I have seen numerous replies in threads that Cortana recieved information about the Forerunners and the Halo rings when her fragment was with Dr. Halsey (Halo: Reach) and that she put in the co-ordinates to the Alpha Halo when the Pillar of Autumn left Reach.
And yet Keyes says that they made a blind jump.
So did they just [i]happen[/i] to have come across the Halo ring or did Cortana lie to the captain?
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It's all said at the end of The Fall of Reach. There's a contradiction, canon wise there because in TFoR Cortana get's the coordinates from the crystal from Sigma Octanus IV, and Reach apparently shows she got it from the Forerunner construct. Anyways, Cortana put the coordinates in for Alpha Halo, not knowing exactly that there would be a Forerunner installation, but since it followed Cole Protocole and was a jump away from Earth, she put it in because her curiosity got the best of her. She never admitted it openly to Keyes. Not even to Chief, that I know of.