Nov. 8, 2019

Two new studies underline that astronomers are still looking for an accurate assessment of the length of a day on Venus and Saturn.


  • Obscured by clouds, it does not present a readily visible surface feature, such as a crater, which could have been a reference point for measuring its rotation period.

  • In 1963, when radar observations broke through the cloud cover, Venus revealed that it rotates in a direction opposite to that for most planets.

  • Studies based on the Magellan spacecraft’s observations concluded that the correct rotation period is 243.0185 days, with an uncertainty of about 9 seconds. Subsequent measurements have, however, given inconsistent values.


  • A gas giant, by definition, it has no solid surface features for scientists to track.

  • For Jupiter, scientists figured out the rotation period by observing patterns in radio signals from there. Saturn has defied such attempts. It emits only low-frequency radio patterns that are blocked by Earth’s atmosphere.

  • A new study led by Duane Pontius of Birmingham-Southern College, US, looked at Jupiter. A key difference is that unlike Jupiter, Saturn has a tilted axis and, therefore, seasons like Earth. Thus, the rotation period cannot be determined from the radio signals.