A figure-of-eight

Exaggerated analemma

Realistic analemma

Q. At 12.00 midday each day (weather permitting) I mark the shadow cast by the top corner of the building. I have been recording this since Sept 23 2005, and the shape has taken a nice elongated figure 8 shape (infinity symbol). On the 23rd June the shape bottomed out, and I expected it to continue the smooth curve as previously established. However it seem to stutter and fell back towards the building in a straight line rather than the expected curve. Now it seems to have returned to the expected curve. Please explain.

A. What you should see is an analemma. This indicates the changing elevation of the Sun above the horizon vertically during the year and horizontally indicates in minutes how much earlier or later than 12 noon the Sun is due north. See the attached horizontally exaggerated curve fom the Wikipedia entry on analemmas. A more realistic version is also attached. Of course they are upside down as they are from the northern hemisphere.

If you do not see a smooth figure-of-eight curve as above all I can suggest is that maybe you were getting a shadow of something else than the corner of the building when the Sun is at its lowest elevation for the year.

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