In astronomy, a transit instrument is a small

telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to obse ...

with extremely precisely graduated

mount Mount is often used as part of the name of specific mountains, e.g. Mount Everest. Mount or Mounts may also refer to: Places * Mount, Cornwall, a village in Warleggan parish, England * Mount, Perranzabuloe, a hamlet in Perranzabuloe parish, ...

used for the precise observation of

star position Star position is the apparent angular position of any given star in the sky, which seems fixed onto an arbitrary sphere centered on Earth. The location is defined by a pair of angular coordinates relative to the celestial equator: right asc ...

s. They were previously widely used in astronomical observatories and naval observatories to measure star positions in order to compile nautical almanacs for use by mariners for

celestial navigation Celestial navigation, also known as astronavigation, is the practice of position fixing using stars and other celestial bodies that enables a navigator to accurately determine their actual current physical position in space (or on the surface o ...

, and observe

star transit A star transit is the passage of a star across the field of view of a telescope eyepiece. The precise observation of star transits is the basis of many methods in astronomy and in geodesy. The measurements can be done in different ways: # visuall ...

s to set extremely accurate clocks ( astronomical regulators) which were used to set

marine chronometer A marine chronometer is a precision timepiece that is carried on a ship and employed in the determination of the ship's position by celestial navigation. It is used to determine longitude by comparing Greenwich Mean Time (GMT), or in the mode ...

s carried on ships to determine

longitude Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek let ...

, and as primary time standards before atomic clocks. The instruments can be divided into three groups: meridian, zenith, and universal instruments.

Types

Meridian instruments

For observation of

s in the exact direction of South or North: * Meridian circles,

Mural quadrant A mural instrument is an angle measuring instrument mounted on or built into a wall. For astronomical purposes, these walls were oriented so they lie precisely on the meridian. A mural instrument that measured angles from 0 to 90 degrees was call ...

s etc. * Passage instruments (transportable, also for prime vertical transits)

Zenith instruments

Zenith telescope A zenith telescope is a type of telescope that is designed to point straight up at or near the zenith. They are used for precision measurement of star positions, to simplify telescope construction, or both. A classic zenith telescope, also know ...

* Photozenith tube (PZT) *

zenith camera A zenith camera is an astrogeodetic telescope used today primarily for the local surveys of Earth's gravity field. Zenith cameras are designed as transportable field instruments for the direct observation of the plumb line (astronomical latitud ...

s * Danjon

astrolabe An astrolabe ( grc, ἀστρολάβος ; ar, ٱلأَسْطُرلاب ; persian, ستاره‌یاب ) is an ancient astronomical instrument that was a handheld model of the universe. Its various functions also make it an elaborate inclin ...

, Zeiss Ni2 astrolabe, Circumzenital

Universal instruments

Allow transit measurements in ''any'' direction *

Theodolite A theodolite () is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying, but it is also used extensively for building and ...

(Describing a theodolite as a ''transit'' may refer to the ability to turn the telescope a full rotation on the horizontal axis, which provides a convenient way to reverse the direction of view, or to sight the same object with the yoke in opposite directions, which causes some instrumental errors to cancel.) * Altaz telescopes with graduated eyepieces (also for satellite transits) *

Cinetheodolite A cinetheodolite (a.k.a. ''kinetheodolite'') is a photographic instrument for collection of trajectory data. It can be used to acquire data in the testing of missiles, rockets, projectiles, aircraft, and fire control systems; in the ripple fir ...

Observation techniques and accuracy

Depending on the type of instrument, the measurements are carried out * visually and manual time registration ( stopwatch, Auge-Ohr-Methode,

chronograph A chronograph is a specific type of watch that is used as a stopwatch combined with a display watch. A basic chronograph has an independent sweep second hand and a minute sub-dial; it can be started, stopped, and returned to zero by successiv ...

) * visually by impersonal micrometer (moving thread with automatic registration) * photographic registration * CCD or other

electro optic Electro–optics is a branch of electrical engineering, electronic engineering, materials science, and material physics involving components, electronic devices such as lasers, laser diodes, LEDs, waveguides, etc. which operate by the pr ...

sensors. The accuracy reaches from 0.2" (theodolites, small astrolabes) to 0.01" (modern meridian circles, Danjon). Early instruments (like the mural quadrants of Tycho Brahe) had ''no telescope'' and were limited to about 0.01°.

References

External links

* * {{cite EB1911 , first=John Louis Emil , last=Dreyer, wstitle=Transit Circle , volume=27 , pages=181–183 , short=x
Great Transit at Lick Observatory, +Photo

Modern roboter telescopes

Transit instruments used by the Survey of India, 1867
Astrometry Geodesy