Star passage

As rating passage (engl. Star transit ) is one in astronomy and Geodesy the passage of the star through the visual field of a telescope referred to. The focus can be on angle or time measurement .

The angle measurement is used for. B. the azimuth and latitude determination as well as the measurement of star locations , double stars and planets , while the registration of exact times is essential for the geographic- astronomical longitude determination and for time services . With astrometry satellites , the angle measurement can be replaced by a scanner process.

Star field (diameter approx. 2 °) and star passages on the vertical thread

What do you see when you pass through the stars?

The picture shows two bright stars on the illuminated vertical thread (movement to the right, telescope magnification approx. 10x):

The star in the upper quarter is exactly on the thread, the lower one about 1 second after the passage. The two weaker stars on the left move along the horizontal thread. With such a weak magnification, the transit time can only be measured to an accuracy of about 0.5 s, which corresponds to around 5 arc seconds .

In the case of a slowly moving star (in the northern sky, extreme case Pole Star ), a trained observer can measure more precisely than the resolution of the telescope. Because the eye recognizes even the smallest deviations from the symmetry ( bisection of the star behind the thread). Faster moving stars "jump" over the thread, which - delayed by an almost constant reaction time - is registered with a hand button or digital stopwatch.

The average accuracy that can be achieved is called the throughput error and is made up of a time and a target error .

Measurement methods

The measurement of such passages through a thread network is the basis of many astrometry and astrogeodesy methods - regardless of how the passages are observed:

visual (mostly earlier): accuracy 0.1 "to 2" depending on the size of the measuring telescope ; Timekeeping with digital stopwatches to about 0.05-0.2 seconds (see also personal equation )
with CCD and other photoelectric sensors : as above, but the time is often more precise
semi-automatic special devices: photography or recording micrometer , about 2 x better than No. 1
by systematically scanning certain areas of the sky: astrometric satellites such as Hipparcos reach around 0.01 ".

Measurements for other purposes

Star passages are also used for other purposes, such as

for demonstrating the rotation of the earth in the planetarium , in public observatories or during star tours
to determine the magnification or the field of view
of terrestrial or astronomical telescopes :
- the stars move because of the earth's rotation through an earth-based measuring device at about 15 "per second sidereal time :
- at the celestial equator with 14.9590435 "/ s or around 1 ° in 4 min, and in a certain declination D by the factor cos (D) slower.

For precise measurement of small angles, for example

with double stars for the mutual distance, or
for the diameter of planets or other celestial bodies.
For example, at 45 "Ø , Jupiter needs exactly 3.008 s to pass a measuring thread of the crosshair.

In astrophysics and space travel to determine particle sizes, spatial distances and atmospheres , for example when a star passes through the ring system of Saturn .

Special instruments

Special astro-geodetic instruments have devices with which such measurements can be semi-automated or fully automated. Throughput measurements are carried out semi-automatically. B. on universal instruments such as the astronomical variant of the DKM3 triangulation theodolite. A moving thread tracks the star, while electrical contacts are continuously triggered and their times recorded.

In the Danjon astrolabe , which u. a. is or was used to measure the world longitude network and the polar movement , the observer keeps the direct constellation and the constellation mirrored on a mercury horizon in line by turning a micrometer, while contacts are also made and recorded on the tape or pressure chronograph . A completely different technology is that of the zenith camera , which is automated by working groups at the Technical University of Vienna and the University of Hanover for the rapid measurement of plumb line deviations .

Some visual instruments have a whole system of parallel measuring threads - for example, which has Ni2 - Astrolab 20 times scribed reticle. If a star passage is now observed through this network and the "single thread " is accurate to ± 0.1 s, then the mean value of 20 threads according to Gaussian law is already 0.022 s (or 0.3 ") accurate 100 years ago, star positions could be measured more precisely than separating a narrow double star in a telescope. In the 1990s, ± 0.1 "was already possible for thousands of stars in the FK5 .

Bright stars can also be observed during the day with a good measuring telescope , which can be used, among other things, to quickly determine the deviation from the perpendicular or for approximate localization on expeditions. Similar measurements are occasionally carried out in the military , for example for the alignment of guns independent of measurements . For this purpose, special developments with CCD sensors should be in development.

A few hundred stars can be seen visually in a deep blue sky if their position is calculated to at least 0.1 ° in advance. In the vicinity of the city, this number is reduced to around 20–50 of the brightest stars of the first to second magnitude .

literature

Karl Ramsayer : Geodetic Astronomy . Surveying Handbook. Volume IIa. JB Metzler, Stuttgart 1969, p. 900 .
Gottfried Gerstbach : Analysis of personal errors in passage observations of stars . In: Geowiss. Announcements from TU Wien . tape 7 . Vienna 1975, p. 51-102 .
Albert Schödlbauer : Geodetic Astronomy - Basics and Concepts . De Gruyter, Berlin / New York 2000, pp. 634 .

Individual evidence

↑ One speaks of a thread because up to around 1900 the finest spun threads were built into the eyepiece. Today, the thread network or the reticle is applied photo-mechanically or, in optoelectronic measuring processes, simulated by fine slits or digital lines.
↑ Star occultations in November 2003 by Saturn, Saturn's rings and Titan .

[1] One speaks of a thread because up to around 1900 the finest spun threads were built into the eyepiece. Today, the thread network or the reticle is applied photo-mechanically or, in optoelectronic measuring processes, simulated by fine slits or digital lines.

[2] Star occultations in November 2003 by Saturn, Saturn's rings and Titan .