The Sidereal and Solar Day Javascript Model illustrates the difference between the sidereal and solar day. The planets of our solar system both orbit around Sun and rotate on their axis. These two rotations allow for multiple definitions of what a day is. The sidereal day is the day according to the stars. It is the time it takes for a point on a planet facing a particular star to then face it again. On Earth, this time is 23 hours and 56 minutes. The unit of time called a solar day is the time it takes for a point on a planet facing Sun to then face Sun again. On Earth, this time is 24 hours. The difference between the sidereal and solar day is due to the orbital motion of the planet. For the planets with their orbital motion in the same direction as their rotation (Mercury, Earth, Mars, Jupiter, Saturn, and Neptune), the solar day is longer than the sidereal day. For Venus, the only planet with retrograde rotation, the sidereal day is longer than the solar day.

This simulation shows the difference between the sidereal and solar day by showing the orbit and rotation of a planet (blue) around sun (orange). It uses an orange arrow to represent where Sun would be seen in the sky relative to the background stars. Users can toggle between the geocentric and heliocentric point of view. The parameters are initially set to model a general direct rotation scenario. The user can also select a planet with a retrograde rotation, and the Earth-Sun system can also be shown which simulates the exact the orbit and rotation rates of Earth.

The Sidereal and Solar Day JS Model was developed using the Easy Java Simulations (EJS) version 5.  It is distributed as a ready-to-run html page and requires only a browser with JavaScript support.