run from Cassiopeia all the way to the Pleiades star cluster in Taurus the Bull.  Once you have identified Perseus use the map linked below to identify the Stars Menkib and Algol.  Once you can do this grab your binoculars and scan back and forth between these stars.  About halfway between them is where you will find the star cluster NGC 1342.  Use the links below help in locating NGC 1342.

Our next Star Party will be held on Friday, January 6, 2016, from dusk until 10 p.m.

As part of the Saint Louis Science Center’s First Fridays, weather permitting, the St. Louis Astronomical Society and the Science Center will set up a number of telescopes outdoors and be on-hand to answer your questions.  Telescope viewing begins once it is dark.  Regardless of the weather on January 6, join us indoors in our planetarium theater for “The Sky Tonight”.  Showtime is at 7 p.m.

This free, indoor star program will introduce you to the current night sky, the planets, and the seasonal constellations. Doors open 15 minutes before show time. Shows begins at 7 p.m. Sorry, no late admissions due to safety issues in the darkened theater.

The St. Louis Astronomical Society helps host the monthly Star Parties at the Saint Louis Science Center which are held on the first Friday of each month. Our Monthly Star Parties are open to the public and free of charge.