Occultation


occultation Event in which one body is obscured by another - in other words, when a distant object is hidden by a nearer one. An obvious example is a SOLAR ECLIPSE, which is, strictly, an occultation of the Sun by the Moon.

The term occultation can be applied to a range of celestial objects, but the most commonly observed are lunar occultations, which occur as the Moon obscures stars during its passage along the band of the ECLIPTIC. The most readily observed lunar occultations are disappearance (immersion) events, occurring on the Moon's leading (easterly) limb before full. At this time, the leading limb is dark; occultations occurring when the Moon is a waxing crescent may be especially favourable for observation, with the limb dimly visible by EARTHSHINE as it approaches the target star. Inexperienced observers are often surprised by the suddenness with which the star vanishes: this is due to the Moon having no atmosphere and the star being at such a great distance that it is virtually a point source of light.

After full moon, reappearance (emersion) events are more readily observed, occurring at the dark trailing lunar limb. As with disappearances, these events occur very abruptly. Successful observation depends on good predictions, including an accurate forecast of the POSITION ANGLE on the Moon's limb at which reappearance will take place.

Visual study of lunar occultations has been undertaken for over a century, and still plays a role in astronomy. Precise timings of events, coupled with precise positional data for the observing location, provide useful information on the Moon's orbital motion. Timing accuracy can be further improved by the application of video recording techniques. High-speed photometry has also been used to obtain light curves for stars during immersion, allowing identification of previously unknown close double stars and measurement of the diameters of some of the nearer stars.

In the course of a year, the Moon may occult more than 4000 stars in the range of a fairly standard amateur telescope. Good star catalogues and modern computing capabilities make the calculation of predictions relatively straightforward. Observational reports are collected by the International Lunar Occultation Centre in Japan for reduction, and the resulting data are available for use by professional astronomers.

Stars are not the only bodies to be occulted by the Moon; on occasion, a planet or asteroid may also be occulted. In the case of Jupiter, the planet's own satellite system can be seen to be occulted. The major planets, being very much closer than the stars, show disks, so that occultations are gradual, lasting several seconds; the brightness of Venus, Mars, Jupiter and Saturn allows events occurring at the Moon's bright limb to be observed quite readily. Occultations of the planets, while rare and yielding less scientific data than those involving stars, can be spectacular to watch in a small telescope or binoculars.

A spectacular planetary occultation was observed from central England on 1980 October 4, when, in the pre-dawn sky, Venus was grazed by the Moon's limb, being only partially obscured. Lunar mountains and valleys were seen in profile against Venus' bright gibbous disk. Graze occultations for a star can be seen along a track no more than 1 km (0.6 mi) wide on Earth's surface and are consequently rare. Teams of observers spread perpendicular to a graze track can obtain multiple timings of events as the Moon's irregular limb alternately covers and reveals the star, gaining much useful information about the lunar profile.

Planets may also, on rare occasions, occult stars. Brightening and fading of stars during immersion or emersion can provide information about planets' atmospheric layering. Among noteworthy events was the occultation of 28 Sagittarii by Saturn and its rings on 1989 July 3. The following night, Saturn's satellite Titan occulted the star in an event seen extensively from Europe, with a brightening at mid-occultation resulting from refraction of starlight by Titan's dense atmosphere. The rings of Uranus were discovered during an occultation of the star SAO 158687 on 1977 March 10.

At certain times, when the plane of their orbits aligns with Earth, Jupiter's Galilean satellites may undergo mutual occultations. They are also, of course, frequently seen to be occulted by Jupiter itself as they pass behind it on their orbits.

Occultations of stars by asteroids are also rare, but provide extremely valuable opportunities to determine the profiles of these small bodies. Essentially, an asteroid casts a 'shadow' equivalent to its diameter (usually less than 200 km/120 mi) as it occults a star. The critical observation in these cases is whether an occultation occurs at all, and its duration (defining a cord across the asteroid's diameter). Secondary occultations have revealed the presence of satellites orbiting some asteroids, including (6) Hebe, (18) Melpomene and (532) Herculina.

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which method relies on the mass of a dark object revealing its presence?...

a: gravitational lensinga.

this is one of the reasons why we think darm matter is out there

Why no planetary occultations visible from Earth between 1818 and 2065, when otherwise comparably frequent?...

This has to do with the plane of the ecliptic. The imaginary line extending from the Sun through the Earth in our Solar System.

Not all planets lie along this

The initial discovery of the rings of Uranus came accidentally during observations of its ... ?...

Uranus’s rings were discovered when the planet passed in front of a star. The star unexpectedly disappeared five times BEFORE it was blocked by the planet and

Th best way to find the exact distance to the Moon is to:?...

dThe best Earth-Moon distance come from laser measures from retroreflectors left on the moon by the Apollo 11, 14, and 15 missions, and also the Soviet Lunokhod

The best way to find the exact distance to the Moon is to:?...

D is the method used by scientists nowadays.f: shoot yourself from earth to the moon with a really really long tape measure.f.) google it?

Rings around Uranus were discovered by?...

meC and later BA and C technically. The initial observation of rings was actually made by William Herschel in 1789, however his observations were not definitive

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