Cook sails to Tahiti to watch a dot

The Endeavour's official mission was to time Venus crossing the face of the Sun, from an island on the other side of the world, at new moon.

Timing the transit from widely separated places was, at the time, the only way to measure the size of the solar system. Observers were sent as far as Hudson Bay and Baja California; Cook's secret orders to go looking for a southern continent came in a sealed envelope.