This is the Saint Louis Science Center’s NIGHT SKY UPDATE for the week of Friday, December 17, 2021.
Information updated weekly or as needed.
Times given as local St. Louis time which is Central Standard Time (CST). For definitions of terminology used in the night sky update, click the highlighted text. If relying on times posted in Universal Time (UT), St. louis is -6 hours when CST.
Public Telescope Viewings
With the changing recommendations from the CDC regarding COVID-19, conversations regarding the return of star parties at the Saint Louis Science Center have begun. We are close to bringing back our public telescope viewings, but a few details still need to be worked out. We will post future updates as we learn more about when we can bring back telescope viewings.
Observing Highlight of the Week
The Sun at local or solar noon on the summer and winter solstice and the fall and spring equinox. The winter solstice is the day the Sun appears the lowest each year and as such did not clear the planetarium building where the image was taken from. Image credit: Eric Gustafson.
This week the Sun will appear its lowest in the sky for the year. This day is called the winter solstice which occurs on December 21, 2021, at 9:59 a.m. Due to Earth’s axial tilt, the northern hemisphere is inclined in a way that limits the amount of direct sunlight we receive. This results in less hours of daytime and a decrease in temperature.
Humans have long been in sync with the Sun’s seasonal cycle. Observing the Sun’s position in the sky was how we learned to track the seasons which became critical knowledge as many societies became agrarian. If you note the Sun’s position each day for a year, you will see a consistent increase or decrease in altitude depending on the season we are headed towards. The winter solstice is lower extreme of the Sun’s annual apparent shift, and the summer solstice is the upper extreme. This high low dance occurs annually because of Earth’s axial tilt and Earth’s revolution of the Sun. Observing the Sun’s apparent motions is how many people learned to understand the changing world. As a result, many traditions celebrated in the world are connected to this cycle of seasons. Yule, Lunar New Year and Soyal are a few examples of traditions determined by the occurrence of the winter solstice.
Many ancient sites still stand today where ancient humans tracked the Sun’s behavior. Stonehenge in England and Pueblo sites in Chaco Canyon are a few famous examples of this. Cahokia Mounds is another place humans were tracking the Sun. If you visit the site today, a structure called Woodhenge was reconstructed on the site where it originally stood. Like Stonehenge, the occupants of Cahokia Mounds used Woodhenge to note where the Sun was rising each day. Typically, the museum hosts an observance of the winter solstice each year on the Sunday nearest the date of the solstice. This year December 19, 2021 is when the winter solstice will be observed at Woodhenge.
If you are interested in observing the sun’s annual shift in the sky, there are several ways to do this. If you have a clear eastern horizon, you can watch were the Sun rises each day. The same could be said for the point of sunset if your western horizon is better. Another simple way is to plant a gnomon in your yard. Gnomons are a simple shadow caster. Any kind of post stuck in the ground will cast a shadow that can be used to study the Sun’s apparent motions. A more complex option is a sundial. To learn more about sundials, I recommend visiting the North American Sundial Society’s website.
The Sun and Moon
The Moon as seen from the International Space Station, on July 31, 2011.
Credit: NASA
Sunrise is at 7:012 a.m. on Friday, December 17 and sunset is at 4:41 p.m. providing us with roughly 9.5 hours of daylight. Even after sunset, the light from the Sun will dimly illuminate our sky for roughly 1 hour and 30 minutes. This period is called twilight, which ends around 6:17 p.m. this week. For those with a sundial, local noon occurs around 11:57 a.m. this week.
| Day | Sunrise | Sunset |
|---|---|---|
| 2021-12-17 | 7:12 a.m. | 4:41 p.m. |
| 2021-12-18 | 7:13 a.m. | 4:42 p.m. |
| 2021-12-19 | 7:13 a.m. | 4:42 p.m. |
| 2021-12-20 | 7:14 a.m. | 4:43 p.m. |
| 2021-12-21 | 7:15 a.m. | 4:43 p.m. |
| 2021-12-22 | 7:15 a.m. | 4:44 p.m. |
| 2021-12-23 | 7:16 a.m. | 4:44 p.m. |
| 2021-12-24 | 7:16 a.m. | 4:45 p.m. |
| 2021-12-25 | 7:16 a.m. | 4:45 p.m. |
Moon
Moonrise for Friday, December 17 occurs at 12:31 p.m. and moonset will occur at 23:51 p.m. On Friday, December 17 the Moon will exhibit a waxing gibbous phase with 99% of the lunar disk illuminated. Full moon for December occurs on December 18, 2021, at 10:36 p.m.
