Researchers from Tel Aviv University have developed a new way to measure the rotation of planets which they have used to determine the length of a day on Saturn. That planet’s day is 10 hours, 32 minutes and 44 seconds long.
The method, by Dr. Ravit Helled of the Department of Geosciences at TAU’s Raymond and Beverly Sackler Faculty of Exact Sciences and Drs. Eli Galanti and Yohai Kaspi of the Department of Earth and Planetary Sciences at the Weizmann Institute of Science, is based on Saturn’s measured gravitational field and the unique fact that its east-west axis is shorter than its north-south axis.
When the researchers applied their method to Jupiter, whose rotation period is already well known, the results were identical to the conventional measurement, reflecting the consistency and accuracy of the method.
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Aientists have had difficulty coming up with a precise measurement of Saturn’s rotation. “In the last two decades, the standard rotation period of Saturn was accepted as that measured by Voyager 2 in the 1980s: 10 hours, 39 minutes, and 22 seconds, ” said Dr. Helled. “But when the Cassini spacecraft arrived at Saturn 30 years later, the rotation period was measured as eight minutes longer. It was then understood that Saturn’s rotation period could not be inferred from the fluctuations in radio radiation measurements linked to Saturn’s magnetic field, and was in fact still unknown.” The Cassini spacecraft had measured a signal linked to Saturn’s magnetic field with a periodicity of 10 hours, 47 minutes and 6 seconds long — slower than previous recordings.
“Since then, there has been this big open question concerning Saturn’s rotation period, ” said Dr. Helled. “In the last few years, there have been different theoretical attempts to pin down an answer. We came up with an answer based on the shape and gravitational field of the planet. We were able to look at the big picture, and harness the physical properties of the planet to determine its rotational period.”
Helled’s method is based on a statistical optimization method that involved several solutions. First, the solutions had to reproduce Saturn’s observed properties (within their uncertainties): its mass and gravitational field. Then the researchers harnessed this information to search for the rotation period on which the most solutions converged.
