Siriometer

The Siriometer is no longer in use astronomical unit from the 19th or early 20th century.

The value of the Siriometers has changed over time.

Herschel Siriometer

Friedrich Wilhelm Herschel defined the beginning of the 19th century, the Siriometer than the distance from the Sun to Sirius.

The unit was created in an effort to measure the distances of the stars visible from Earth. There were no method known by which the distance could be measured directly, Herschel wanted to at least identify with each other, the relative distance of the stars.

Starting from the assumption that all the stars were bright, more or less the same, Herschel took the brightest star in the night sky, Sirius, as a reference. By comparing the brightness of the other stars, he tried to estimate the distance: The brightness decreases with the square of the distance, a star which has a quarter of the brightness of Sirius, will be twice as far away.

Because of the inaccurate estimate of the brightness and the fact that the stars have yet very large differences in brightness, its results were not very accurate.

How big is a Siriometer really is, is not identified Herschel's lifetime. The first absolute distance measurements were carried out in 1838 by Friedrich Wilhelm Bessel, 16 years after Herschel's death. Today we know that Sirius 8.6 light years away, or about 81 trillion kilometers.

Charlier Siriometer

1911 suggested Carl Charlier, set the Siriometer than 1 million astronomical units. This happened in the course of that debate about the definitions of a useful measure of cosmic magnitude. From this debate, the parsec is ultimately emerged victorious.

The new value corresponds to about 149.5 trillion kilometers, almost twice as large as the original definition.