The above applet is based on the Planetary theory of Ptolemy, as described in the Almagest. It is intended to perform two functions: it allows the user to get an intuitive sense of Ptolemy's model that static diagrams do not allow, and it is capable of calculating the positions of the planets to within a few minutes of longitude.
The left bar contains the settings for the applet. The first section contains the general settings for the applet. These boxes allow the user to toggle display of the sun, mean sun, and construction lines (the geometrical objects that define the model) respectively. The sun and mean sun are treated seperately from the other planets mainly because it is often useful to calculate the sun's position and a planet's position at the same time (for example, when calculating oppositions), and the applet does not allow the simultaneous display of two different planets. Also, with respect to the construction lines, the program does not display the circle on which the centre of the eccentre moves or the line that connects the apogee with the earth. These were too small to display properly and provided very little information, given the slow speed at which the apogee moves.
The 'planets' section allows the user to select which planet to display. One can also choose which model of the planet to display. The models are from the simplest (regular motion on a circle) to the complex 'final' model which Ptolemy uses to find the position of the planet at a given time.
Also, on a practical note, some browsers do not render the applet correctly when the window is scrolled, causing the image to 'smear' across the rest of the page. The simplest cure for this problem is to refresh the page while the applet is centered, though be aware that this will reset the applet's settings.