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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?

enter image description here

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  • $\begingroup$ Cool question! Any idea what instrument was Carrington using, and at what focus? Was it a fixed telescope with a coeleostat/siderostat/heliostat or refractor/reflector/Coudé? Source of the image? $\endgroup$
    – uhoh
    Oct 29, 2021 at 4:22
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    $\begingroup$ Original article and a nice paper about it. (Spoiler: the answer is not there!) $\endgroup$
    – Jean-Marie Prival
    Oct 29, 2021 at 8:24
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    $\begingroup$ Thanks for that! I am currently looking through Carrington's book on sunspots, to see if I can find anything. $\endgroup$
    – Phiteros
    Oct 29, 2021 at 8:26
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    $\begingroup$ At first I thought the "P" could stand for "Ponante", which is a synonym of West, but then the corresponding East synonym would be "Levante", which does not begin with an "F"... $\endgroup$
    – Jean-Marie Prival
    Oct 29, 2021 at 8:27
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    $\begingroup$ The best we could think of was "Past" and "Future", perhaps referring to the sun's rotation or movement across his screen. $\endgroup$
    – Phiteros
    Oct 29, 2021 at 8:27

1 Answer 1

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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.

https://britastro.org/node/10014

Alternatively, see page 27 of Norton's Star Atlas, 20th edition.

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  • $\begingroup$ This sounds pretty plausible. Do you have a source? I'd like to read more about this. $\endgroup$
    – Phiteros
    Oct 29, 2021 at 21:35
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    $\begingroup$ @Phiteros - done $\endgroup$
    – Dr Chuck
    Oct 30, 2021 at 20:56
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    $\begingroup$ It is always okay to quote from an article, provided you deal fairly. You can quote "Because objects exit the field of view on the west and enter on the east, these two directions are sometimes referred to as preceding and following. These terms can also be incorporated into the definition of position angle so, in the above galaxy example, its position angle could be stated as north following/south preceding or, as more usually written, nf/sp." while citing the british Astronomical Association website (as above). No permission is required to make a reasonable quote. $\endgroup$
    – James K
    Oct 31, 2021 at 15:42

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