transit instrument


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transit instrument

or

transit,

telescope devised to observe stars as they cross the meridian and used for determining time. Its viewing tube swings on a rigid horizontal axis restricting its movements to the arc of the meridian. In the field of view of the eyepiece are threads of spider web or fine lines ruled on thin glass. The threads or lines are parallel in a north-south direction and odd in number. Precise adjustment places the middle line exactly on the meridian. After the observer has noted the times at which each line is passed by the star, he averages them to learn the instant at which the star was on the meridian. In modern transits, known as meridian circles or meridian telescopes, the observer merely presses a button as the star crosses each line. Electrical impulses are recorded on a revolving drum at one or two second intervals as they pass through a chronograph. The meridian circle is equipped with precisely graduated circles mounted on the horizontal axis. Stationary verniers, or reading microscopes, mounted on the fixed supports of the telescope enable the observer to read the circles. The meridian telescope gives the altitude of a star as well as the transit time. This information yields the right ascension and declination, i.e., the location of the star in the celestial sphere. The meridian circle has largely replaced the transit as the equipment of observatories, although the older transit instrument is still used to some extent for determining sidereal timesidereal time
(ST), time measured relative to the fixed stars; thus, the sidereal day is the period during which the earth completes one rotation on its axis so that some chosen star appears twice on the observer's celestial meridian.
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. For a discussion of the transit used by engineers, see surveyingsurveying,
method of determining accurately points and lines of direction (bearings) on the earth's surface and preparing from them maps or plans. Boundaries, areas, elevations, construction lines, and geographical or artificial features are determined by the measurement of
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.

Transit Instrument

 

an astrometric instrument used to determine the transit times of heavenly bodies in their apparent diurnal motion through a given vertical. The transit—more precisely, its line of sight—usually has its line of collimation in the plane of the meridian in order to obtain the right ascensions of stars and to determine corrections to the clock. To obtain stellar declinations and local latitude, the line of collimation should be in the plane of the prime vertical.

The transit instrument was invented in 1689 by the Danish astronomer O. Romer. A stationary transit consists of an astronomical telescope having an objective lens with a diameter of about 18 cm and a focal length of about 2 m. The instrument is mounted on a horizontal axis of rotation (about 1 m) and rests on pillar supports. Smaller, portable transits are used for time services. An ocular micrometer having a reticle with vertical and horizontal wires is situated in the focal plane of the transit objective. The times at which the image of a star passes across the vertical wires are registered by a chronograph. In the mid-19th century, keys depressed by the observer at the appropriate time were used to make the recordings. In modern transits a registering micrometer, invented in the late 19th century, is used for visual observations.

Two Soviet astronomers, N. N. Pavlov and V. E. Brandt, developed a method of photoelectrically registering the transits of stars, thereby increasing the precision with which clock corrections could be determined by time services and eliminating human error from transit observations. The accuracy of a single calculation of the right ascension of a star using a stationary transit is about ±0.015 sec, while the precision of a single photoelectrically determined clock correction is about ±0.005 sec.

REFERENCES

Podobed, V. V. Fundamental’naia astrometriia, 2nd ed. Moscow, 1968.
Bakulin, P. I., and N. S. Blinov. Sluzhba tochnogo vremeni. Moscow, 1968.

V. V. PODOBED

transit instrument

[′trans·ət ‚in·strə·mənt]