John Kerr and Pine Gap surveillance base (Image: National Archives of Australia; Wikimedia)

It was Monday November 3, 1975, and on a practice track somewhere in Melbourne, champion horse Think Big was being taken for a light canter.

It was the day before the Melbourne Cup, and the 1974 winner was widely tipped to get the double. Think Big — she was a crowd favourite and the name was much in the air. Was it on the minds of the nation’s leaders that morning, as they descended on Melbourne for the race that stops a nation?

Sir John Kerr and Gough Whitlam would all be in town that momentous day, as Sir John would tell the palace in his letter about that day.