On the Low and High Sun. Letter II.

then be measured against the foot-rule mentioned before. So that you may also have some small, yet recorded, variation of the shadow when the Sun is low and high, consider this: when I was writing to our friend Naudé: Gabriel Naudé (1600 to 1653), a French librarian and scholar who served as a physician to Louis XIII in 1636, I had recently completed various observations on this matter, particularly those conducted in the month of July. Among others, at sunrise on the 12th day, when the Sun was already three degrees high, the shadow cast by the upper pinnacidium: a sight or vane on an astronomical instrument onto the lower one was measured at 3 digits, 3 lines, and 6 parts. original: "digitorum 3. lin. 3. partic. 6." These are subunits of the Royal foot. When it was 5 degrees high, the shadow was 3 digits, 3 lines, and 0 parts. At 8 degrees high, it was 3 digits, 2 lines, and 8 parts. Thus it continued decreasing until, at an altitude of about 15 degrees and at midday itself, the shadow was 3 digits, 2 lines, and 4 parts. I should also mention that it increased in the same sequence after midday, until the Sun had sunk to 3 degrees above the horizon, where the shadow was again 3 digits, 3 lines, and 6 parts. I mention these things so that if you ever wish to repeat the observation yourself or through friends, you have a definite point of reference to aim for, and you will not regret having placed your trust in these findings.

III.

As for the explanation you then provide regarding the First Cause, it has caused no small amount of trouble. I regret that I was not clear enough, and thereby gave you the opportunity to connect causes that I intended to keep separate. Specifically, since there seemed to be two conflicting effects, one in the eye and the other in the shadow: in the former, the appearance of the Sun is larger when it is low than when it is high; in the latter, the spreading of the shadow is less when the Sun is high than when it is low. My intention was to explain the causes of each separately, so that when speaking of the variation in the eye, I would mix in nothing about the shadow cast by an opaque object; and regarding the variation in the shadow,