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An occultation , covering or eclipse is the passage of one apparently larger celestial body in front of another.

In the opposite case, when the covered celestial body is the apparently larger one, one speaks of a transit or passage .

The most common of these phenomena are occultations by the moon, which is already using a small telescope for amateur astronomers can observe every few days. Occultations of planets by the moon are rarer. Coverings of bright stars by planets or mutual coverings of planets are extremely rare celestial events. In purely geometric terms, solar eclipses are also among these phenomena.

Saturn coverage by the moon on November 3, 2001 (time lapse)


The occultation is a special case of a conjunction . The apparent angular distance between the two celestial bodies is so small that the closer celestial body partially or completely covers the more distant one from the perspective of the observer.

A special case of the occultation is the eclipse , if the covered object is so bright that this naming is appropriate, in particular the solar eclipse (the occultation of the sun by the moon ), but in a certain sense also the lunar eclipse (the occultation of the Sun through the earth , namely seen from the moon). However, lunar eclipses are real eclipses because the moon moves through the earth's shadow , whereas a solar eclipse is strictly speaking a "solar cover" or an "earth eclipse" because part of the earth's surface is darkened by the moon's shadow.

While star occultations by the moon and occultations of the planetary moons by their mother planets occur relatively frequently, star occultations by planets and asteroids are very rare occurrences from a fixed observation point. Mutual planetary occultations are even rarer . Since 1818 no one planet has been covered by another, the next one will not take place until 2065.

The Bessel elements are used to describe the geometrical properties of a cover of stars or planets observed from Earth by the moon , as well as for solar eclipses.

Star occultation

Star occultation by the moon

The occultation of a fixed star by the Earth's moon , but also by other bodies in our solar system such as planets or asteroids , is called star cover . For the observer on earth, the star suddenly disappears behind the celestial body - which is definitely an element of tension - and later reappears just as suddenly on the other side.

Star occultations by the moon are most common (at locations in Europe about 5–10 per month up to stars 6th mag ). They take about 55 minutes for central occultations, and correspondingly shorter for grazing ones. The occultation of bright stars can also be observed with open eyes, especially at the dark edge of the moon, but it can be more precisely captured telescopically .

Time measurements with a good stopwatch reach about 0.1 seconds (electro-optical sensors are even more precise), from which the moon orbit and the height of the moon mountains were determined until recently . Longer series of measurements result in a few decameters in height.

Occultations by planets, however, are much less common. Star occultations are also observed from the orbit of the Hubble Telescope .

In the case of planets and asteroids, important data of the celestial body can be calculated from the duration of the eclipse and the course of the measured light curve , e.g. B. Size and shape of the asteroid or the density and composition of the planet's atmosphere . Depending on the gases it contains, it absorbs certain parts of the starlight so that it is not covered suddenly.

Optical occultation

Occultation of Rhea by Dione (each of Saturn's moons )

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

Through the behavior of the starlight at the moment of obscuration (sudden disappearance of the starlight in bodies without atmosphere such as the moon, gradual disappearance of the same in bodies with an atmosphere such as in Venus ) one could make statements about possibly existing atmospheres of the celestial bodies, which cause the obscuration.

The exact determination of the contact times when the moon is covered with stars enables an exact measurement of the lunar orbit. In some cases, it is also possible to directly determine the diameter of the covered star by observing the moon's occultations with high temporal resolution.

Based on grazing Occultations in which the edge of the Earth's moon obscured one star, the profile of the moon's surface can be determined relatively accurately. Grazing star occultations are often observed and evaluated by amateur astronomers . Analogous to this, the darkening of fixed stars by asteroids is used to learn more about their shape. Organized via the Internet, several astronomers look at the eclipse at the same time in order to then recalculate the shadow cast by the asteroid from its local distribution and display it graphically.

The rings of the planet Uranus , which are not directly visible even with the most powerful telescopes in the world , were  discovered on March 10, 1977 when this planet covered the star SAO 158687 (HIP 71567). Before and after the passage of the planet, the light of the star behind was briefly darkened several times.

