In Richard Carrington's drawing of sunspots, he has a compass labeled "N", "P", "S", and "F". If "N" is North and "S" is South, what do "P" and "F" mean?
Also, why is the compass not oriented in the same way as the lines of longitude/latitude?
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P is preceding and F is following.
This nomenclature dates from when all observations were visual. I think the point was that the observer didn't want the complication of figuring which way was east or west (and that depends on the convention used), but relied on which object, including the edge of the sun or planet, came into view first (preceding), or last (following).
Similarly if trying to measure right ascension, the observer would use cross hairs or a reticle to measure the time difference for the objects being measured to pass over the same point, which would give the difference in right ascension.
Here is a link to an article from the British Astronomical Association, which supports my answer. I won't copy/paste as I'm not sure of the copyright position.
Alternatively, see page 27 of Norton's Star Atlas, 20th edition.