What is time anyway? This morning, as I worked on this show, I looked around my office and realized it is fossilized to January 2020. Calendar on the wall: December 2019. Yearly passes attached to lanyards: 2019-2020 year. Everything just kind of stopped as every day became blursday. 

As humans, we measure the passage of time in different ways; many humans consider “today” to be from the moment of waking to the moment of sleeping, even if that “today” spans multiple calendar days. A year for a teacher or professor is September to August. Time, how we divide it up at least, is just an arbitrary set of decisions. For the purposes of science, a day is the amount of time it takes for a single point on a world to go from noon to noon. Except when it isn’t. 

Here on Earth, we rotate so that the Sun moves across our sky daily, for folks not at the poles, and we have a close to 24-hour day. For Uranus, the planet is flipped sideways, so the solar day and the orbital period are the same (84 Earth years), but relative to the stars, a day would be measured in Earth hours. Of course, exactly how you measure that on a gas world is a bit hard. What do you measure it against? With Saturn, folks have decided to measure a day based on the ripples its gravity creates in the rings. By the way, Saturn’s day, it’s ten Earth hours and 33 Earth minutes long. Gas planets… they are just hard to sort.

Filled under “why is this so hard?”, it turns out that cloud-covered solid planets can actually hide the length of their day pretty effectively. While Venus is in many ways a twin to Earth – similar size, similar original composition – the world had a rather catastrophic history that isn’t explained by just its closer distance to the Sun. Understanding what happened actually requires understanding how Venus rotates and how that reflects its internal structure. The thing is that we can’t see its surface. 

While orbiting satellites can get some information on Venus’s day length from radar images, their fast orbits and the influence of Venus’s shape on their orbits make precision hard. In fact, past measurements didn’t match each other. So scientists started hitting Venus with radar from Earth, and over long periods found that: The new radar measurements show that an average day on Venus lasts 243.0226 Earth days — roughly two-thirds of an Earth year. What’s more, the rotation rate of Venus is always changing: A value measured at one time will be a bit larger or smaller than a previous value. The team estimated the length of a day from each of the individual measurements, and they observed differences of at least 20 minutes. 

When your average day is 243 Earth days, a variation of twenty minutes actually isn’t a lot, but how Venus got that way is a serious mystery that we can’t solve today.

More Information

How Long Is One Day on Other Planets? (NASA)

UCLA press release

“Spin state and moment of inertia of Venus,” Jean-Luc Margot et al., 2021 April 29, Nature Astronomy