International Space Station (ISS) Observing
Visible passes of ISS from St. Louis for the week of December 17 occur during morning hours. The best passes this week occur on the mornings of December 20 and 21. Use the table below for information about these and other visible passes this week.
Catch ISS from St. Louis starting Friday, December 17
| Date | Starts | Max. altitude | Ends | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Time | Alt. | Az. | Time | Alt. | Az. | Time | Alt. | Az. | ||
| 18 Dec | -1.7 | 06:05:27 | 10 | SSW | 06:08:26 | 30 | SE | 06:11:26 | 10 | ENE |
| 19 Dec | -0.8 | 05:18:52 | 10 | S | 05:21:09 | 17 | SE | 05:23:28 | 10 | E |
| 20 Dec | -3.9 | 06:06:53 | 10 | SW | 06:10:14 | 81 | NW | 06:13:37 | 10 | NE |
| 21 Dec | -3.2 | 05:22:24 | 50 | S | 05:22:50 | 55 | SE | 05:26:09 | 10 | ENE |
| 22 Dec | -0.6 | 04:37:50 | 14 | ENE | 04:37:50 | 14 | ENE | 04:38:29 | 10 | ENE |
| 22 Dec | -2.8 | 06:10:52 | 21 | WNW | 06:12:16 | 29 | NNW | 06:15:17 | 10 | NNE |
| 23 Dec | -2.4 | 05:25:58 | 30 | NNE | 05:25:58 | 30 | NNE | 05:28:00 | 10 | NE |
| 24 Dec | -1.9 | 06:13:52 | 15 | NW | 06:14:31 | 16 | NNW | 06:16:40 | 10 | NNE |
| 25 Dec | -1.4 | 05:28:37 | 15 | NNE | 05:28:37 | 15 | NNE | 05:29:30 | 10 | NNE |
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 www.heavens-above.com
Detailed information regarding all unmanned exploration of our universe, missions past, present, and planned, can be found at Jet Propulsion Laboratories:
The Visible Planets
Looking South, at 5:30 p.m. December 17, 2021. Credit: Stellarium, EG
Looking east, at 6:30 am, December 25, 2021. Credit: Stellarium, EG
This week, four naked eye planets are visible. Venus can be found in the south southwest after sunset. Jupiter and Saturn can be found in the south after sunset. Mars is still tough to see but it is steadily climbing out of the Sun’s glare in the east before sunrise.
Venus
Venus is nearing the end of another evening apparition. It will be visible in the southwest as the sun sets. Venus will set at 6:54 p.m. Venus is on its way back towards the Sun’s glare as it approaches inferior conjunction on January 8, 2022. Venus is now bright enough to be seen naked eye during the day. If you have never attempted to spot Venus in the day, near sunset is the time to begin. As always, use precaution when trying to find something in the day as the Sun will be in the sky. Remember to never look directly at the Sun.
Mars
A new apparition for Mars has begun. Mars is becoming visible before sunrise. Look for the Red Planet 9° above the eastern horizon around 6:30 a.m. Mars will continue to climb out of the Sun’s glare as we see the end of 2021. Mars reaches opposition next year on December 7, 2022.
Jupiter
Jupiter has passed opposition and as such will be visible after sunset. Look for Jupiter in the south as it starts to darken outside. Jupiter sets at 9:31 p.m. and will set about 30 minutes earlier each week. Jupiter reaches superior conjunction on March 5, 2022. This means we can count on seeing Jupiter in the southern skies for the rest of this year and well into 2022.
Saturn
Saturn is past opposition which means it will be visible about 20 minutes after sunset. Right now, it can be found in the south after sunset, but as we continue through the year, Saturn will set about 30 minutes earlier each night. Saturn sets at 8:03 p.m. Saturn will reach superior conjunction on February 4, 2022.
James S. McDonnell Planetarium
Night Sky Update: December 17 – December 25, 2021