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Updated: May 5, 2025
Just about this time, however, a great optician came to the aid of the astronomers. Joseph Fraunhofer perfected the refracting telescope, as Herschel had perfected the reflector, and invented a wonderfully accurate "heliometer," or sun-measurer. With the aid of these instruments the old and almost infinitely difficult problem of star distance was solved.
The Transit of Venus in 1874, in which the present writer assisted, overthrew this delusion. In 1877 Sir David Gill used Lord Crawford's heliometer at the Island of Ascension to measure the parallax of Mars in opposition, and found the sun's distance 93,080,000 miles.
This interesting, though inconspicuous object, then, was chosen by Bessel to be put to the question with his heliometer, while Struve made a similar and somewhat earlier trial with the bright gem of the Lyre, whose Arabic title of the "Falling Eagle" survives as a time-worn remnant in "Vega."
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