I play and teach piano, and I would like to learn the harp. Does my knowing piano give me an advantage in playing the harp, or does it not matter? Are they anything alike? Thanks a million! Btw I am speaking of a small harp, not a huge pedal harp, something with about 22 strings.
Just for supplement: Non-pedal harps are either tuned to a fixed key, or have a mechanism (called hook) for each string, to modify its pitch a half tone upwards. This takes some preparation, so chromatic changes are not possible on the fly, but only in longer breaks. Harps with hooks were often manufactured in Bohemia.
Contrary to the piano the strings also sometimes require to be muffled by the palm for which there is a special notation, as I learned from a recent question.
Also note, that tuning a harp is frequently necessary and due to the high number of strings takes considerable time.
Harps are written in concert pitch on a grand staff. If you read piano music, you can read harp music. Musical concepts like phrasing, articulations, dynamics, etc. will be similar on the harp and the piano (and other instruments for that matter.)
Technical details are very different. A harp is quasi-diatonic (at least the big orchestral harp) in that it has 7 notes per octave. There are 7 pedals that raise notes by a half-step; the "native" tuning is in Cb. A pedal raises or lowers all notes with the same name by a half-step. This limits the type of phrases easily played. Also, harpists do not use the little finger which means that common piano idioms are not playable.
One similarity between harp and piano that hasn't been mentioned is the spacial distribution of the tones. Like a piano, and unlike most other instruments, on the harp you have a scale under your fingers going from low to high in one direction, with the spacing not all that different from a piano. So if you can plunk out a scale on the piano, you can pluck out a scale on a harp, pretty much.
On the other hand, as mentioned above, there are lots of differences. Another one is that, with the harp, both hands have the thumb playing higher notes (for a given hand position) than the fingers, unlike the piano where the right hand has the thumb playing lower.