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Emily LakdawallaOctober 15, 2009

A brief word on Saturn's radius

I've had two people write in to correct my Phoebe ring post from yesterday. In that post I said

You can measure the extent of the rings in kilometers, but astronomers and Cassini mission people both seem to find "Saturn radii" to be a handier unit that helps them have a better intuitive feel for the scale of things. One Saturn radius, abbreviated as R-subscript-s but usually written just as Rs to make things easier, is 60,330 kilometers. Beginning at Saturn's center, one radius gets you to Saturn's cloud tops.

The two readers both pointed out that 60,330 kilometers is not the accepted radius for Saturn. The value that is typically used for Saturn's equatorial radius is 60,268 ±4 kilometers; this is based on a report published in Celestial Mechanics and Dynamical Astronomy in 2007 by P. K. Seidelmann et al.: "Report of the IAU/IAG Working Group on cartographic coordinates and rotational elements: 2006," volume 98, pp. 155-180. This is the equatorial radius of Saturn at the 1-bar level, that is, where its atmospheric pressure is about equal to Earth's at sea level.

I sent off this question to Dave Seal, and confirmed that the 60,330 radius is the one used by the Cassini mission when they speak of distances in Rs; the same number was used by the Voyager mission. It is the radius of Saturn at the 100 millibar altitude. He didn't say why they choose the 100 millibar level instead of the 1 bar level.

In any case, because the difference is only important at the fourth significant digit, it doesn't affect any of the distances I quoted in the Rs unit in my story on the Phoebe ring, which I reported to at most three (and sometimes fewer) significant digits!

Read more: Saturn

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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