Uranus passes the star HIP 71567 on March 10, 1977 (simulation)

Planetary coverings by the moon and mutual coverings of planets are of little scientific importance, since the contact times are difficult to determine when the luminosity of the covered object is lower.

Radio occultation

See main article: Radio occultation

Radio occultation is a new way of studying the atmosphere of planets. The radio signal from a satellite disappearing behind a celestial body is observed. GPS satellites are suitable transmitters on earth . From the exact knowledge of the transmitted signal, one obtains indications of the properties of the atmosphere by comparing it with the received signal that made its way through the Earth's atmosphere. For example, the scientific program of the CHAMP satellite was expanded to include this type of remote sensing of the atmosphere in order to obtain information about the temperature and water vapor distribution of the earth's atmosphere.

Mutual coverings

Mutual occultation in an eclipsing star

Two celestial bodies can only cover each other if one can step both in front of and behind the other as seen by the observer. Since in almost all known cases the two bodies are not the same size, one of these coverings is a passage .

The following cases of mutual cover exist:

  • eclipsing stars
  • outer planets and their moons (transits of larger moons can be observed)
  • Dwarf or minor planets and their moons
  • Moons of a planet (e.g. mutual coverings of the moons of Jupiter )
  • Mercury and the Sun ( Mercury transit , occultation of Mercury by the Sun)
  • Venus and the sun ( Venus transit , covering of Venus by the sun)
  • Mercury and Venus (only planets that can cover each other)

Frequency / occurrence

Coverings of fixed stars and planets by the moon occur relatively frequently because the moon has an angular diameter of 30 arc minutes and, as a body close to the earth, has a large horizontal parallax . Since the nodes of the moon move retrograde through the ecliptic in a period of 18.6 years , practically all stars that are located in an area 6 degrees north or south of the ecliptic will be covered at some point. Of course, not all of these eclipses can be observed from a given location, because they can take place at a time when the moon is below the horizon, or the eclipse can only be observable in other regions due to parallax . Another limitation is that some occultations cannot be observed or are extremely difficult to observe if they take place in the daytime sky.

It is not uncommon for the sun to cover stars and planets, but they are uninteresting because of their unobservability (at least with the usual optical devices, not necessarily with radio astronomical methods, provided the object to be covered is a radio source). However, covers of radio sources ( e.g. quasars ) by the sun have been used to test general relativity .

Coverings of bright stars by planets, moons of other planets or asteroids as well as mutual coverings of planets are extremely rare celestial events. Because of the planetary parallax, they are not necessarily observable everywhere where the planet is above the horizon at the time of the eclipse. Such events are so rare because planets move more slowly across the sky than the moon and they also have a much smaller angular diameter than the moon. In addition, there are far greater restrictions as to which stars can be covered at all, because in contrast to the moon, the orbital nodes of the planets move only very slowly (orbital period> 10,000 years versus 18.6 years for the moon). This means that not all fixed stars that are within the value of the maximum ecliptical latitude that a planet can reach can also be covered. In the period from approx. 5000 BC No planet covers the fixed star Aldebaran until 5000 AD . The other three first magnitude stars near the ecliptic ( Antares , Spica and Regulus ) can be traced back to 5000 BC. BC and 5000 AD Antares can only be covered by Venus , because only this planet can pass Antares both north and south. During this period, Spica and Regulus can only be covered by the lower planets Mercury and Venus, as only these planets can pass these two stars both north and south. In the distant past and distant future this changes because of the node migration (and possibly also due to the proper movement of the fixed stars). Nunki is currently the brightest fixed star that can in principle be covered by an upper planet , namely by Mars. However, this was last done on September 3, 423.

