This is the Saint Louis Science Center’s NIGHT SKY UPDATE for the week of Saturday, July 5, 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, August 1, 2025, held in association with the St. Louis Astronomical Society.
Observing Highlight of the Week
Animation showing the observations of Comet 3I/ATLAS by the ATLAS survey telescope in Chile when it was discovered on July 1, 2025. Image credit: ATLAS/University of Hawaii/NASA.
On July 1, 2025, scientists working with the ATLAS telescope in Rio Hurtado, Chile, reported observations of a new object moving through our solar system. The object was originally given the designation of A11pl3Z which was a place holder until scientists could learn more about the object. Eventually, it was renamed Comet 3I/ATLAS and C/2025 N1 ATLAS. Both names reveal the nature of what it is.
C/2025 N1 ATLAS is a designation that implies the object is a comet. The Prefix “C” indicates the object is a long period comet, 2025 is the year it was discovered and “N1” indicates it was the first comet discovered in the first half of July (July 1-15). Finally, the comet was discovered by the survey telescope called ATLAS (Asteroid Terrestrial-impact Last Alert System).
Comet 3I/ATLAS is far more interesting name as it indicates the object is from beyond the solar system. The prefix “I” in the name means interstellar. Scientist have only seen two other objects known to be interstellar; the first was 1I/Oumuamua and the second is Comet 2I/Borisov. Astronomers know this comet is interstellar due to the shape of its orbit.
Not much is known about Comet 3I/ATLAS at this time. They know it is interstellar due to its trajectory. Instead of following a closed orbit around the Sun, Comet 3I/ATLAS’s trajectory is hyperbolic. They know it is a comet due to its behavior. Comets are icy bodies that are left over bits from a solar system’s formation. If they get close enough to a star, they heat up and start to sublimate. The gas and dust released through this process form a gaseous envelope around the comet’s nucleus which is called a coma.
The size of 3I/ATLAS is not well known due to its comet-like behavior. Early analysis suggests it is around 10 km in diameter or 6.1 miles. If you live in St. Louis, that is similar to the distance from Forrest Park to the Gateway Arch on the riverfront.
The trajectory of the comet suggests it is coming from the direction of the galactic center. The comet poses no threat to Earth as its closest approach will be only 1.8 AU. The comet is moving about 61 km per second as it heads towards perihelion on October 30, 2025.
At the time of discovery, 3I/ATLAS had an apparent magnitude of +20 which is much too faint for most backyard telescopes. Currently, it is about +18 magnitude and is expected to peak at +11 in October. At peak magnitude, the comet should be in reach of moderate sized telescopes however it will be difficult to see. Your best chance to see this comet will be through imaging.
3I/ATLAS passing through our solar system gives scientists a unique opportunity to study other solar systems. It is also an important reminder that there is plenty out there that we don’t know about. It is crucial that we continue to scan the skies looking for potential hazard to Earth. Pan-STARRS, Catalina Sky Survey and ATLAS are just a few examples of programs that are actively doing this.
Titan Shadow Transit 7/18/2025
The transit occurring on July 18, 2025, begins at 02:00 CDT and ends at 07:00 CDT. From St. Louis, we will see about 2/3 of the transit before the Sun rises. When the transit begins, Saturn will be roughly 28° above the eastern horizon around 118° azimuth.
You will need a telescope to see Titan’s shadow transit across the clouds of Saturn. While the transit is underway, you will also see Titan to the east of Saturn. Saturn’s rings will still appear thin however we are starting to see the underside of the ring system. If you would like to plan for future transits of Titan’s shadow make sure to read Bob King’s article on Sky and Telescope which can be found here.
T CrB Nova Update 7/6/2025
f 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 is overhead once it is dark. By 03:00, the constellation Corona Borealis will be found low in the western sky. 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 10.0.
The Sun and Moon
The Moon as seen from the International Space Station, on July 31, 2011.
Credit: NASA
Sun
Sunrise is at 05:42 on Saturday, July 5 and sunset is at 20:28 providing over 14.5 hours of daylight. Even after sunset, light from the Sun will dimly illuminate our sky for about 2 hours. This period is called twilight, which ends around 22:25 this week. For those with a sundial, local noon occurs around 13:06 on July 5, 2025.
Moon
Moonrise for Saturday, July 5, is at 16:12 and moonset occurs at 02:00 the following morning. On July 5, 2025, the Moon will exhibit a waxing gibbous phase with 77% disk illumination. By July 13, 2025, the Moon will exhibit a waning gibbous phase with 90% disk illumination. Full moon occurs on July 10, 2025, at 15:37 CDT.
International Space Station (ISS) Observing
Visible passes of ISS from St. Louis for the week of July 5 occur during morning and evening hours. The table above 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 west at 21:15 on July 6, 2025. Credit: Stellarium, EG
Looking east at 04:00 on July 7, 2025. Credit: Stellarium, EG
Three naked eye planets will be visible this week. Mars is visible in the west after sunset once it is dark. Venus and Saturn 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 03:00. Look for Venus in the east around 04:00 as it clears tree lines.
Mars
Look for Mars in the western sky after sunset. The current apparition of Mars will continue throughout the rest of 2025. As the year continues, Mars will slowly appear closer to the western horizon each night. Mars sets around 23:23.
Saturn
Saturn can be found in the east before sunrise. Saturn rises around 00:17 this week. The planet should be high enough to see by 01:30. If you missed the last Titan shadow transit, the next one occurs on July 18, 2025.
Our next Star Party will be held on Friday, August 1, 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 August 1, 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 on the 3rd Sunday of each month. 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: July 7-13, 2025
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