In astronomy, a transit instrument is a small

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

with an extremely precisely graduated mount used for the precise observation of star positions. They were previously widely used in

astronomical observatories An observatory is a location used for observing terrestrial, marine, or celestial events. Astronomy, climatology/meteorology, geophysics, oceanography and volcanology are examples of disciplines for which observatories have been constructed. Th ...

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 ...

, and observe star transits 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), and the time at t ...

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 lett ...

, 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 star transits in the exact direction of South or North: * Meridian circles, Mural quadrants etc. * Passage instruments (transportable, also for

prime vertical A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways ...

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 known ...

* Photozenith tube (PZT) * zenith cameras * Danjon

astrolabe An astrolabe (; ; ) is an astronomy, astronomical list of astronomical instruments, instrument dating to ancient times. It serves as a star chart and Model#Physical model, physical model of the visible celestial sphere, half-dome of the sky. It ...

, 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) * Cinetheodolites

Observation techniques and accuracy

Depending on the type of instrument, the measurements are carried out * visually and manual time registration (

stopwatch A stopwatch is a timepiece designed to measure the amount of time that elapses between its activation and deactivation. A large digital version of a stopwatch designed for viewing at a distance, as in a sports stadium, is called a stop clock ...

, 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 hour and minute hands on the main dial to tell the time, a small seconds hand to tell that the watch is running, and ...

) * visually by impersonal micrometer (moving thread with automatic registration) * photographic registration * CCD or other electro optic 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 Tycho Brahe ( ; ; born Tyge Ottesen Brahe, ; 14 December 154624 October 1601), generally called Tycho for short, was a Danish astronomer of the Renaissance, known for his comprehensive and unprecedentedly accurate astronomical observations. He ...

) 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