Occultation

An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks from view (occults) an object in the background. In this general sense, occultation applies to the visual scene observed from low-flying aircraft (or computer-generated imagery) when foreground objects obscure distant objects dynamically, as the scene changes over time.

If the closer body does not entirely conceal the farther one, the event is called a

.

The symbol for an occultation, and especially a solar eclipse, is (U+1F775 🝵).

Occultations by the Moon

The term occultation is most frequently used to describe lunar occultations, those relatively frequent occasions when the

or 0.28 μrad, a star that is occulted by the Moon will disappear or reappear in 0.1 seconds or less on the Moon's edge, or limb. Events that take place on the Moon's dark limb are of particular interest to observers, because the lack of glare allows easier observation and timing.

The Moon's orbit is inclined slightly with respect to the

, can be occulted by the Moon.

Within a few kilometres of the edge of an occultation's predicted path, referred to as its northern or southern limit, an observer may see the star intermittently disappearing and reappearing as the irregular limb of the Moon moves past the star, creating what is known as a grazing lunar occultation. From an observational and scientific standpoint, these "grazes" are the most dynamic and interesting of lunar occultations.

The accurate timing of lunar occultations is performed regularly by (primarily amateur) astronomers. Lunar occultations timed to an accuracy of a few tenths of a second have various scientific uses, particularly in refining our knowledge of

.

Several times during the year the Moon can be seen occulting a planet.^[4] Since planets, unlike stars, have significant angular sizes, lunar occultations of planets will create a narrow zone on Earth from which a partial occultation of the planet will occur. An observer located within that narrow zone could observe the planet's disk partly blocked by the slowly moving Moon. The same mechanism can be seen with the Sun, where observers on Earth will view it as a solar eclipse. Therefore, a total solar eclipse is essentially the Moon occulting the Sun.

Occultation by planets

Stars may also be occulted by planets. Occultations of bright stars are rare. In 1959,

.

In rare cases, one planet can pass in front of another.^[5] If the nearer planet appears larger than the more distant one, the event is called a mutual planetary occultation. The last occultation or transit occurred on 3 January 1818 and the next will occur on 22 November 2065, in both cases involving the same two planets—Venus and Jupiter.

Jupiter rarely occults Saturn. This is one of the rarest events known,^[6] with the next occurrence on February 10, 7541. This event is visible worldwide since the duo would be positioned almost in opposition to the sun, in the border line between the constellations of Orion and Taurus. In some areas this occultation cannot be seen, but when viewed through even small telescopes, both gas giants appear to be in the same part of view through the eyepiece. The last one occurred in 6857 B.C.E.^[7]

Occultations by smaller bodies

A further set of occultations are those when a small Solar System body or dwarf planet passes in front of a star, temporarily blocking its light as seen from Earth.^[8] These occultations are useful for measuring the size and position of body much more precisely than can be done by other means. A cross-sectional profile of the shape of a body can even be determined if a number of observers at different, nearby, locations observe the occultation. Occultations have been used to calculate the diameter of trans-Neptunian objects such as 2002 TX₃₀₀, Ixion and Varuna. Software for coordinating observations is available for download at http://www.occultwatcher.net/

In addition, mutual occultation and eclipsing events can occur between a primary and its satellite. A large number of moons have been discovered analyzing the photometric light curves of small bodies and detecting a second, superimposed brightness variation, from which an orbital period for the satellite (secondary), and a secondary-to-primary diameter-ratio (for the binary system) can often be derived.

Examples

Notable occultations of asteroids

Name	Chords	Measured profile (km)
704 Interamnia	35	350×304
39 Laetitia	~16	219×142
94 Aurora	9	225×173
375 Ursula	6	216±10
444 Gyptis	6	179×150
48 Doris	4	278×142

Asteroids

• On 29 May 1983,
  Galileo in October 1991).^[9]
• On 12 March 2009 nine asteroids (
  2448 Sholokhov, 1746 Brouwer, and 191 Kolga) occulted notable magnitude stars, viewed from given places on the Earth.^[10]
• According to the 1998 European Asteroidal Occultation Results from Euraster, 39 Laetitia was observed by over 38 observatories in one occultation on 3 March 1998, which resulted in many chords being determined.^[11]
• The star
  Hudson River Valley, with the center of the shadow path following a line roughly connecting New York City, White Plains, Newburgh, Oneonta, Rome, and Pulaski before crossing into Canada near Belleville and North Bay, Ontario.^[12][13] Bad weather obscured the occultation.^[14]

