When Venus is in the House of Games
July 7, 2025 at 1:01 PM by Dr. Drang
I had a touch of insomnia last night and tried to cure it by watching YouTube videos. The algorithm suggested a compilation of quiz questions from Richard Osman’s House of Games, a British game show that I used to watch regularly back in 2020, when I was trying to take my mind off of things I had no control over.
The cure didn’t work, because the video I watched was this one, and the last question—which starts at about the 26:35 mark—gave what I was sure was the wrong answer. So instead of falling asleep, I went off to look up the right answer and then got caught up in how to calculate it.
The question was “How many hours are there in one day on the planet Venus?” and the answer they gave was 5,832. Now, I am by no means an expert on Venusian planetary motion. I know its year is somewhat more than 200 Earth days, that it rotates backwards, and that its days are quite long. But in my mind, “quite long” is nowhere near 5,832 hours.
So I went to NASA’s Venus Fact Sheet, which told me that its day is 2,802 hours, roughly half of what House of Games said. But the number 5,832 did appear in the section on Venus’s orbital parameters:
It’s fourth from the bottom in the row labeled Sidereal rotation period. I took the negative sign to be an indication of Venus’s backward rotation, similar to the obliquity being given as 177.36° instead of 2.64°.
(By “backward rotation” I mean that Venus rotates in the opposite direction of its revolution around the Sun.)
So now I felt compelled to figure out how the -5,832.6 hours of sidereal rotation related to the day length of 2,802.0 hours. Which kept me awake longer.
First, I noticed—or rather, PCalc told me—that the sidereal orbital period of 224.701 Earth days is 5392.824 hours, a number that’s fairly close (in magnitude) to the sidereal rotation period. I imagined looking “down” on a solar system in which a planet revolved counter-clockwise around its sun and rotated clockwise about its axis at the same speed. It would look like this at various points in its year:
The black line on the planet is a meridional stripe that runs from pole to pole to help see the rotation. For every 30° of counter-clockwise revolution of the planet about its sun, there is 30° of clockwise rotation about its axis. This is all relative to us, who are looking at this from the point of view of the stars (sidereal).
As you can see, midnight to midnight on the planet’s meridional stripe takes half a year. That’s going from the 3:00 position around to the 9:00 position. So for these conditions—a planet rotating clockwise at the same speed it revolves counter-clockwise—there will be two days in a year.
So in rough numbers, there are two Venusian days in a Venusian year. With a Venusian year being about 5,400 hours, that makes its day about 2,700 hours long. But let’s do a more precise calculation to see if we can get all the numbers in the fact sheet to line up.
Taking Venus to move in a circular orbit, its orbital speed is
Its rotational speed about its axis is
So Venus will have gone through one day when
That works out to
or
which matches the value in the table.
This correct answer, by the way, would not have changed who got the points in the game. Both teams were orders of magnitude too low. And with my curiosity satisfied and my phone turned off, I went back to sleep.