Bark of the byte

Running with bytes and other wild creatures

The Solar System Clock

I was thinking about the daily cycles of the different planets and ended up programming the clock at the top of the page. The Solar System Clock is actually a really good way to visualise the rotations of the planets, the movement of the sun and timekeeping in general. 

What is it?

The clocks basically show us the progression of a day on other planets relative to earth. If we compare the time it takes for a day to pass on another planet in terms of earth time we can create a comparative clock by splitting that time into hours, minutes and seconds and showing it on a clock face. 

The purpose of the clock is not to provide a timepiece that would be useful for us to keep time on other planets. For example, the passing of a day on Mercury is so slow that it would not be useful for keeping track of short intervals (such our earth seconds) unless the Mercurians themselves were very slow ponderous creatures. The purpose of the clock is to provide a visual representation which immediately makes the daily cycle on each planet obvious. 

The result is a set of clocks that provide some interesting insights. Lets have a look at a few:


It might seem strange at first that Mercury has such a slow clock. Mercury is the closest planet to the sun and has the shortest year (time it takes to orbit the sun). Mercury’s year is roughly 88 earth days. That is, every 88 days on earth Mercury will orbit the sun once. You might think that shorter years would mean shorter days, but this is not the case. In fact, Mercury only completes a full day once every 2 years. That means that Mercury has to Orbit the sun twice before the sun returns to the same position in the sky. Mercury has much more to offer in terms of weirdness than mere long days. At some vantage points on Mercury the sun moves backwards for a period of time before regaining composure and moving in the correct direction. See a visualization of this here along with an explanation.


Jupiter’s clock is moving very fast compared to earth. This is because Jupiter rotates much faster than earth. Jupiter takes much longer to orbit the sun (around 4333 earth days). So, if Jupiter rotates faster than the earth, but takes longer to go around the sun, it seems logical that there would then be more days in a Jupiter year than on earth. This is correct, there are around 10476 Jupiter days in a Jupiter year compared with our 365.On Jupiter we would have to wait almost 29 earth years before our first birthday. For more details, a visualization and explanation see here.

Earth and Mars

Notice how close Earth and Mars’ clocks are. The earth and Mars days are very similar. However, Mars takes twice as long to orbit the sun as earth. Since the period of the day is similar for Earth and Mars, this means that there are almost twice as many days in a year on Mars. Still, of all the planets, Mars has a day/night cycle that would be most familiar to ‘would be’ colonists.

Venus, Uranus and Pluto

Notice anything strange about the clocks for Venus, Uranus and Pluto? The hands are going backwards (anticlockwise). This is because they rotate in the opposite direction to the other planets. Note that all planets orbit the sun in the same direction, but these planets rotate in a different direction from the others. Another way of saying this is that all planets follow the same direction in their journeys around the sun, but these planets spin in the opposite direction. [*]

Next - What is in a day? 

We have only scratched the surface. In the next post we ask the question 'What is in a day?' which throws up an interesting curveball - a day and a rotation are not the same. Click here to go to the next post.

References and further reading
This page was the source of the values that I used in programming The Solar System Clock. The whole site is an excellent resource for information about astronomy. 
In particular, for information on the rotations of the planets, see


* I was initially going to leave direction out of The Solar System Clock because I worried that it would be too much information. However, after seeing how clearly it demonstrates the difference and since this is such a fundamental component of the movement of the planets I decided to keep it in.