Occultation
From ArticleWorld
An occultation is the hiding of one celestial body by another. To the viewer of an occultation, this event may be called an eclipse. This is the case in a solar eclipse, which is, in reality, an occultation of the sun and a very small, partial eclipse of the earth by the moon. Occultations only occur when three or more celestial bodies are in a straight line. Occultations in our solar system occur at regular intervals and can be predicted with quite a bit of accuracy.
The word ‘’occultation’’ is generally used to describe a situation where the ‘’occulting’’ or light-blocking body has an angular size that is the same or larger than the light source that is being covered.
In cases where the occulting body is smaller than the background disc, the event is called a ‘’transit’’. In the cases of both transits and occultations, every planet, light source, moon, or star involved must be in a straight line in space. This situation in astronomy is called syzygy. Whether or not a celestial event is called an eclipse, occultation or transit depends on the size of the bodies involved and the location and position of the viewer.
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Transits
When the occulting body has a smaller angular size than the background body, the event is a transit. This is the case when small inner planets like Mercury and Venus pass in front of the nearer gas giants such as Jupiter and Saturn. In the case of the farthest planets, even a small body like Mercury has a large enough apparent size to completely cover the disc of a large body like Uranus. Thus, when Mercury passes in front of Uranus, it is an occultation.
The next transit predicted to occur will be that of Venus across the face of Jupiter in November of 2065. Several more occultations and transits will follow over the following decades. The solar system will have a very high incidence of such alignments after a gap of nearly 250 years since the last slew of such phenomena in the 18th and 19th centuries.
Types and causes
There are many common types and causes of syzygy that can be observed from earth.
Solar transit
Transits of the sun can only be caused by the inner planets, mercury and Venus. In order for a solar transit to occur, the transiting planet must be at an intersection with the earth’s ecliptic. These intersections between orbital planes are called nodes.
Lunar occultation
The most commonly observed and discussed examples of occultation are those that occur between the moon and the stars. Several large stars and other astronomical bodies are at an ecliptic altitude small enough that they can be regularly occulted by the moon. These include Antares, Aldebaran, Regulus and Spica as well as the Crab Nebula and the Pleiades cluster
Planetary occultations and transits
Occultations and transits between planets are rare, but occultations of the stars by planets occur regularly. Several such events have occurred during the 20th century. The natural satellites of planets can also be seen to occult and transit the satellites of others. This has been seen between the moons of Saturn and Jupiter when their orbits simultaneously intersected with the earth’s ecliptic.