Length determination

Longitude determination is the abbreviation for calculating the astronomical or geographical longitude of an observation point by measuring time and angle according to celestial bodies . Because of the earth's rotation, it is closely linked to time measurement .

The oldest - and at the same time the simplest - method is to measure the true noon (greatest height of the sun ) and to determine the associated time. If you had a good clock available, you could control it according to the true local time and after a change of location you could compare it with the true local time of the other point. The difference corresponds to the length difference, the difference to the time at the prime meridian of the geographical longitude itself.

However, such precise watches “ chronometers ” were only developed by John Harrison between 1735 and 1770. Until then, different local times or differences in length could only be determined through the moon's star cover - the so-called lunar distances .

Classic methods of determining length

The best methods of determining length only became practical in the last 200 years. In particular:

• The measurement of the meridian passages (meridian passage) of known stars. The respective sidereal time then corresponds to the RA coordinate of the star and can be converted into civil time (local time). The difference in length to the comparison location results from a time comparison.
• The method of the astronomical stand line (English Sumner line ) - in which the heights of two stars are measured and compared with those at the cast (assumed) location. The associated stand lines are entered on the nautical chart (or processed digitally) and intersect at the real location of the observer.
• the method of equal heights - in which three or more stars are measured and their circles of distance are calculated. They intersect in the place of the observer according to the principle of the arc cut (see also astrolabe and Ni2 astrolabe )
• the circummeridian method (equal star heights on both sides of the meridian)
• the time and altitude measurement in the first vertical
• the simultaneous measurement of time and azimuth in the vertical of a terrestrial target.

Accuracy, measurement and instrument errors

The astronomical-geodetic methods have an accuracy potential of about 1 millisecond (0.001 s ~ 0.01 ″ = 20 cm) at observatories if a few repeated measurements are carried out (see also throughput errors ). To achieve this, all other effects down to 0.01 ″ must be taken into account, which is not entirely possible.

For measurement points (in an open field), latitude and longitude determinations to about 0.1 ″ to 0.5 ″ are possible within 1 hour. In addition, however, the time errors when measuring the star passages must not exceed a few hundredths of a second, which can only be achieved with digital clocks and some experience of the observer.
With controlled tracking (e.g. recording micrometer , Danjon astrolabe) or devices for automatic time measurement ( optoelectronic observation, recordings with CCD sensors , etc.), the time errors can be reduced to a few milliseconds with appropriate effort.

Instrumental errors can also noticeably influence the length determination. These include B. the synchronization of the clocks, the tilt axis error and the influence of temperature on the measuring instrument.

Non-astronomical methods of location determination

Most newer location methods determine latitude and longitude at the same time . They include:

• the radio navigation with Loran , Decca , Omega , Consol u. Ä.
• the dead reckoning to inertial navigation
• the satellite geodesy - today v. a. with GPS
• radio interferometry ( VLBI )

See also

• Determination of longitude , latitude
• Astrogeodesy , astronomical navigation , plumb direction
• Dead reckoning (control of length determination), bearing
• Astronomical triangle , hour angle , great circle
• Length problem