5 Dollars - Elizabeth II Captain Cook: Transit of Venus

The 1769 transit of Venus was the scientific event that brought Cook to the Pacific in the first place — specifically to Tahiti, where the Royal Society had dispatched HMS Endeavour to record the planetary crossing as part of a global effort to calculate the Earth-Sun distance through parallax. The measurements, taken simultaneously from multiple world locations, ultimately proved less precise than hoped due to the "black drop effect," an optical distortion that blurred the exact contact timings.

Cook Islands has issued commemoratives under its own authority since 1972, despite using New Zealand currency in practice.