Is there an echo in here? I wonder how many stories have used this device:
Any reader of The Hobbit remembers this. In their quest for the Arkenstone, the dwarves and one hobbit make use of the prediction that on Durin’s Day, the last light of the setting sun will shine directly upon a keyhole enabling entry to a secret passage into the mountain.
In The Raiders of the Lost Ark, Indiana Jones makes use of ancient instructions to find the location of the Well of Souls, where the Ark is kept hidden. The light of the rising sun, passing through the headpiece of the properly positioned Staff of Ra, shines directly onto the location of the Well of Souls in the fabled Map Room.
In the 1959 movie Journey to the Center of the Earth (which I started watching while feeling under the weather today), the location of the passage leading into the Earth is revealed when the sun shines through an opening in a nearby peak, directly onto the mouth of the passage. (In the Verne original, I believe, the mechanism is similar, but less cinematic.)
Reuse of plot devices is a time-honored tradition among storytellers, of course. How many other stories have used this device? If you can think of examples, list them here in the comments!