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ASNSW's Occultation Timing Section

Section Leader: Chris Douglass

An Occultation is the term given to an event where a body (like the moon or an asteroid) moves between an observer and a distant referencing point (usually a star), thereby obscuring or blocking the light from the distant point.

If an observer notes the instant the event occurs then information can be gleaned concerning all three players.

Just consider, a Solar Eclipse is actually a Lunar Occultation and it is unlikely that anyone would consider a Solar Eclipse boring by any stretch of the imagination.


Lunar Occultations are the most common events to observe because the moon is nearby, is about ½ degree in diameter and will occult many stars during its passage through the sky every month. They are pretty much unaffected by light pollution as most times the moon itself is the brightest thing in the sky and so the observation of these events is an ideal time for the city or suburban observer to get ‘Telescope Time’. The easiest events to observe occur at the dark limb and disappearance events occur with a waxing moon and reappearances occur at the waning moon. Lunar Grazing events occur at lunar polar regions.

The observation of Asteroid Occultations is an ideal opportunity to measure a chord across an asteroid with great precision. If many observers obtain positives for the same event then the shape of the asteroid can be determined. The astrometry obtained can be used to define the orbital elements of the asteroid that are orders of magnitude better than the best astrometry available from conventional means. Sateroids, that is the satellites of Asteroids have been discovered by the observation of these ‘Asteroidal’ events.

The observation of planetary occultations can be used to discover and analyse the composition of the atmospheres, measure the thickness and composition of the rings of Saturn and Uranus.

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