This is the Saint Louis Science Center’s NIGHT SKY UPDATE for the week of Saturday, September 6, 2025.
Information updated weekly or as needed.
Times given as local St. Louis time this week will be in Daylight Saving Time. 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. Times posted in the Night Sky Update are in the 24-hour format.
Join us for our next star party, Friday, October 3, 2025, held in association with the St. Louis Astronomical Society.
For details, see the information at the bottom of this page or visit: https://www.slsc.org/explore/mcdonnell-planetarium/public-telescope-viewings
Observing Highlight of the Week
A comet discovered earlier this year might become bright enough to see from St. Louis in October. The comet’s name is C/2025 A6 (Lemmon) which currently appears in the constellation Cancer. Its current magnitude is roughly 10.3 making it a difficult target for those observing in light pollution. As the comet nears its closest approach to Earth on October 21, 2025, it is expected to reach a peak magnitude of 4.4. Keep in mind this is only an estimate and should not be taken as a guarantee.
How bright an object appears to us on Earth is a measure of its intrinsic luminosity, its distance and any extinction of its light caused by dust or atmosphere. The value measured is an object’s apparent magnitude. While 4.4 magnitude is within naked eye visibility, a given brightness value for a bright star-like object will appear brighter than an extended object like C/2025 A6 (Lemmon) with the same magnitude. The reason for this is the brightness measurement of an object is based on the amount of light emitted across the entire object. Extended objects find their light spread across a larger area resulting in a diffuse appearance.
If C/2025 A6 (Lemmon) reaches 4.4 magnitude in October, it is possible we might see the comet naked eye. Last year around this same time, comet C/2023 A3 Tsuchinshan-ATLAS remained visible to the naked eye as a 5th magnitude target. City lights and atmospheric conditions will be major factors in this possibility. When at peak magnitude most viewers will find binoculars to be the best option to see C/2025 A6 (Lemmon). Any binocular between 7×35 and 10×50 should be sufficient to see the comet provided it continues to increase in magnitude.
Currently the comet has an apparent magnitude of 10.2 and is found in the constellation Cancer, not far from the location of Jupiter. At this magnitude, the comet will be very difficult to find even while using a telescope. If you want to try finding the comet this, look east around 5am. At this time look for Jupiter high in the east about 7° south of the bright star Pollux. If you draw a line from Jupiter to Pollux, C/2025 A6 (Lemmon) will be found about 8° beyond Pollux. As we progress through September and October, the comet’s path will take it through the constellations Leo, Leo Minor, Lynx, Ursa Major and Boötes. As we approach the comet’s closet approach to Earth we will update this post with new information.
T CrB Nova Update 9/7/2025
If you are still interested in seeing the recurrent nova T Coronae Borealis, there is still hope. The nova has not yet occurred meaning we are in a waiting game. It could happen this year or it could happen next year. We will have to wait and see. If you are still interested in this event, the part of the sky the nova will appear in the west once it is dark. By midnight, the constellation Corona Borealis will be hard to find due to tree lines or buildings. Spaceweather.com is still posting magnitude estimates from the AAVSO. This is the resource I recommend following to keep up with any changes in the star’s brightness. Current magnitude of the system is 10.1.
The Sun and Moon
The Moon as seen from the International Space Station, on July 31, 2011.
Credit: NASA
Sun
Sunrise is at 06:35 on Saturday, September 6 and sunset is at 19:23 providing a little less than 13 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 20:54 this week. For those with a sundial, local noon occurs around 12:59 on September 6, 2025.
Moon
Moonrise for Saturday, September 6, is at 19:00 and moonset occurs at 06:20 the following morning. On September 6, 2025, the Moon will exhibit a waxing gibbous phase with 99% disk illumination. By September 15, 2025, the Moon will exhibit a waning crescent phase with 35% disk illumination. Full moon occurs on September 7, 2025, at 13:09 CDT and last quarter phase occurs on September 14, 2025, at 05:33 CDT.
International Space Station (ISS) Observing
Visible passes of ISS from St. Louis for the week of September 6 occur during evening hours. The table below lists the best of these passes. 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.
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 east at 21:00 on September 6, 2025. Credit: Stellarium, EG
Looking east at 05:30 on September 7, 2025. Credit: Stellarium, EG
Four naked eye planets will be visible this week. Mars is visible in the west after sunset once it is dark. Saturn is visible around 22:00. Venus and Jupiter are visible before sunrise in the east.
Venus
Venus is found in the east before sunrise for the rest of the year. It currently rises around 04:08. Look for Venus in the east around 05:00. Venus reaches superior conjunction on January 6, 2026.
Mars
Look for Mars in the western sky after sunset. The current apparition of Mars will continue throughout the rest of 2025 however twilight will make finding Mars increasingly difficult. Mars will slowly appear closer to the western horizon each night. Mars sets around 20:44 this week. For many viewers, Mars may already be lost to tree lines our buildings.
Jupiter
This week, Jupiter rises around 01:59. Look for Jupiter in the east around 03:00. Jupiter reaches opposition on January 10, 2026. As we continue towards this date, Jupiter will continue to rise earlier each week.
Saturn
Saturn is now visible in the southeast around 21:00. It will be easy to spot as it is the brightest object in that part of the sky. Saturn rises this week around 20:05. Saturn reaches opposition this month on September 21, 2025.
Our next Star Party will be held on Friday, October 3, 2025, from 6:30 pm until 9:30 pm
On the first Friday of each month, the St. Louis Astronomical Society and the Saint Louis 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 3, join us indoors in our planetarium theater for our free monthly sky lecture.
The St. Louis Astronomical Society helps host the monthly Star Parties at the Saint Louis Science Center. In addition to our nighttime viewings, they also help facilitate our daytime event called Solar Sundays. These daytime viewing sessions occur every third Sunday each month from 11:00-15:00 weather permitting. Visit SLAS’s website linked above to learn about other telescope events SLAS hosts around the St. Louis area.
James S. McDonnell Planetarium
Night Sky Update: September 6-15, 2025
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