This is the Saint Louis Science Center’s NIGHT SKY UPDATE for the week of Saturday, September 23, 2023.
Information updated weekly or as needed.
Times given as local St. Louis time which is Central Daylight Time (CDT). 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 -5 hours when CDT.
Public Telescope Viewings
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 October 6 , join us indoors in our planetarium theater for “The Sky Tonight”. Showtime is at 7 p.m. There is no First Friday event in July.
Observing Highlight of the Week
Annular eclipse seen from Hsinchu City, Taiwan on May 21, 2012. Image credit: Teddy 190902.
On September 23, 2023, the Sun will cross our celestial equator at its descending node. For us in the northern hemisphere, the September equinox marks the start of fall, while in the southern hemisphere it is the beginning of spring. The Sun crosses the celestial equator twice each year, first in March and later in September. During an equinox, the Sun illuminates both hemispheres equally.
An equinox is when the Sun will rise and set due east to west. If you watch sunrise or sunset throughout the year, you will quickly see that the Sun does not rise or set in the same place each day. Because Earth is tilted on its rotational axis, the sub-solar point on Earth shifts north half of the year and then shifts south for the other half. The most northern and southern points of this behavior are the June and December solstices. Our ancestors were well aware of the Sun’s annual shift long before modern civilization existed, which led to many great discoveries.
These discoveries were a product of careful observation of celestial mechanics along with the behaviors of several individual objects that moved independently from background stars.
Watching the sky taught us it behaves in predicable ways which in turn led to several great discoveries. Time keeping, the cycle of seasons and the discovery of our solar system are just a few examples.
Eclipses are another astronomical phenomenon we understand thanks to careful observations of how the Moon and Sun behave. The oldest possible record of a solar eclipse is found in Ireland at an ancient site called Loughcrew. This site that dates back to the 4th millennium BCE, contains several cairns or passage burials. Inside Cairn L is a petroglyph that contains two sets of concentric circles with one overlapping the other. As Loughcrew appears to have been used for time keeping and tracking the seasons, the concentric circle design has been interpreted as a solar motif. Are the two sets of concentric circles meant to indicate the Moon passing in front of the Sun; we will never know for sure, but it has been calculated that on November 30, 3340, a solar eclipse would have been visible at Loughcrew.
Over time, ancient humans discovered that lunar and solar eclipses occur over a cycle which today we call a saros. The next solar eclipse visible in North America is annular eclipse occurring on October 14, 2023. Most will only see a partial eclipse on this date, but those willing to travel can catch a glimpse of the annular eclipse. If you travel to a narrow path which runs from Oregon to Texas, you will see annularity. An annular and total solar eclipse differ due to the variation in the Moon’s distance from Earth. As the Moon orbits Earth, it has a far and near point. These are called perigee (near) and Apogee (far). From our view on Earth, this causes the Moon to look slightly larger or smaller. When near perigee, the Moon looks roughly the same size as the Sun. This makes it possible for the Moon to completely cover the solar disk at new moon. When an annular eclipse occurs, the Moon is near apogee and as such it looks smaller than the Sun. The result is a ring of sunlight remains visible. The last annular solar eclipse visible from St. Louis occurred on May 10, 1994.
On October 14, 2023, those viewing from St. Louis, Missouri, will see close to 54% of the Sun eclipsed. While it is not the best view it will still be spectacular. In the upcoming weeks we will continue to explore the October 14 eclipse to help prepare people interested in viewing the eclipse. One of the best resources available to us is the Missouri eclipse task force. Other states have similar resources and there is a national task force as well. Below you will find links for both the Missouri and national eclipse task force.
https://eclipse.aas.org/about-us
Also, of note this week, the NASA mission called OSIRIS-Rex is scheduled to return samples collected from the surface of an asteroid called Bennu. This is expected to occur on the morning of September 24, 2023. NASA-tv coverage begins at 9:00 a.m. CST. You can learn more about OSIRIS-Rex at https://www.nasa.gov/osiris-rex
The Sun and Moon
The Moon as seen from the International Space Station, on July 31, 2011.
