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Ephemeris: 09/02/2025 – 3I/ATLAS, interstellar visitor

September 2, 2025

This is Ephemeris for Tuesday, September 2nd. Today the Sun will be up for 13 hours and 10 minutes, setting at 8:17, and it will rise tomorrow at 7:07. The Moon, 2 days past first quarter, will set at 1:54 tomorrow morning.

Interstellar visitor 3I/ATLAS was discovered a couple of months ago. We are lucky because it was discovered on its way into the inner solar system. The first interstellar visitor, 1I/‘Oumuamua, we didn’t spot until it had already passed and on its way out. It turns out that the chemical composition of this interstellar visitor, which astronomers think is a comet, has a great deal of carbon dioxide in its atmosphere compared to water. In the universe water is the third most common molecule after diatomic hydrogen and diatomic oxygen. But this comet appears to have about 8 times more carbon dioxide than water. Some astronomers think the 3I/ATLAS is 3 billion years older than the solar system making it 7 1/2 billion years old.

The astronomical event times given in this blog are for the Traverse City/Interlochen area of Michigan (Lat 44.7° N, Long 85.7° W; EDT, UT – 4 hours) unless stated otherwise. Times will be different for other locations.

Addendum

Interstellar comet 3I/ATLAS imaged by the Hubble Space Telescope. The telescope was tracking the comet during the time exposure, causing the stars to trail. Credit NASA/HST.
A diagram showing the position of 3I/ATLAS for the 2nd of July 2025. Its designation here is the discovery designation: C for comet, 2025 the year, and N for the first half of July, 1 the first object discovered in that period, and the name of the person or organization that discovered it . Note that the comet will come fairly close to Mars. The Mars Reconnaissance Orbiter is going to be observing the comet as it passes Mars next month. Credit: NASA/JPL.
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