Although there are several methods of timing occultations, only two will be described here. However all methods rely on the observer having access to an accurate timebase.
The best timebase to use is the shortwave radio time signal station WWVH (Hawaii). This station continuously broadcasts one second time pips, and thousands of people worldwide use these accurate time signals to make observations in all branches of science.
WWVH broadcasts on frequencies of 2.5, 5.0, 10.0, 15.0 and 20.0 MHz with time pips every second. WWVH provides voice announcements of the time before every minute beat, starting at the 45 second mark, while the 59th second beat is omitted and the minute tone is at a higher frequency than the other beats.
WWVH can be picked up easily in New Zealand, and less well so in parts of Australia, using only a cheap shortwave receiver. The best frequency to use will depend very much on your own location and on many other factors - you should experiment. You should however listen carefully to the signal as both stations can sometimes experience interference from the Japanese station JJY. For more information about how this interference manifests itself, as well as comprehensive additional information about timebases, please visit Alfred Kruijshoop's New Zealand/Australia Time Resources Page.
If you don't have a short wave radio you can still make occultation timings, although it is less convenient to do so. Be warned however that although time pips are broadcast by a few AM and FM radio stations on the hour, we have found these to often be unreliable. For example, the time signals on many commercial stations in both Australia and New Zealand can be out by many seconds, while signals on National Radio in New Zealand may now be sometimes out by a few tenths of a second because of a change in the way the signals are distributed. Within Australia we understand that not many ABC stations now carry time pips.
Australian observers should also avoid using the Telstra talking clock as this has been found to be unreliable with random errors of as much as 3-6 seconds. Within New Zealand Industrial Research Ltd offers an accurate talking clock by dialing 0900-45678. This will provide about two minutes worth of time pips.
It is ironic that as technology has advanced, our ability to obtain accurate time has declined. Future timebases will most likely
rely on GPS navigation satellites, although even here, it has been noted that
the time output from some cheaper GPS devices can be as much as 2 seconds late
because internal processing priority is given to the determination of latitude and longitude,
not the time. Several stand-alone accurate timebase devices based on GPS technology are
under construction at present (visit http://videotimers.com/home.html
for details of the IOTA VTI system .
Please email Director
for further information about this topic.
TAPE RECORDER AND TIME SIGNAL
Using a tape recorder and timebase is the easiest and by far the most popular method to time occultations. In this method you position a small tape recorder next to your telescope; it needs to be positioned so that it can pick up both the shortwave radio station time pips, and your own voice comments made at the telescope. When you see the occultation occur yell out "off" (if a disappearance of the star), or "on" (if a reappearance) so that your voice is recorded on the tape along with the time pips.
Later, you can play back the tape and, by listening carefully, determine the exact time at which the occultation occurred. (You will need to listen to the tape several times before you get the time exactly). On the first pass, listen for the time signal station voice announcement at each minute and note the last minute beat before you hear your own voice. This tells you the hour and minute in which your observation was made. Now, while listening to the background time signals, count the number of second beats from the minute beat up until the time of your own voice announcement. You now have the hour, minute and second of your observation. Finally, listen to your own voice announcement several times and try to estimate approximately where, between each second beat, it occurred. Although this may seem difficult the first time you do it, with practice you will be able to determine your occultation timing to an accuracy of about 0.1 or 0.2 seconds.
Occultation timings should be reported to 0.1 second precision if possible. Do not report timings to any greater precision (e.g. 0.01 second) as the last digit is meaningless. The reason for this is explained under Reaction Time below.
STOPWATCH AND TIME SIGNAL
If you don't have a cassette tape recorder you can time occultations with a stopwatch. However you will still need access to a timebase (e.g. WWVH or a telephone talking clock). You can either start your stopwatch at a known time signal and then stop it when you see the occultation. Or you can start the watch at the occultation and stop it at a subsequent known time signal. From the elapsed time on the watch you can then work out the time of your occultation.
When you make an occultation timing there will always be some delay between when the event occurs and when you yell out "off" or "on" (or push the button on your stopwatch). This delay is your reaction time or Personal Equation. You should always try to minimise your reaction time wherever possible. You can do this by being fully prepared, and maintaining peak concentration around the predicted time of the occultation. Experienced observers can sometimes estimate their own reaction times with reasonable accuracy - for example, the average reaction time for a moderately experienced observer is around 0.4 seconds. If you do have a good estimate of your reaction time you can subtract this from your observation and note it on the report form.
However... it has been shown that reaction times for less experienced observers are usually MUCH longer than 0.4 seconds. Even when the observer feels that he or she has made a "good" timing, often that timing may be out by between one and two seconds. Newer observers usually don't realise how long they take to react to an event. For this reason, unless you feel very sure about your own reaction time, you should not attempt to report it.
Various computer programs exist to help you estimate what your reaction time might be. For one such program click here. However even these programs are of only limited use because the concentration you experience sitting comfortably at a computer keyboard is usually vastly different to what you experience bent over a telescope on a cold and blustery night!
It is because of this inherent reaction time error that visual occultation timings should not be quoted to any more than 0.1 second precision. If you quote your timing to 0.01 second precision just because your stopwatch measures down to this level, the second digit is meaningless.
[Top of Page][Return to Home Page]