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.

TIMING METHODS

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.

USEFUL HINTS

1. Set up your telescope as close as possible to the position assigned by your graze leader. If an exact position has not been assigned, set up your station at a point whose geographical position can be easily determined afterwards (e.g. a road intersection) and make a careful note of this.
2. Note that BEFORE FULL MOON the star will approach the Moon from its DARK limb side. AFTER FULL MOON the star approaches across the LIGHTED limb (which usually makes it more difficult to find).
3. "Lock on" to the star at least 15 minutes before graze time. The moon moves its own diameter in an hour, so the star might appear a surprising distance from the Moon at first. If you cannot find the star, check that you are looking near the correct lunar cusp. If necessary ask for help.
4. Make sure well before the graze that your telescope is stable and at a comfortable height. Check the dewcap (if any) is secure.
5. Radios often function better when off the ground and on a box or stool. If reception is poor, try hitching your aerial to the nearest piece of fence wire (so long as it is not electrified!)
6. Make sure you are at the telescope, observing with maximum concentration, from at least three minutes before the central graze time predicted for your location, to at least three minutes afterwards.
7. Remember that the mountains behind which the star will disappear are not usually sunlit, and hence cannot be seen. Therefore events can occur SUDDENLY and observers should be prepared for this.
8. It is always possible for a star involved in a graze to be an undiscovered double. Hence always be prepared for unusual multiple events. (E.g. a stepwise Disappearance or Reaprearance; gradual fading; etc).

Also see the Other Points section on the "Planning A Grazing Occultation" page.

WHAT TO BRING

1. WARM clothes
2. Telescope, sturdy tripod, and medium power eyepiece
3. Short wave radio (with NEW batteries)
4. Tape recorder (with NEW batteries)
5. Blank cassette tape (plus a spare)
6. Stopwatch (if applicable)
7. Torch
8. Clipboard (optional) and paper
9. Pen/pencil

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