Distant objects

• Preliminary results of a 6 November 2010 occultation by the dwarf planet Eris of a magnitude 17 star in the constellation of Cetus placed an upper limit on Eris's diameter of 2320 km, making it almost the same size as Pluto.^[15] Due to their slower movement through the night sky, occultations by TNOs are far less common than by asteroids in the main-belt.
• The dwarf planet Haumea was observed in a stellar transit on 21 January 2017, identifying a ring.^[16]
• On 3 June 2017, a star was occulted by the Kuiper belt object 486958 Arrokoth, the first such occultation detected.^[17] The multi-faceted campaign involved cooperation from the Argentinian government (including local governments – a major highway was closed for two hours, and street lights were turned off, in order to preclude light pollution), three spacecraft, 24 portable ground-based telescopes, and NASA's SOFIA airborne observatory in "the most challenging stellar occultation in the history of astronomy," in an effort spanning six months.^[18]

Double occultations

The Moon or another celestial body can occult multiple celestial bodies at the same time.

Because of its relatively large angular diameter the Moon, at any given time, occults an indeterminate number of stars and galaxies. However the Moon occulting (obscuring) two bright objects (e.g. two planets or a bright star and a planet) simultaneously is extremely rare and can be seen only from a small part of the world: the last such event was on 23 April 1998 when it occulted Venus and Jupiter for observers on Ascension Island.

Artificial occultations

The Big Occulting Steerable Satellite (BOSS) was a proposed satellite that would work in conjunction with a telescope to detect planets around distant stars. The satellite consists of a large, very lightweight sheet, and a set of maneuvering thrusters and navigation systems. It would maneuver to a position along the line of sight between the telescope and a nearby star. The satellite would thereby block the radiation from the star, permitting the orbiting planets to be observed.^[19]

The proposed satellite would have a dimension of 70 by 70 metres (230 ft × 230 ft), a mass of about 600 kg, and maneuver by means of an

engine in combination with using the sheet as a light sail. Positioned at a distance of 100,000 km from the telescope, it would block more than 99.998% of the starlight.

There are two possible configurations of this satellite. The first would work with a

of the orbit, the satellite would remain relatively stationary with respect to the ground, allowing longer exposure times.

An updated version of this design is called the

See also

References

Further reading

• Meeus, Jean (1995). Astronomical Tables of the Sun, Moon and Planets. Richmond, Virginia: Willmann-Bell, Inc.
  ISBN 0-943396-45-X
  .
• (in German) Marco Peuschel – Astronomische Tabellen für den Mond von 2007 bis 2016, Mondphasen, Apsiden, Knotendurchgänge, Maximale und minimale Deklinationswerte und Sternbedeckungen sowie ausführliche Ephemeriden für jeden Tag des Jahres, inkl. Mondauf-und Untergänge und physische Daten.

External links

• Occultation Studies at Williams College, www.stellaroccultations.info
• International Occultation Timing Association (IOTA), with lunar occultations
• Moon, planets, asteroid occultations, www.occultations.net
• Mutual occultations (and conjunctions) of planets Archived 11 July 2011 at the Wayback Machine, TAU astronomy Club
• Planetary Conjunctions and Mutual Planetary Occultations stevealbers.net
• Discovery Hints at a Quadrillion Space Rocks Beyond Neptune, www.space.com, August 2006
• Scientists Use Novel Technique To Watch Occultations By Planets Archived 31 October 2018 at the Wayback Machine, The Starry Mirror, 2007
• Asteroid Occultation Updates, www.asteroidoccultation.com, 2018
• Occultations by Phoebe, www.occultations.net
• Combining asteroid models derived by lightcurve inversion with asteroidal occultation silhouettes, arXiv, 2011
• Occultations of bright stars by planets between 0 and 4000, Il cielo ed i suoi femomeni (in Italian)
• Lunar Occultation Workbench, (software), Durch Occultation Association
• Occult, (software), www.lunar-occultations.com