Equipment needed to successfully time an occultation of star by the moon includes a suitable telescope, a stopwatch (mechanical or digital), and a short-wave receiver. The stop watch should be of good quality. The short-wave receiver can be any type as long as it can receive the WWV time signals located on 5, 10 and 15 MHz.
You are going to also need to know your coordinates within 50 feet of a known location. A GPS unit, which is allowed to run for 3-5 minutes will normally give you this longitude and latitude coordinate that you will need for reporting your observation. If you do not have a GPS, email me with your address, and I will compute your coordinates from a USGS Survey map.
Set the telescope up outside, far enough in advance to allow the optics to come to ambient air temperature (15-30 minutes before the event).
Locate the moon and star to be occulted. The star should be visible near the moon's edge 4-5 minutes before the event, in the same field of view of the eyepiece.
When the star is about 1 minute away from being occulted, begin watching as the limb of the moon creeps toward the star.
At the time of disappearance, start the stopwatch.
Let the stop watch run, while you go in, turn on the radio, and watch when the clicks or tones are heard on the radio in relation to the tenths of a second on the stopwatch. This will take practice! Write this tenth second number down on a sheet of paper. At the next full minute tone, stop the stopwatch, and write down the full minute time. DO NOT RESET THE STOPWATCH! We are not finished!
Now we are going to "reduce" the observation into a meaningful time. Look at the stopwatch, and you will see a certain amount of time has elapsed. Let's say it was 1 minute 23.7 seconds. The tenth second you wrote down earlier was .5 second, and the full minute tone you wrote down was 2:36:00. We will now compute the actual time the star disappeared.
The 1 minute 23.7 seconds you have showing on the stopwatch is not entirely correct. Remember the .5 second mark you observed while listening to the second clicks or tones? That was the actual tenth second. So the 1 minute 23.7 seconds (1:23.5) is now 1:23.5
Now let's do our calculation:
|The full minute tone you wrote down was:||2:35:00.0|
|Subtract the stopwatch time:||0:01:23.5|
|New corrected time:||2:33:36.5|
We are not finished though. You need to correct for your reaction time in starting the stopwatch at the time of disappearance. Most generally, a "normal" reaction time is .3 of a second. This could be longer depending on how tired you are, how old you are, you general well being, etc. So, by subtracting your reaction time from the "raw" time, will give the actual time the star disappeared. Using .3 seconds, the raw time now becomes 2:33:36.2, which is the actual event time.