The Astronomical Society has a number of members who are actively involved with the observation of occultations by the Moon, as well as, occultations by minor planets.
 
 

An occultation occurs when one celestial body passes in front of another, whereby a distant object is hidden by a nearer one. Strictly, therefore, a solar eclipse is really an occultation of the Sun by the Moon. But occultation observing involves two main types of events:

 1.) Occultations of stars (and occasionally, solar system objects) by the Moon.

 2.) Occultations of stars by minor planets (asteroids), and occasionally by the major planets.

 The most important information that observers obtain through watching occultations is a record of the times of the events. These timings are sent to relevant national and international organisations which keep records of the observations and make use of them for various projects.
Lunar occultations can be used to determine, with great accuracy, the positions of the stars and the Moon, and improve our knowledge of the Moon's orbit around the earth. The occultation of a star by a minor planet assists in improved determinations of the positions of the two bodies and, importantly, gives us information on the exact size and shape of the minor planet. Attempts can be made to search for minor planet satellites using this method.
To make serious occultation observations, the basic equipment requirements are a TELESCOPE (the larger the instrument, the more observations you will be able to make), a SHORT WAVE RADIO (timing of the events must normally be quoted correct to 0.1 second, so you must have an accurate time signal such as Australia's VNG or America's WWV/WWVH), and SOME MEANS OF RECORDING THE TIMINGS ( a tape recorder, synchronised stopwatch or similar).

LUNAR OCCULTATIONS

 1.) TOTAL occultations

 2.) GRAZING occultations

 A total occultation is one in which the star disappears at the eastern limb of the Moon and reappears some time later at the western limb. Before full Moon, disappearances take place at the dark limb and reappearances at the bright limb; after full Moon the situation is reversed. Dark-limb events are easier to observe because the star can be seen more easily, and disappearances are easier to observe than reappearances because the observer can see the star before the event takes place.

 The dark limb is not actually dark, but is illuminated by the EARTHSHINE - light is reflected onto the the Moon's surface from the Earth -which, unless the Moon is nearly full, allows the limb to be seen through the telescope. The larger the telescope, the easier it will be to see the dark limb. Earthshine is relatively easy to see through any instrument when the Moon is less than half full. As had been mentioned above, disappearances at the dark limb are the easiest type of event to observe. An interesting effect when watching these occultations is the star apparently 'waiting' on the Moon's limb for some time before it finally (and suddenly) disappears: this occurs when the magnification of the image no longer allows the difference in position between the star and the Moon's limb to be detectable.

 A graze occurs when the occultation takes place very close to the Moon's northern or southern edge. The time between disappearance and reappearance is very short, and as the Moon 'grazes' the star, the latter will occasionally reappear and disappear as its light shines past low-level areas on the Moon and is blocked by areas of high elevation. Any particular graze can only be seen from a particular track on the Earth's surface. For example, if the occultation occurs at the southern edge of the Moon, observers too far north of the track will see a total occultation, while those south of it will see no occultation at all - a 'miss'.

 Grazing occultation tracks are typically only a few kilometres wide, and so to observe a graze it is generally necessary to travel. For this reason, portable equipment is very useful.

 Graze timings are very valuable as they provide, among other things, a very useful way of determining north or south shifts in the Moon's position as well as an improved determination of the shape of the Moon's limb.

 The AST provides detailed predictions of both total and grazing lunar occultations, which have been calculated by the International Lunar Occultation Centre in Japan. The easiest events to observe are listed in the AST Bulletin. These are selected from very extensive lists: for serious observers, far more information is available than is actually printed in the regular "Lunar Occultations" article.

MINOR PLANET OCCULTATIONS

 Minor planets have much smaller apparent size, but occultations by them are easier to observe because the combined magnitude of the star plus minor planet is temporarily reduced to the magnitude of the minor planet alone. Provided that the drop is at least a magnitude or so, the event can be timed visually. If the minor planet is too faint to be seen, the star will simply disappear

 Although occultations by the Moon take place frequently, minor planet occultations worth attempting from a given location take place only a few times each year. Such events are only observable from narrow bands across the Earth's surface; the position and width of each band depends on the exact positions of the Earth, star and minor planet and the minor planet's diameter.

 Predictions of these events is a difficult task because, usually, the each position of the star and minor planet are not known with sufficient accuracy. Small errors may result in the 'shadow' of the minor planet missing the Earth altogether! Because of the uncertainty in the position of the path across the Earth, it is of value to obtain as many observations of each '' (close approach) as possible. The probability of a particular observer actually seeing an while monitoring an appulse is small, but 'negative' observations ARE useful: they provide information on locations from where the occultation was NOT seen.

 Observation of an appulse between a star and a minor planet is quite simple once you have found the star: you simply watch the star and time any disappearances and reappearances that occur. Some observers may report 'secondary' occultations, which may be due to 'satellites' of the main minor planet.

The AST receives predictions of minor planet occultations from the Royal Astronomical Society of New Zealand. In addition, it is possible to time eclipses of the satellites of Jupiter and Saturn, which are also very interesting and worthwhile. Opportunities to observe such events for Saturn occur only twice in a Saturnian year when we have a near-edge-on view of the planet's equator.

For more information on upcoming occultation events visit:

The Occultation Section of the Royal Astronomical Society of New Zealand (RASNZ).

 International Occultation Timing Association (IOTA).

 Sky & Telescope's Occultation Page

 World Asteroidal Occultations

New minor planets are still discovered on a regular basis. Most are very small, and some are only detected because they come close enough to the Earth.
Information on these Near Earth Objects can be found at The NEO Page.