In this illustration, a planet (in a manner similar to Earth) moves with the usual double motion: it orbits a distant Sun while rotating about its own axis. A white dot marks a single, "stationary" observer on the planet's surface. The times are shown in hours. How long are one solar day and one sidereal day for this planet? Which is longer? Why?

Step by Step:

1. Pause and reset your animation to time t = 0. Note the position of the observer on the planet's surface. Now, play the animation and see how long it takes until the observer has made one complete rotation with respect to the fixed, distant stars.
 
2. What, then, is the length of one sidereal day for this observer? 
hours 
3. Now replay the animation. The gray line runs from the center of the planet to the center of its Sun, and thus passes through the observer's point at exactly solar noon each day. For this observer, what is the length of one solar day?
hours
4. Which, then, is the longer day: solar or sidereal? 
  
Food for Thought:

For Earth, the difference between one solar and one sidereal day is a tiny fraction (about 4 minutes in a 24-hour day). What is it about the planet in this illustration that so exaggerates that difference?

Reference:

Chaisson and McMillan, Astronomy Today, Fifth Edition, Section 1.3

 

Illustration written by Steve Mellema