This is the Saint Louis Science Center’s NIGHT SKY UPDATE for the week of Saturday, December 2, 2023.
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.
Observing Highlight of the Week
Mercury seen by NASA’s Messenger mission. Messenger’s mission lasted 10 years. It orbited Mercury for four years starting in 2011. The mission ended in 2015 when the spacecraft crashed into the surface of Mercury. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institution of Washington.
The highlight for this week is the planet Mercury. On December 4, 2023, Mercury reaches greatest eastern elongation. Elongation is the angle between an object in the solar system and the Sun. Greatest elongation is the maximum angular separation between an inferior planet and the Sun. When viewed from Earth, this pertains to the planets Mercury and Venus.
Inferior planets are those closer to the Sun than you are. From Earth this includes Mercury and Venus. From our perspective, they never stray far from where the Sun is. This is why Mercury and Venus are only seen in the east before sunrise or in the west after sunset. As a planet elongates from the Sun becoming visible, it starts an apparition. Inferior planets alternate between morning and evening apparitions exhibiting one of each during a synodic period. A synodic period is the interval of time between superior solar conjunctions. Venus’s synodic period is roughly 584 days while Mercury’s is about 116 days.
Mercury will exhibit six apparitions and a partial apparition each year. Unfortunately, they will not all be favorable. This depends on two factors, the inclination of the ecliptic to the horizon and Mercury’s eccentric orbit. The ecliptic is the Sun’s apparent path amongst the background stars. It represents the plane of Earth’s orbit, which is why it is often called the plane of the ecliptic. Throughout the year, the ecliptic meets the horizon at varying angles which vary seasonally. If the angle is low, Mercury’s elongation needs to be large for us to see it above the horizon. When the angle is nearly perpendicular, Mercury can be seen much higher in the sky. For observers in the northern hemisphere, the angle of the ecliptic at sunset is greatest near the vernal equinox and it is greatest at sunrise near the autumnal equinox.
The other factor which contributes to the quality of an apparition is the shape of Mercury’s orbit. Like all planets, Mercury’s orbit is elliptical. Because of this, there are two extreme points in Mercury’s orbit called perihelion and aphelion. These represent when Mercury is nearest and farthest from the Sun. The difference in distances between these extremes varies depending on the eccentricity of the orbit. Circular orbits have low eccentricity, so the difference is low, while non-circular orbits have higher eccentricity, and the difference is larger. Mercury’s orbit is the most non-circular in the solar system and its distance from the Sun at aphelion is over 50% greater than the distance at perihelion. Because of this, Mercury will appear farther from the Sun during apparitions that are near aphelion.
The current evening apparition of Mercury is not favorable. On the night of December 4, 2023, Mercury will only be about 6° above the western horizon, 30 minutes after sunset. To have any chance of spotting Mercury, you will ne a western horizon free of trees and buildings. Additionally, you want to start looking about 30 minutes after sunset as this is when it is just dark enough to spot the planet. Most of us will likely need to use binoculars to see Mercury as the contrast between it and the bright twilight sky will be low. Around 5:15 p.m., Mercury’s azimuth will be roughly 229° so you want to look southwest to find Mercury.
The next morning apparition of Mercury will be better than the current evening apparition. Mercury reaches maximum western elongation on January 12. 2024. On this night Mercury will be roughly 10° above the eastern horizon with an azimuth of 129°. You will find Mercury in the southeast this morning just below the bright planet Venus. The pair will be separated by roughly 11°. An easy way to estimate degrees in the sky is to use your hand. At arm’s length and one eye closed, your fist from side to side will cover about 10° of sky. While this is also not the best apparition, Mercury will be easier to see than the current apparition.
The Sun and Moon
The Moon as seen from the International Space Station, on July 31, 2011.
Credit: NASA
Sun
Sunrise is at 7:00 a.m. on Saturday, December 2 and sunset is at 4:40 p.m. providing us with roughly 9 hours and 40 minutes of daylight this week. Even after sunset, light from the Sun will dimly illuminate our sky for about 1 hour and 30 minutes. This period is called twilight, which ends around 6:14 p.m. this week. For those with a sundial, local noon occurs around 11:49 a.m. this week.
| Day | Sunrise | Sunset | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 2-Dec | 7:00 a.m. | 4:40 p.m. | ||||||||||
| 3-Dec | 7:01 a.m. | 4:39 p.m. | ||||||||||
| 4-Dec | 7:02 a.m. | 4:39 p.m. | ||||||||||
| 5-Dec | 7:03 a.m. | 4:39 p.m. | ||||||||||
| 6-Dec | 7:04 a.m. | 4:39 p.m. | ||||||||||
| 7-Dec | 7:05 a.m. | 4:39 p.m. | ||||||||||
| 8-Dec | 7:06 a.m. | 4:39 p.m. | ||||||||||
| 9-Dec | 7:07 a.m. | 4:39 p.m. | ||||||||||
| 10-Dec | 7:07 a.m. | 4:39 p.m. |
Moon
Moonrise for Saturday, December 2 was at 9:35 p.m. and moonset occurred at 12:05 p.m. the following day. On Saturday, December 2, the Moon will exhibit a waning gibbous phase with roughly 70% disk illumination. By the end of the week the Moon will exhibit a waning crescent phase with 5% disk illumination. Last quarter moon occurs on December 4, 2023, at 11:49 p.m.
International Space Station (ISS) Observing
There are several visible passes of ISS from St. Louis for the week of December 2. They occur during evening hours. The table below lists the best of these passes that will be seen from St. Louis. If you do not live in the area, you can use https://heavens-above.com/ to set your viewing location and get times for where you are.
Catch ISS from St. Louis starting Saturday, December 2, 2023
| Date | Starts | Max. altitude | Ends | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Time | Alt. | Az. | Time | Alt. | Az. | Time | Alt. | Az. | ||
| 04 Dec | -3.2 | 18:17:06 | 10 | WNW | 18:20:23 | 55 | SW | 18:21:45 | 30 | SSE |
| 05 Dec | -3.9 | 17:28:34 | 10 | NW | 17:31:56 | 79 | NE | 17:35:16 | 10 | SE |
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°.
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 southeast at 6:00 p.m. on December 3, 2023. Credit: Stellarium, EG
Looking east at 4:45 a.m. on December 4, 2023. Credit: Stellarium, EG
This week, three naked eye planets will be visible. Saturn and Jupiter are visible once it is dark with Saturn in the south and Jupiter in the east. Venus will be easy to find in the east before sunrise.
Venus
Venus rises this week around 3:31 a.m. It will be easy to spot for most by 4:30 a.m. If you have a clear view of the planet, telescopes will reveal a gibbous phase with roughly 68.9% disk illumination. Now that Venus is past dichotomy, it is beginning to head back towards the Sun from our perspective. As this continues, Venus’s disk illumination will increase as it heads towards superior conjunction on June 4, 2024.
Jupiter
Jupiter has passed opposition and as such it will rise before the Sun sets. Jupiter will be easy to spot in the east once it is dark outside. If you have considerable trees or buildings to the east, you may have to wait about an hour after sunset to spot this giant world. Jupiter will set around 4:08 a.m.
Saturn
Start looking for Saturn in the south about 30 minutes after sunset. If you have lots of trees or buildings, you may have to wait an hour or so after it rises before it will be visible. Saturn will set around 10:43 p.m.
James S. McDonnell Planetarium
Night Sky Update: December 2-10, 2023