The software can be downloaded at:

as well as a number of other factors.

Human reaction time is the single largest cause of error in occultation timings, and its effects can have serious consequences, especially in the case of minor planet occultations where reaction times (differing between Disappearance and Reappearance) can be a substantial fraction of the occultation duration. When minor planet occultations are observed from multiple sites, large differences in the reaction times of the observers lead not only to uncertainty in the actual length of the chords, but also to large offsets between adjacent chords. It is for this reason that we encourage observers to move to non-eyeball forms of observing (e.g. video).

The International Lunar Occultation Centre (ILOC) in Japan has determined, thorugh the analysis of many tens of thousands of lunar occultation observations over many years, that the average reaction time for a lunar occultation disappearance timed by an experienced observer is of order 0.4 seconds. Experienced observers timing reappearances have an average reaction time of order 0.6 seconds. For less experinced observers studies have shown that reaction times are frequently many times these values.

Nevertheless, visual occultation timings still have value and will continue to do so. (It was, after all, the analysis of tens of thouands of visual occultation timings that showed that the Moon is slowly spiraling away from the Earth). With practice, human reaction times can be minimised.

Various computer programs have been written to help observers estimate - and reduce - their reaction times. The program available here - REACT - has been kindly made availale by John Talbot for free download.

REACT takes the form of an MS Excel spreadsheet containing macros written in VBA. (You must have Excel installed to be able to run this, and you must allow the REACT macros to be run).

REACT simulates a realistic occultation, by producing a random star field on your screen with a target star whose brightness varies randomly on each run. If your speakers are turned on you will hear one-second pips that simulate the signals from time-signal station WWVH. The star will disappear at some random time and reappear some time later. You can make an "occultation" timing by hitting the ENTER key at each of these events. Your reaction times for both the disappearance and reappearance are measured over several runs, and the average value and standard deviation calculated. Note that while this program does simulate what happens in a real occultation, your reaction time at a true occultation event will also be affected by whether you're standing in the freezing cold, at 3 a.m. with a strong wind blowing....