Occultation

As occultation, occultation or eclipse is called the passing of a seemingly larger celestial body in front of another, with the ratio refers to the apparent size. In the opposite case, when the overcast sky body which is apparently larger, it is called a transit or passage.

Definition

The occultation is a special case of a conjunction. The apparent angular distance between the two celestial bodies is so low that the nearer celestial bodies more distant from the perspective of the observer partially or completely covered.

A special case of the occultation is the darkness ( eclipse ) when the covered object is so bright that this naming is appropriate, especially so the eclipse ( the occultation of the sun by the moon ), but also the lunar eclipse ( the occultation of the Sun by the earth seen from the moon ). However, is a lunar eclipse is a real eclipse of the moon, because then this is located in the shadow of the Earth, whereas an eclipse actually a " solar cover " or a " Erdfinsternis " is because a portion of the earth's surface is eclipsed by the moon's shadow.

While occultations are relatively common by the moon, and occultations of the planets moons by their mother planet occultations are seen by planets and asteroids from a fixed observation of very rare events. Even rarer are mutual planetary occultations. Since 1818 no cover of a planet by another took place, the next will not take place until 2065.

To describe the geometric properties of an observable from Earth occultation of stars or planets by the moon the Bessel elements are used, as well as for solar eclipses.

Occultation

As the occultation occultation of a fixed star is designated by the Earth's moon, but also by other bodies in our solar system, such as planets or asteroids. For an observer on Earth, the star disappears abruptly behind the celestial bodies - which certainly represents a tension member - and later emerged on the other side just as suddenly back on.

Occultations of stars by the moon are most common ( at locations in Europe about 5-10 per month to stars 6.Größe ). You take correspondingly shorter at central coverages about 1 hour, at grazing. The occultation of bright stars can be observed freiäugig, especially at the dark edge of the moon.

Time measurements with a good stopwatch reach about 0.1 seconds ( electro-optical sensors in more detail ), from which it has determined until recently the moon's orbit and the amount of the Mountains of the Moon. For longer measurement series a few tens of meters result in height.

Occultations by planets are, however, much less common, those with small planets than once per year. Also from the orbit of the Hubble telescope occultations are observed.

For planets and asteroids can from the duration of the eclipse and the magnitude of the measured light curve important data of the celestial body can be calculated, eg Size and shape of asteroids or the density and composition of the planetary atmosphere. It absorbs depending on the gases contained certain amounts of starlight, so that the cover rather than abrupt.

Optical occultation

Occultations of fixed stars by the moon, planets, planetary moons or asteroids were especially before the age of space travel a great scientific importance, because you can by measuring its duration determine the diameter of these celestial bodies.

Due to the behavior of the star's light at the moment of coverage ( sudden disappearance of the starlight at airless bodies such as the moon, the gradual disappearance of the same case of bodies with atmosphere similar to Venus ) you could make statements about any existing atmospheres of the heavenly bodies, which cause the cover.

The accurate determination of the contact times for stellar occultations by the Moon enables accurate measurement of the moon's orbit. Due to high temporal resolution observations of star occultations by the moon, it is also possible in some cases to determine the diameter of the star covered directly.

On the basis of grazing occultations in which the edge of the Moon obscured a star, the profile of the lunar surface can be determined relatively accurately. Grazing occultations are frequently observed and evaluated by amateur astronomers. Be Analogously, obscurations of fixed stars used by asteroids to learn more about its shape. Organized over the Internet, several astronomers consider the coverage at the same time, and then from their local distribution recalculated the cast by the asteroid shadow and depict.

The self is not directly visible with the strongest telescopes in the world rings of Uranus were discovered at a coverage of the star SAO 158687 (HIP 71567 ), by that planet on March 10, 1977. Before and after the passage of the planets of the light behind the star was repeatedly briefly darken.

Planetary occultations by the moon and mutual occultations of planets only have a low scientific significance, because the contact times to determine bad at lower luminosity of the covered object.

Radio Occultation

See main article: Radio Occultation

The radio occultation is a new method to study the atmosphere of planets. The radio is a satellite signal is observed, which disappears behind a celestial body. At the earth GPS satellites are suitable transmitters. From the exact knowledge of the transmitted signal is obtained by comparing with the received signal, that took its way through the Earth's atmosphere, clues to the properties of the atmosphere. For example, the scientific program of the satellite CHAMP has been extended to this type of atmosphere remote sensing to obtain information about the temperature and water vapor distribution of the earth's atmosphere.

Mutual occultations

Mutual occultations of two celestial bodies are only possible if one can be seen from the observer occur both before and behind the other. Since almost all known cases, the two bodies are not the same, one of these coverings is a passage.

The following cases of mutual occultations exist:

• Eclipsing stars
• Outer planets and their moons ( transits larger moons can be observed)
• Dwarf or minor planets and their moons
• Moons of a planet (eg mutual occultations of Jupiter's moons )
• Mercury and the Sun (Mercury transit, coverage of Mercury in the sun )
• Sun and Venus (Venus Transit, coverage of Venus by the sun )
• Mercury and Venus (the only planet that can cover each other )

Frequency / occurrence

Occultations of fixed stars and planets by the moon are relatively common because the moon an angular diameter of 30 arc minutes and as near-Earth body shows a large horizontal parallax. Since the nodes of the moon walking in a period of 18.6 years once retrogradely through the ecliptic, during this period, virtually all stars that are located north or south of the ecliptic in an area of ​​6 degrees, eventually covered. However, from a given location, of course, not all of these coverages are observed, because they can take place at a time when the moon is below the horizon, or the cover may be observable parallax due only in other regions. Another limitation is added by the fact that some coverages are not or only extremely difficult to observe when they take place in the daytime.

Occultations of stars and planets from the sun are not exactly rare, but because of their unobservability (at least with the usual optical devices, not necessarily with radio astronomical methods if the object to be covered is a radio source ) uninteresting. However, occultations of radio sources (eg quasars ) uses the sun to check the general theory of relativity.

Occultations of bright stars by planets, moons of other planets or planetoids and mutual occultations of planets are extremely rare celestial events. They are also not observable everywhere necessarily because of Planetenparallaxe where the planet is at the time of cover above the horizon. Such events are so rare, because planets slowly drag across the sky as the moon, and they also have a much smaller angular diameter than he. There is also far greater restrictions on what stars can be covered at all, because in contrast to the Moon, the railway junction of the planets migrate very slowly (current time > 10,000 years versus 18.6 years for the moon). Thus, not all the fixed stars, which are located within the value of the maximum ecliptic latitude, which can reach a planet, are also covered. So can cover the fixed star Aldebaran in the period from about 5000 BC to 5000 AD, not a planet. Of the other three ekliptiknahen stars 1 Size ( Antares, Spica and Regulus ) can be covered in the period from 5000 BC and 5000 AD Antares only by Venus, because only this Planet Antares both north and south pass can. Coverages of Spica and Regulus are in this period only by the inferior planets Mercury and Venus, as only these planets can pass both north and south of these two stars. In the distant past and distant future this will change because of node migration (and possibly also by the proper motion of the fixed stars ). Currently, Nunki the brightest star that can be in principle covered by an upper planets, by Mars. However, this was last done on 3 September 423

Occultations of bright fixed stars (<4 mag) and planet by planet 1800-2100

These events are not visible everywhere where are the specified time the luminaries on the horizon. The observation of some of these events is highly complicated by the standing nearby in the sky the sun.