A passage instrument (also: passage instrument, passage instrument or midday pipe ) is a measuring instrument of astrogeodesy and astrometry , which defines any vertical circles with its horizontal axis . It is used to observe the times of star passages (i.e. of "passages") on a vertical thread network and is transportable to a limited extent due to its construction, which is very compact compared to the meridian circle .
The principle of the instrument was invented by Ole Rømer in 1689 and used to determine time by observing the meridian passages of stars. To do this, he mounted a telescope that could be rotated around a fixed, precisely horizontal east-west axis ( tilting axis ) and registered the star passages by means of the beats of a precision pendulum clock . Since the end of the 19th century, the construction of a broken telescope has prevailed.
All astronomical objects pass through the celestial meridian horizontally . With a telescope aligned with the meridian and a recording chronometer , one can precisely determine the right ascension of a star . The declination can also be determined from its apparent height during passage if the pole height is known and a vertical circle is available.
The star passages are measured by a parallel thread network (in the past it was actually spider threads of special spider species ) or, to increase accuracy, with an optical micrometer and electrical contacts (“ impersonal micrometer ”, see also: universal instrument ). The time registration took place until 1980 with tape or printing chronographs , digital in today's applications.
The measured times or angles must (as with every precision measurement) be corrected ( reduced ) for the influence of small axis inclinations , which is done with 2-3 vials and the Mayer formula (according to Tobias Mayer ).
The measuring accuracy is about 0.1 " , for averaging a number of star passages correspondingly higher. The first device designed by Ole Römer around 1700 reached around 1 ″. The time measurement is visually accurate to about 0.02 seconds and even better with a micrometer.
In both cases, however, the “ personal equation ” ( reaction time ) must be taken into account. It averages one tenth of a second (0.05–0.2 s depending on the observer), but is constant to a few hundredths of a second. The artificial star was developed to detect any small changes in the reaction time .
The passage instrument is a smaller form of the meridian circle , with which one can measure the transit times of the stars through the meridian , and with additional devices also the zenith distance . It is primarily used for determination
- of star words - see star catalogs FK3 , FK4 and Fundamentalsystem
- to determine time and length - see atomic time , UTC and deviation from the perpendicular
- for monitoring the Earth's rotation - see Pole Movement
- (less often) for measuring astro-geodetic azimuths. In this case, the instrument is not used in the meridian, but in the vertical circle of the measurement point ( Niethammer method ).
The instrument is too heavy for a tripod , although its size is not significantly larger than a precision theodolite . Therefore, a measuring pillar is required , such as those built on fundamental points or to mark out long tunnels .
Because of their compact design, passage instruments were not only used in observatories , but also occasionally in field stations until around 1980. Some of these instruments had a slightly different design of the vertical circle or dragonflies and could e.g. Some of them can also be used outside the meridian (for azimuth and zenith distance measurements and deviation from the perpendicular).
A well-known manufacturer of the instruments was the Askania company in Berlin and until around 1920 other companies such as Starke & Kammerer (Vienna), Johann Georg Repsold (Hamburg) or English workshops.
Radio telescopes are also sometimes set up as straight-through instruments, as they then have to be made movable in only one axis. Radio sources can then only be observed once a day. One example of this is the Arecibo observatory built into a basin on Puerto Rico .
- Karl Ramsayer : Geodetic Astronomy . In: Wilhelm Jordan, Otto Eggert, Max Kneissl (Hrsg.): Handbuch der Vermessungskunde . Volume 2a, 10th edition. JB Metzler, Stuttgart 1970
- Albert Schödlbauer : Geodetic Astronomy - Basics and Concepts. De Gruyter, Berlin / New York 2000, ISBN 3-11-015148-0 .
- Passage instrument . In: Brockhaus Konversations-Lexikon 1894–1896, Volume 12, p. 934. - Facsimile with illustration
- The response time is much here than, for example, in road traffic (0.5–1 s) because the star does not pass the measuring threads surprisingly, but in a very predictable way.