Jump of the thumb Jump of the thumb: The left edge of the thumb coincides with the left or right edge of the house, depending on the eye used.

The thumb jump is a rule of thumb to estimate the distance to an object or its size. The rule states that the width by which the thumb “jumps” on the observed object when changing eyes (in the picture around the width of the house) is one tenth of the distance.

The parallax of the directions of sight of both eyes over the side edge of a thumb set up on the extended arm is used.

background Theorem of rays
a = arm length; b = eye relief; c (+ a) = object distance; d = object size; K = crossing point

With this estimate, your own body serves as the measuring standard for an angle . The thumb aimed over the shoulder on the outstretched arm of an adult is 60 to 70 centimeters from the face, while the eye relief is 6 to 7 centimeters. The length ratio varies from person to person in the range from 1: 7 to 1:12 and must be determined individually for the best measurement results. The transfer of the length ratio to the object space is an application of the ray theorem, see figure. A ratio of 1:10 corresponds to an angle of approximately 5.7 °.

Carrying out a distance estimate

• Extend one arm completely, also turn the shoulder towards the target, make a fist, "put your thumb up" (better a pen, stick, etc., as it increases the accuracy).
• close one eye and use the other, open eye to aim at the target with your thumb
• now close the open eye, open the other
• the thumb has now apparently jumped to the side
• Now you estimate the distance between “Thumb 1” and “Thumb 2” at the target. A known comparison value (e.g. car length) can be helpful.
• this distance is multiplied by 10 and you get roughly the distance.

example

In the graphic above, the base length of the house “randomly” fits between the left edge of “Thumb 1” and “Thumb 2”. The house width is 7 meters. Then the estimated distance to the house is 70 meters.

Angle measurement

In order to measure the horizontal angular distance between two distant points, several thumb jumps can be added by repeatedly advancing the thumb (in front of a structured background that offers sufficient clues) and possibly further turning the body, thus generating multiples of the angle of about 6 °. By tilting the head and thumb, the angles can be aligned in different directions, for example to roughly determine the azimuth and elevation angles of stars or the angular distances between two stars.

Military use

The technique is also used when addressing the target : Two enemy shooters jump the thumb to the left of the individual tree .

literature

• Werner D. Bockelmann: eyes - glasses - car . See better - drive safely, second completely revised edition, Springer Verlag Berlin Heidelberg, Berlin Heidelberg 1987, ISBN 978-3-642-93317-2 .