Star Occultations by Minor Planets

Application for Determining Reaction Time (Personal Equation)

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 occultations can often be observed with small instruments, e.g. binoculars. The predicted times frequently differ from the real ones. The event can sometimes come unexpectedly, making it difficult to obtain the correct time.

This applet, written and provided by Klaus Nagel, simulates an occultation. It shows a view of the night sky through a telescope. One of the stars will be occulted, within a two minute time period, for a few seconds. Place the mouse pointer anywhere within the simulated field of view and click the mouse when you notice the occultation, and then again when you notice the star reappear. The real occultation times, your measurements, and the deviations will be displayed three seconds later. This gives an idea of your Personal Equation (PE) and its variance. This can then be applied to refine a real occultation timing, and therefore obtain a more accurate result.

The star to be occulted is located near the centre. It will be marked red for three seconds when you push the button Display. The other button will Start a new trial.

If you require specific occultation data for your locality, or details of occultations involving fainter stars, please get in touch with the Director of the Occultation Section giving your geographical location. Graphical representations of each occultation will be available at the Occultation Sections web-site before the event is due to take place. Please visit the UK's definitive site for lunar and asteroidal occultations: www.asterocc.org.uk

Jon Harper

Email: occultation@popastro.com

Address in Popular Astronomy

• Occultation Section home page