Reply To: Partial solar eclipse of 2022 October 25

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Duncan Hale-Sutton

I am going to stick my neck out here but I wondered if observers actually calculate the obscuration of the sun by the moon from the pictures that they take of the partial eclipse or does this have no value because the earth/moon system dynamics is so well known? I think that the obscuration percentage is given by the simple formula 100(w – sin w)/pi if you assume that the sun and moon appear to be the same anglular size. If you draw a chord between the points where the moon’s shadow touches the sun’s edge (the chord PP’ in the attached diagram) the angle w (in radians) is the angle subtended by P and P’ from the centre of the sun’s image. If you have an image of the partial eclipse you can measure (in pixels) the diameter of the sun’s disc and the length of the chord l and then w = 2 arcsin (l/d).

For example, for my image of the partial eclipse taken at 09:45 UT on the 25th from 52 degrees 44 minutes north and 1 degree 28 minutes east, l was 209 pixels, d was 311 pixels and so the obscuration was about 15% at this time (this was about 15 minutes from maximum coverage).

Even if the calculation has no scientific value, it may still be interesting to calculate and compare with prediction.