The Rainbow Star, or the Mystery of Hartmann's Celestial Object

Updated on November 9, 2017
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Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly improve it.


Our mystery began on May 11, 1835, when Niccolo Cacciatore, a former assistant of Piazzi, was hunting for stars that showed proper motion aka movement. He found a 7.8 magnitude star near the “17th star, 12th hour of Piazzi’s Catalogue,” and measured the distance between them. After confirming his finding a few nights later, he found the star again 3 months later and recorded the new distance between the stars. The observations showed the star had moved east since the last time it had been seen. Rain followed for many nights, preventing more observations but by the time Cacciatore had another chance the object had fallen into twilight and became obscured. However, that meant that the object had travelled 10 seconds over a 3 day period, placing it beyond Uranus (Baum 268).

Cacciatore wrote to W.H. Smyth on September 19, 1836 about this weird object he saw but Smyth has no luck finding it. But before this, on February 15, 1836 word of mouth got to Jean Elias Benjamin Valz, then at the Marseille Observatory. He felt that maybe this mystery object was a planet, for Uranus still had orbital perturbations that remained unsolved. But after this mention and a brief reference in Smyth’s A Cycle of Celestial Objects, no further word was mentioned until years later. Why? Because no one could find the object again (268-9).

And then Neptune was found, and Valz remembered the mystery object and went back to his letter from Cacciatore to refresh himself and see if he could spot it again. He made sure that Cacciatore hadn’t mistaken an asteroid for the object, then investigated if it had been a comet or even a trans-Uranian object. He discusses with others and in a July 4, 1878 Nature, scientists agree that the chances of it being a new planet are slim to none (269-71).

Was this what was seen back then? Image of Neptune.
Was this what was seen back then? Image of Neptune. | Source

And that should have been that, until findings from a different scientist raised an interesting mystery. After the return of Halley’s comet in 1835 was off of its predicted path because of an unknown gravitational tug, Louis Francis Wartmann, an astronomer at the Geneva Observatory, mentioned to fellow astronomer Arago about observations he had started in the summer of 1831 that might help resolve the gravity problem. He was looking at Uranus, which at the time was in the Capricornus constellation at 313 degrees’ right ascension when at 10:10 pm he spots an 8th magnitude star located at 315⁰ 27’ right ascension and 17⁰ 28’ declination. Over a few nights, it exhibits a westward retrograde motion and a pale-white color yet it doesn’t scintillate like a star. After a few weeks, Wartmann spots the star again at 7 pm on a moonless September 25th. The object was near star 481, had a yellowish color, and was moving towards the sun. The next time Wartmann spotted his star, it was October 15th and further developments ensued. Now the star was orangish 9-10 magnitude object and had moved further west. His final definite observation was on November 1 when it was a yellow-orange hue and even fainter than before (Baum 271-2, Hubbell 265-6).

Based on the angular motion of the object as well as Bode’s Law, he computed the object was at 38.8 AU away from the sun and orbited with a period of 243 years. Wartmann then sent his findings to Alfred Gauter, the director of the observatory, and also to Van Zach, another astronomer. But the papers became misplaced and Wartmann died the next year, hence why it had taken so long for the findings to become publically known. But how do we explain this mystery object as seen by two different people? (Baum 272-3)

Some wondered if the rainbow star could have been Neptune spotted accidentally. Benjamin Pierce raise the idea during the 298th American Academy of Arts and Sciences meeting on November 12, 1846. However, based upon backtracking and comparison, Neptune was too far away for the observed angular changes seen by Wartmann to hold true. About 40 years later, Theodor von Oppolzer feels that a mistake was made on Wartmann’s part, for the nearly circular orbit projected by the angular distance measurements happen to line up with Uranus rather nicely. However, we still have the right ascension and declination variances to deal with. Also, what was up with the color changes? Like most aspects of science, the easy answers allude us... (Baum 273-4, Hubbell 266)

Works Cited

Baum, Richard. The Haunted Observatory. Prometheus Books, New York, 2007: 268-74. Print.

Hubbell, John G. and Robert W. Smith. “Neptune in America: Negotiating a Discovery.” Journal for the History of Astronomy, Vol. 23, No. 4, 1992: 265-6. Print.

© 2017 Leonard Kelley


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