Solar compass

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Faroe Islands stamp pad from February 11, 2002: Vikings sail on the North Atlantic with a Viking ship , left sun compass (type: equatorial sundial )

As sun compass one is portable sundial used, from the position of the sun rather than the hour of the day on the north-south direction at the observation site is closed. The instrument is rotated around a vertical axis until it shows the known time of day; so the hour line for noon (12 o'clock) is in north-south direction.

A sun compass, instead of a magnetic compass to be used when the Earth's magnetic field is disturbed at the observation by magnetic fields, or its declination is unknown at this point.

function

In the position of the sun in the sky, the time of day ( hour angle of the sun), the calendar date ( declination angle of the sun) and the geographical latitude of the observation location are coded. If the geographical latitude is known, one of the other two quantities can be used to determine the north-south direction at the observation site.

In a sundial , the latitude of the place where it is used is “built in” (given structural size). Since it always shows at least the time of day, the reverse can be drawn from the known time of day to the north-south direction for each of its types. Each of its types can thus be used as a sun compass.

Sundials with point-shaped shadow objects ( Nodus ) also show the calendar date approximately, so that the north-south direction can be determined in a second way. When using a sundial with a pole rod , the calendar date is not displayed. The achievable accuracy is not only lower because of the low resolution between the date lines, but also because the shadow point in the middle hours of the day cuts such a line by grinding when the instrument is rotated around a vertical axis. Before there were wheel clocks - a portable wheel clock (pocket watch) did not exist until the end of the Middle Ages, i. H. a few centuries later as stationary wheel clocks (tower and table clocks) - one was dependent on the sundial as a timekeeper. There was no second measuring device for the time of day, so it could only be used as a compass using the known calendar date, i.e. with less precise results.

A sun compass is usually a simple sundial and shows the true local time . In order to be able to orientate yourself as precisely as possible, the geographical length of the observation location and the current value of the equation of time must also be known. The time of day read by the watch that is carried is the mean local time at the reference longitude (e.g. 15 ° East for CET / CEST ). The sun compass user must first convert this with the longitude difference and the value of the time equation into the true local time.

Portable sundials with adjustable geographical latitude are preferably suitable for use both as a sundial and as a sun compass. You can use them at a greater latitude from their "home town".

use

middle Ages

It is assumed that the sun compass was already used by the Vikings on their voyages (see figure above).

Modern times

Since the magnetic compass in jet fighter jets such as the Me 262 produced deflections and clear errors due to high field strengths , the navigation instrument company C. Plath in Hamburg constructed a sun compass based on an automated 24-hour clock. This was used in the Second World War by the Air Force and in vehicles of the Africa Corps .

The US armed forces also used solar compasses during the Second World War to orient their land vehicles used in North Africa.

Using a wrist watch as a sun compass

When the sun is shining, any normal wristwatch with an analog display can be used as a compass:

(Instructions for the Northern Hemisphere)

- Keep the face of the watch horizontal.

- Point the hour hand towards the sun.

- Halve the angle between the hour hand and the number 12 - this line points south.

Web links

  • Otto Lueger: Lexicon of the entire technology and its auxiliary sciences. Vol. 9. Stuttgart / Leipzig 1914, p. 736. (facsimile)
  • IKARUS , a commercially available sundial that can also be used as a sun compass.
  • Handicraft instructions on www.wandernonline.de , greatly simplified (no indication that an equator-parallel setting is required)

literature

  • Kuno Gross: The Bagnold Sun-Compass: Long Range Desert Group. 5th edition. Books on Demand, ISBN 3-8423-3702-7 .

Single receipts

  1. Applies directly to a horizontal sundial ; general: The shadow object and midday line are in the meridian plane .
  2. C. Sun compass from Plath (Hamburg) and Junghans : Description with photos
  3. René RJ Rohr: The sundial. History, theory, function. Callwey, Munich 1982, ISBN 3-7667-0610-1