As noted in some comments, there are certain phrases that give interesting results when input to the diary. "Diadem" and "Dragon" give some interesting answers back.
Of particular interest is this:
Following up on the text for "dragon", which references "liondragons" or, as they're also known in the Potterverse, "Chinese Fireballs" ...
Following clues for "Diadem", which mentions disambiguation, leads me to walking the Wikipedia disambiguation page for Diadem. Eventually, the reference to Diademed Monkey gave a diary response that suggests a "different celestial body", which takes me to "Diadem (star)", the diary response for which suggests using the full proper name. Hence:
From the Geocaching page for the search challenge,
we find provided coordinates (need geocaching.com account to view) -
N 39° 03.506 W 077° 15.733
Note it is specified that these are not the actual coordinates, so are presumably a starting point to find the actual coordinates.
Using the diary's text,
"Chinese Fireball" gives an algorithm to find the coordinates; it tells us we'll need:
- "the key, 13 digits hidden in some text"
- some "numbers...originally posted" which need to be decrypted
- a "secret passphrase...with the same number of characters as the encrypted numbers"
We have the key (
1026042050833, from text for Alpha Comae Berenices).
We have the posted numbers (
3903506 07715733 - provided coordinates) - 15 digits.
The passphrase should be something with 15 characters; the diary hints at "Chinese Fireball", but following the algorithm in that text gives coordinates in the ocean off the coast of Newfoundland.
We seem to be missing just that one remaining piece.
What other 15 letter passphrase might it be?
Things I've tried that don't appear to work: