When a minor planet (asteroid, planetoid) passes in front of a star, the star will be occulted for a short while, typically for a few seconds. Exact measurements from various places make it possible to determine the asteroid's size and shape. Such an occultations can often be observed with small instruments, e.g. binoculars. Frequently the predicted times differ from the real ones. The event comes unexpected, making it difficult to take the correct time.
This applet simulates an occultation. It shows a view of the nightly sky through a telescope. Within two minutes one of the stars will be occulted for a few seconds. To measure the time , click the mouse, when you notice the occultation and again when the star reappears. Three seconds later the real times, your measurements and the deviations will be displayed. That gives hints about your personal equation and its variance.
The star to be occulted is located near the center. It will be marked red for three seconds, when you push the button Display. The other button will Start a new trial.