RASNZ Occultation Section - Grazing Lunar Occultations

GRAZING LUNAR OCCULTATIONS

Occasionally an observer will see a star just seem to skim along the Moon's northern or southern edge. As it does so, the shadow of the mountains on the edge of the Moon is cast onto the Earth. Someone within this band of shadow will see the star alternately disappear behind lunar mountains and reappear within lunar valleys. Such grazing occultations can be very exciting to watch!

Because the shadow of the Moon’s mountains cast onto the Earth is only a few kilometres wide, observers only a few hundred metres apart (measured perpendicular to the direction of motion of the shadow) will each see the star occulted at a different time. This is because each observer sees an occultation by a slightly different part of the same lunar mountains. When results from several observers are combined an extremely accurate picture of the edge of the Moon can be built up. In fact, this technique enables us to gather data about the mountains on the Moon's limb far more accurately than by any other method.

Unlike total lunar occultations, a specific grazing occultation can only be observed from a very narrow (but very long) band on the Earth's surface. It is unusual for the track of a grazing occultation to pass across a fixed observatory site. So graze observers must be prepared to use portable telescopes and to travel to wherever the graze is predicted to occur.

PREDICTIONS

Grazing occultation prediction data is prepared by the International Occultation Timing Association in Europe and the U.S., and predictions are computed by Alfred Kruijshoop in Melbourne, Australia. Alfred distributes data to observers in Australia, while the RASNZ Occultation Section passes on information to New Zealand observers. In addition the OCCULT software package allows anyone to compute grazing occultation data on their PC.

For grazing occultation predictions for your region you will need to send us the following information:

Approximate Longitude
Approximate Latitude
Telescope Aperture (cm)

Email or street address

Observers in Australia should send their requests by regular mail to Alfred Kruijshoop, while New Zealand predictions can be obtained from Graham Blow or Brian Loader

Click here for a summary of forthcoming bright grazing occultations. (Predictions for some bright events are available on-line).

Click here for information on how to use graze predictions.

TIMING AND REPORTING GRAZE OBSERVATIONS

For general information on how to time a grazing occultation, click here.
For specific information on how to set up to observe a grazing occultation, click here.

Occultation timings are collected by the Section’s co-ordinator in Victoria, Australia, who batches them with others and sends them on to the International Lunar Occultation Centre (ILOC) in Japan.

For information on how to report your timings, click here.
For a copy of the grazing occultation report form, click here.

REDUCTIONS

The greatest value in observing grazing occultations comes from the very detailed picture of the edge of the Moon they can provide us. Timings from a group of nearby observers can be plotted and then combined in a "join the dots" fashion to provide extremely accurate information about the mountains on the edge of the Moon.

Here are the results from a grazing occultation of Eta Tauri (one of the Pleiades stars) observed near Wellington, New Zealand, on 25 October 1991. Distance in kilometres on the ground is plotted vertically, and time horizontally. The plot is drawn with north towards the top, so since this was a graze at the southern edge of the moon, the moon's mountains protrude "downwards". (i.e. solid moon is towards the top; deep space is towards the bottom).

Eta Tauri Grazing Occultation - 1991 October 25 [17K]

On this plot:

The "Smooth Moon Profile" is drawn - i.e. where the edge of the moon would be if it were a perfect sphere with no mountains. (On this plot the smooth moon profile does not look circular because the horizontal scale has been compressed by about 20 times).
The irregular dotted line shows the expected shape of the moon's limb in the occultation region.
The solid horizontal lines represent the actual observations made by the observers, who were positioned roughly along a north/south line. Where the lines are thick the observers were still able to see the star. Where the lines are thin the star has disappeared from view.
By interpolating and drawing lines between the places where the star disappeared or reappeared for adjacent observers, the actual profile of the edge of the moon can be deduced. In this case, the observed profile is quite a bit different from the predicted profile.

For plots of other recent events, click here.

This page was last updated on 11 March 1999.
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