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Total Occultation Predictions
Lunar Occultations of Double Stars

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When the Moon passes in front of a background star, the shadow of the Moon cast by the star sweeps across the Earth. When the leading or trailing edge of the Moon's shadow crosses an observer, the observer sees the star "disappear" or "reappear". These events are usually very sudden, and timing the instant of occultation is an important astronomical measurement.

Timings of total lunar occultations can be made almost every night of the year. All you need is some very basic equipment - a small telescope, some form of accurate timebase, and a stopwatch or tape recorder. To find out how to make timings, click here.


Observing lunar occultations is important because the results improve our knowledge of the position and motion of the Moon. For example, when you time the disappearance of a star behind the edge of the Moon to 0.1 second accuracy (a value easily attainable), you are actually fixing the position of the Moon's edge in space to an accuracy of about 80 metres. i.e. you are making a measurement with a precision of only 80 metres over a distance of 384,400 km. This is one of the most accurate measurements an amateur observer can make in any branch of science!

Combining many such measurements of the Moon's position over a long time gives astronomers new information about the Moonís motion and orbit. For example, total occultation observations have shown that the Moon is spiralling away from the Earth at a rate of a few centimetres per year.

Total lunar occultations have also been used to provide valuable information about star positions, about the hills and valleys on the edge of the Moon, and to discover new double stars.


Predicting total lunar occultations is not a simple exercise. Until recently such predictions were computed only by the International Lunar Occultation Centre (ILOC) in Japan and the International Occultation Timing Association (IOTA) in the United States. Now however, software is available which allows anyone with a computer to predict occultations on a night by night basis.

The program Win-OCCULT, authored by David Herald in Australia, provides accurate predictions of all types of occultations and related phenomena. You can obtain a copy of Win-OCCULT by downloading it from here, or by requesting it on a CD from the RASNZ Occultation Section. If you opt to download Win-OCCULT note that the files are now very large!

Software supplied by CD will also include the combined UCAC2/Tycho2 star catalog (about 55Mb). The CD costs $5 (Australian or New Zealand) to cover the cost of the media, postage and packing. Although Win-OCCULT is now freeware, we do recommend that you also consider enclosing a donation which we will pass to David Herald. (Suggested donation: $A20).

The RASNZ Occultation Section will compute limited predictions for you on a one-off basis. We will do this to give new observers an opportunity to see an occultation and to try their hand at timing one. To obtain predictions for your site send the following information to Brian Loader:

Predictions will be computed for up to two months and sent to you.


For information on how to time a total occultation, click here.

Occultation timings are collected by the Sectionís co-ordinators who perform an initial reduction of your observations, and then batch them with others to send to the International Occultation Timing Association's archive to be made available for research purposes.

For more information on reporting your timings, click here.


Once you have made an occultation observation it must be "reduced" - that is, the timing is used to compute the position of the edge of the Moon at the instant of the occultation. This quantity is then compared against the Moon's position computed from our knowledge of its motion. The difference between the two, measured in seconds of arc, is called your residual "O-C", or observed minus calculated. Combining O-Cís from many thousands of occultations, observed all around the edge of the Moon and from a large number of sites worldwide, allows astronomers to make very fine corrections to their theories of how the Moon moves. You can view the reductions of occultation timings made around the world at the International Lunar Occultation Centre. Once ILOC has computed residuals of occultation timings they are returned to observers by email or disk.

The OCCULT program also allows you to compute your own preliminary residuals, which is a useful way to check that your timings are in the right ball park. Remember however that for your timings to contribute to improving our overall knowledge of the Moon they must be reported to ILOC.

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