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'A Scale for the Solar System' printed from https://nrich.maths.org/
Lukaz Smith, from Parkside School, sent in
the following correct approach to finding the ratio of the
Earth-Sun and Venus-Sun distances (diagram added to help clarify
the solution)
If you said the Earth to sun distance was 1 astronomical unit ($a$
in diagram), then you could simply measure the angle ($\theta$ in
diagram) between Sun, Earth and Venus, when you knew
that there was a 90 degree angle between Earth, Sun and
Venus. Then you just find the tangent of the angle, so it's 1 :
$\tan \theta$ as the ratio of Earth to Sun distance and Venus to
Sun distance.