Credit: NASA
Sun
Sunrise is at 6:49 a.m. on Saturday, September 23 and sunset is at 6:56 p.m. providing us with roughly 12 hours of daylight. 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 8:26 p.m. this week. For those with a sundial, local noon occurs around 12:53 p.m. this week.
The northern hemisphere’s autumnal equinox occurs on September 23, 2023, at 1:49 a.m. At this time, the Sun will cross the celestial equator at the descending node. Later this week on September 26, 2023, equilux occurs. This term refers to the day with roughly equal amounts of day and night. This does not occur on the equinox due to atmospheric distortions. You can learn more about the difference between equinox and equilux at
https://www.timeanddate.com/astronomy/equilux.html
| Day | Sunrise | Sunset | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 23-Sep | 6:49 a.m. | 6:56 p.m. | ||||||||||
| 24-Sep | 6:50 a.m. | 6:55 p.m. | ||||||||||
| 25-Sep | 6:51 a.m. | 6:53 p.m. | ||||||||||
| 26-Sep | 6:52 a.m. | 6:52 p.m. | ||||||||||
| 27-Sep | 6:53 a.m. | 6:50 p.m. | ||||||||||
| 28-Sep | 6:54 a.m. | 6:48 p.m. | ||||||||||
| 29-Sep | 6:55 a.m. | 6:47 p.m. | ||||||||||
| 30-Sep | 6:55 a.m. | 6:45 p.m. | ||||||||||
| 1-Oct | 6:56 a.m. | 6:44 p.m. |
Moon
Moonrise for Saturday, September 23 is at 3:35 p.m. and moonset occurs at 12:39 a.m. on the following day. On Saturday, September 23, the Moon will exhibit a waxing gibbous phase with 61% disk illumination. Full moon occurs on September 29, 2023, at 4:58 a.m.
This year, September’s full moon is called the Harvest Moon. This is because it is the nearest full moon to the autumnal equinox. Most years, the Harvest Moon is in September, but it can occur in October.
International Space Station (ISS) Observing
There are several visible passes of ISS from St. Louis for the week of September 23. They occur during evening hours. Use the table below for information about the best of these passes.
Catch ISS from St. Louis starting Saturday, September 23
| Date | Starts | Max. altitude | Ends | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Time | Alt. | Az. | Time | Alt. | Az. | Time | Alt. | Az. | ||
| 29 Sep | -2.5 | 20:05:20 | 10 | NNW | 20:08:16 | 27 | NNE | 20:08:38 | 26 | NE |
| 30 Sep | -1.7 | 20:53:36 | 10 | WNW | 20:55:23 | 29 | WNW | 20:55:23 | 29 | WNW |
| 01 Oct | -3.8 | 20:05:21 | 10 | NW | 20:08:41 | 69 | NE | 20:09:17 | 53 | 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°.
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 11:00 p.m. on September 23, 2023. Credit: Stellarium, EG
Looking southeast at 5:30 a.m. on September 24, 2023. Credit: Stellarium, EG
This week, three naked eye planets will be visible. Saturn will be found in the southeast once it is dark. Jupiter is easy to find in the east by 11:00 p.m. Venus will be easy to find in the east before sunrise.
Venus
Venus rises this week around 3:30 a.m. It will be easy to spot for most by 5:30 a.m. If you have a clear view of the planet, small telescopes will reveal a crescent phase with roughly 31.5% disk illumination.
Jupiter
Jupiter rises at 8:52 p.m. About one hour after Jupiter rises, it should be easy to spot. Jupiter will rise about 30 minutes earlier each week. The ideal time to start looking for Jupiter will be after 11:00 p.m.
Saturn
Start looking for Saturn in the southeast 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.
James S. McDonnell Planetarium
Night Sky Update: September 23-October 1, 2023