Time Delay

Humanity has probably known about the planet Saturn since prehistoric times, although obviously we have no idea what they thought about it. The ringed planet can be very faint, although it brightens when its rings are face-on to the Earth and it is the most distant planet visible to the naked eye. Babylonians, Greeks and most other ancient civilizations show it on their maps of the heavens.

But even once we invented telescopes that could see the gas giant up close there was something about it we did not know, and in fact have never really known until quite recently: How long was a Saturnine day?

Since it's a gas giant, there's no solid surface to measure rotational speed. Some parts of the outer cloud layer that we could see moved faster than others. Jupiter has the same issue, but scientists could track its radio emissions and get a solid number. Saturn's magnetic field acts weird, so that method was out. Venus is also covered by clouds, but it's close enough that radar signals bounced off its surface can be measured and features of that surface identified to determine rotational speed (Turns out that Venus rotates backward and very slowly; a Venusian day takes almost 250 Earth days).

But as the Astronomy Today story at the link notes, Saturn does have those amazing rings. And it oscillates as it rotates, which sets up ripples in the rings that were measured by the Cassini probe. Christopher Mankovich, a graduate student at the University of California Santa Cruz studied the ripples and developed computer models of what Saturn would have to be doing, rotation-wise, to produce them. Rotational speed was one of the variables in the models, and by studying them researchers could see which elements best matched reality and then figure out how fast Saturn was rotating.

The estimate is 10 hours, 33 minutes and 38 seconds -- which is a little speedier than one offered by radio signals used in the 1981 Voyager flyby. Saturn's magnetic pole, though, lines up almost perfectly with its actual north pole and makes the radio-timing trick a tough one and very likely to be off a little.

To me one of the most fascinating things about this discovery is the time it took to happen. Human beings have known about Saturn for as long as we've been looking at the sky and watching its objects move. We've known about its rings and its moons for almost 400 years. But we weren't sure how fast it turned on its axis until just a few days ago (Mankovich published his research on January 17).

It's a pretty cool universe that always has something new to find out, even when that new something is directly connected to a very very old something.