Compared with other planets in the solar system, the Earth rotates rapidly on its axis. It takes about 24 hours for any point on the Earth to return to the same position relative to the sun. We call each rotation a "day". Earth completes about 365 days during one orbit around the sun (that is, one "year"). But Mercury's rotation is much slower: Mercury completes only one and one-half rotation during one orbit around the sun.
Going Around the Sun presents a graphic representation of Mercury's orbit around the sun. This interactive moves the planet through its orbit; by watching two points on the planet's surface, you can watch the changes in these points' positions relative to the sun. A table below the graphic shows the elapsed time, relative to Mercury and to Earth.
The interactive starts with Mercury directly above the sun. Students can start Mercury in its orbit by clicking PLAY, and stop it at any point by clicking STOP. The movement will stop by itself at various points.
As the orbit proceeds, students should keep an eye on the small numbers 1 and 2 on either side of the planet. These mark fixed positions on the planet's surface. The numbers help students keep track of these locations' positions relative to the sun.
The table below the graphics shows the numbers of rotations, orbits, and solar days that have elapsed since the start of the animation. (Students can reset these numbers and start again by clicking RESET.) A second table shows how many Earth rotations, orbits, and solar days take place during the same movements of Mercury—a good way to relate Mercury's movements to their own experience.
You can use this interactive to add some perspective to a discussion of the Earth's orbit and rotation. If possible, run the interactive on a large screen so that the whole class can discuss it. Each time Mercury stops in its orbit, ask students about the numbers in the table. How far does Mercury get in its orbit after one half Mercurian day? (About 1/3.) After a complete orbit, are the "1" and "2" on the planet's surface back in the same position? (They look like they are, but compare them to the original position: after one complete orbit, the numbers are reversed!) After one rotation, can students predict how much further Mercury will travel before the numbers are back where they started from? Remind students that Mercury's "solar day" is not complete until both numbers are back in their original positions.
So how long does it take for Mercury to complete a "solar day"? The answer may be surprising!