Solar Drawings and Position Measurements
The cheapest and most convenient way of recording the Sun’s appearance each day is to draw it. Drawings can also be important in determining sunspot statistics, as I shall describe in Chapter 5. Photography and CCD imaging are in theory more accurate than drawing, but they are less practical, for several reasons. It is not as easy to define the cardinal points (north, south, east and west) on a photographic image as accurately as on a good drawing. Secondly, it is difficult to derive precise positions of sunspots from a photograph. Thirdly, a photograph taken at a scale small enough to show the whole Sun on one frame — essential for measuring the positions of sunspots — has a relatively limited resolution and small spots which can be picked up visually can be missed on a photograph. Photographs taken at a small scale such as this sometimes suffer from distortion towards the edge of the frame, which can compromise the accuracy of any sunspot position measurements. Finally, a drawing is ready for analysis as soon as it has been completed, whereas photographs and electronic images both have to be processed. This can be time-consuming, especially if you are sending results in to an amateur observing organisation that requires observations on a monthly basis.
KeywordsPosition Measurement Rotation Number Sunspot Group Central Meridian Solar Image
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