Occultations of bright fixed stars (<4 mag ) and planets by planets
between 1800 and 2100

Day Time (WZ) Covering planet Covered object Elongation towards the sun
December 9, 1802 7:36 a.m. Mercury Akrab 16.2 ° West
December 9, 1808 8:34 pm Mercury Saturn 20.3 ° West
December 22, 1810 6:32 am Venus Xi 2 sagittarii 11.1 ° East
January 3, 1818 9:52 pm Venus Jupiter 16.5 ° west
July 11, 1825 9:10 a.m. Venus Delta 1 Tauri 44.4 ° West
July 11, 1837 12:50 p.m. Mercury Eta Geminorum 17.8 ° West
May 9, 1841 7:35 pm Venus 17 Tauri 9.2 ° East
September 27, 1843 6:00 p.m. Venus Eta Virginis 3.2 ° West
December 16, 1850 11:28 am Mercury Lambda Sagittarii 10.2 ° East
May 22, 1855 5:04 am Venus Epsilon Geminorum 37.4 ° East
June 30, 1857 12:25 am Saturn Delta Geminorum 8.4 ° East
December 5, 1865 2:20 pm Mercury Lambda Sagittarii 21.0 ° East
February 28, 1876 5:13 am Jupiter Akrab 97.6 ° West
June 7, 1881 8:54 pm Mercury Epsilon Geminorum 21.2 ° East
December 9, 1906 5:40 pm Venus Akrab 14.9 ° West
July 27, 1910 2:53 am Venus Eta Geminorum 31.0 ° west
December 16, 1937 6:38 pm Mercury Omicron Sagittarii 11.6 ° East
June 10, 1940 2:21 pm Mercury Epsilon Geminorum 20.1 ° East
October 25, 1947 1:45 a.m. Venus Zuben-el-dschenubi 13.5 ° East
July 7, 1959 2:30 p.m. Venus Regulus 44.5 ° East
September 27, 1965 3:30 p.m. Mercury Eta Virginis 2.6 ° West
May 13, 1971 8:00 p.m. Jupiter Akrab 169.5 ° West
April 8, 1976 1:00 a.m. Mars Epsilon Geminorum 81.3 ° East
17th November 1981 2:27 pm Venus Nunki 47.0 ° East
November 19, 1984 1:32 a.m. Venus Lambda Sagittarii 39.2 ° East
4th December 2015 4:14 pm Mercury Theta Ophiuchi 9.6 ° East
November 17, 2035 3:19 pm Venus Pi Sagittarii 42.1 ° West
October 1, 2044 22:00 O'clock Venus Regulus 38.9 ° West
February 23, 2046 7:24 pm Venus Rho 1 Sagittarii 45.4 ° west
November 10, 2052 7:20 a.m. Mercury Zuben-el-dschenubi 2.8 ° West
November 22, 2065 12:45 p.m. Venus Jupiter 7.9 ° West
July 15, 2067 11:56 am Mercury Neptune 18.4 ° West
August 10, 2069 8:25 pm Venus Zavijava 38.4 ° East
October 3, 2078 22:00 O'clock Mars Theta Ophiuchi 71.4 ° East
August 11, 2079 1:30 a.m. Mercury Mars 11.3 ° West
October 27, 2088 1:43 pm Mercury Jupiter 4.7 ° West
April 7, 2094 10:48 am Mercury Jupiter 1.8 ° west

These events are not visible wherever at the specified time both stars are above the horizon. Observing some of these events is extremely difficult due to the nearby sun in the sky.

See also


  • Wolfgang Held: Solar and lunar eclipses - and the most important astronomical constellations up to 2017 . Free Spiritual Life Publishing House, Stuttgart 2005. ISBN 3-7725-2231-9 .
  • Marco Peuschel: Conjunctions, Coverings and Transits: The Little Almanac of the Planets . Self publication . Engelsdorfer Verlag, 2006. ISBN 978-3939144663 .

Web links

Commons : Occultation  - collection of images, videos and audio files

Individual evidence

  1. The rings of Uranus , JL Elliot, E. Dunham & D. Mink, Nature 267, 328-330 (May 26, 1977)