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Ephemeris: 12/29/2025 – 3I/ATLAS, Comet of the Year

December 29, 2025 Comments off

This is Ephemeris for Monday, December 29th. Today the Sun will be up for 8 hours and 50 minutes, setting at 5:10, and it will rise tomorrow at 8:19. The Moon, 2 days past first quarter, will set at 4:04 tomorrow morning.

One of the more interesting events that occurred this year was the discovery at midyear of an interstellar visitor to the solar system, a comet called 3I/ATLAS. Last week as I heard in breathless news reports that the comet came its closest to the Earth. Well yeah, it was its closest to the Earth, but it wasn’t all that close. It was actually farther from us than the Sun is. We were nowhere close to the comet at anytime in its orbit, passing through the inner solar system. The closest it ever got to anything was to Mars, in October. Now as 3I/ATLAS is moving out of the inner solar system the Hubble and James Webb space telescopes can take a good look at it. Despite what the cranks say, it’s still just a comet, but from another solar system.

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; EST, UT – 5 hours) unless stated otherwise. Times will be different for other locations.

Addendum

A diagram of the solar system inside the orbit of Jupiter, showing for 7 PM EST, December 29th 2025, 3I/ATLAS heading out towards Jupiter on its path through the solar system.
A diagram of the solar system inside the orbit of Jupiter, showing for 7 PM EST, December 29th 2025, 3I/ATLAS heading out towards Jupiter on its path through the solar system. The comet is denoted here and as C/2025 N1 (ATLAS), it’s discovery designation as the first new comet discovered in July of 2025. It was given the designation 3I because its orbit is hyperbolic in that it came from interstellar distances which the “I” signifies. 3 means that’s the third such object discovered. Credit: NASA/JPL Small-Body Database, direction arrow added.
Categories: Comet, Ephemeris Program, Interstellar visitor, Year review