... The Round Earth . 8a. The Horizon . ... 9a. Earth orbits Sun? . ... What is the distance D to the horizon? ... Your line of sight to the horizon is a tangent to the Earth--a line which touches the sphere of the Earth at just one point, marked B in the drawing here. ... From the top of Mauna Kea on Hawaii, an extinct volcano about 4 km high (also the site of important astronomical observatories), the horizon should be about twice as distant, 226 km. ... One more: Distance to the horizon on Mars . ...

Hipparchus, who used an eclipse of the Moon to deduce the precession of the equinoxes ( here ), used a total eclipse of the Sun --probably in 129 BC--to estimate how far the Moon was. But he also derived the Moon's distance from a lunar eclipse--see here .) That eclipse was total at the Hellespont--the Dardanelles, part of the narrow strait that separates the European and Asian parts of Turkey--but only 4 / 5 of the Sun were covered in Alexandria of Egypt, further to the south. ...

Mathilde . ... On June 27, 1997 the Near-Earth Asteroid Rendezvous (NEAR) spacecraft flew flew past asteroid 253 Mathilde. ... On June 27, 1997 NEAR made a spectatular 25 minute flyby of Mathilde which resulted in more than 500 images of a dark, crater-battered little world that dates to the beginning of the solar system. ... This first image of asteroid 253 Mathilde, returned by the NEAR spacecraft just before 10:00 AM EDT on June 27, 1997, was taken from a distance of 1800 kilometers (1120 miles). ...