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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

Using a Stopwatch and Time Signal


  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.


  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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