On a sunny day, a gnomonist in Utrecht stands on the analemmatic sundial on the Janskerkhof. Not only does he read the time, but he also watches his shadow moving about the sundial. It moves clockwise, and he had not expected otherwise.
However, this little experiment in Capetown would show the shadow to move counter-clockwise.
It is even more fun between the tropics.
The shadow moves clockwise or anti-clockwise, depending on date. On some days, it moves clockwise, then anti-clockwise, then clockwise again - or exactly the other way around.
We call this change of motion retrogradation.
Would you like to see this phenomenon for yourself without setting off to the tropics?
Then construct a model of an analemmatic sundial for a latitude of, say, 18 degrees north, and incline it to an angle equal to your own latitude minus those 18 degrees. Your model is now parallel to the horizontal plane at 18 degrees north latitude, and it will show you whatever happens there.
Notice how the date strip on this analemmatic sundial is partly outside the ellipse. On these dates, retrogradation will occur.
After sunrise, the shadow will move clockwise until it touches the ellipse. Then it will move anti-clockwise until it touches the ellipse on the other side and finally clockwise until sunset. The sequence of figures below show this nice game, but it is even more fun to see it happen on a model.
Retrograde motion is also mentioned in the Bible; see 2 Kings xx:
Hezekiah's sickness, and the message of the prophet to him, to prepare for death, 1.
His distress and prayer to God, 2, 3.
The Lord hears, and promises to add fifteen years to his life, and Isaiah prescribes a means of cure, 4-7.
Hezekiah seeks a sign; and to assure him of the truth of God's promise, the shadow on the dial (ma’aloth, usually rendered steps or degrees) of Ahaz goes back ten degrees , 8-11.
[However, if the dial of Ahaz had been constructed as described above, the effect must have been generally known; and Hezekiah would never have taken that for a miracle which he and all his courtiers must have observed as an occurrence which at particular seasons, took place twice every day. And that the matter was known publicly to have been a miracle we learn from this circumstance: that Merodach-baladan, king of Babylon, sent his ambassadors to Jerusalem to inquire after the wonder that was done in the land, as well as after Hezekiah's health: see 2 Chron. xxxii.]
Fer de Vries
Italic text by Ruud Hooienga