A grazing occultation occurs when the northern or southern edge of the Moon passes very close to a star. A graze can be thought of as the star's light casting a shadow of the mountains on the edge of the Moon down onto the Earth. As the Moon moves past the star, this area of shadow, which is generally a few kilometres wide, will speed across the Earth in a roughly east-west direction.
By situating several telescopes at different places in a roughly north-south direction within this moving shadow, different observers will see the star pass behind different parts of the lunar mountains. That is, each observer will see the star alternately DISAPPEAR and REAPPEAR from behind mountains on the Moon's limb. The star may also BLINK, as it is momentarily occulted by the tip of a lunar peak, FLASH as it momentarily reappears in a lunar valley, or DIM as it passes extremely close to the lunar surface. Because each graze observer is at a different position with respect to the shadow of the lunar mountains, each will time different occultation events at different times. Later plotting of these times versus location allows a picture of the mountains on the edge of the Moon to be built up.
You should aim to record your timings of the occultation events to at least 0.5 sec, and preferably 0.1 sec precision.
Using a Short-Wave Radio and Tape Recorder
This method is by far the most popular timing method, as it requires only one person per observing team. In this method the observer tunes a short-wave radio to either of the time signal stations VNG AUSTRALIA (on 2.5, 5.0, 8.638, 12.984 or 16.0 MHz), or WWVH HAWAII (on 5.0, 10.0 or 15.0 MHz). The one second time pips from these stations need to be clearly audible. (Note: Be alert for interference from the Japanese station JJY). A tape recorder is then positioned in such a way that it will clearly pick up both the radio time pips and the comments the observer at the telescope makes as the graze progresses.
As each occultation event occurs the observer calls out in a loud voice "OFF" for a disappearance, "ON" for a reappearance, "BLINK" for a momentary disappearance, or "FLASH" for a momentary reappearance. (Rarely, "DIM" indicates that the star has faded although not completely disappeared). Having recorded the voice comments on tape, superimposed over the radio time signals, the tape can later be played back and the time of each comment determined.
Note: WWVH provides voice announcements before each minute beat. VNG provides voice announcements less often (and sometimes the time stated is in error even though the time pips are not), so it is important that you clearly state a reference time onto the tape for at least one minute marker.
Using a Stopwatch and Time Signal
This method generally requires an observer, timekeeper and recorder (although sometimes only two are necessary); it was more popular in the days before portable cassette recorders became widely available.
In this method the timekeeper accurately starts the stopwatch at some known time shortly before the graze begins (e.g. at a known radio time signal), and the recorder notes this time. The timekeeper then sits with his/her eyes glued to the watch face, so that as the observer calls out "off", "on", "blink", "flash" or "dim" he or she can immediately note the elapsed time on the watch, and call this out to the recorder who immediately notes it down. (If the stopwatch is digital, the LAP function can be used to assist). After the graze is over the watch is then accurately stopped to a known time signal, and both this and the elapsed time noted. It is then easy to work out the true time of each event from the notes made.
Note: For this method to be accurate the stopwatch (especially if analogue) should be calibrated beforehand - i.e. its rate of gain or loss over one half or one hour should be determined.
Also see the Other Points section on the "Planning A Grazing Occultation" page.
WHAT TO BRING
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