Meridiana (part one)

To paint a ,,meridiana’’ (or sundial) on your south-facing wall during summer-recess, first condition to be fulfilled: you need the sun, at least when you do it in the non-mathematical, trial & error method. A first attempt was made during the Easter holiday. Every hour between 11 a.m. and 7 p.m., I put a mark on an improvised wooden board, which was placed parallel with our façade (south-southwest oriented). Afterwards I copied the design on the façade itself.

Only the iron had to be placed in the right angle on the wall: work for our roof-specialist, who planned a revision of our roof in the month of June. But my instructions were obviously not clear enough: our mason didn’t manage to put the iron or shadow-casting gnomon correctly as I asked. So I had to redesign and paint the dial plate in July. Fortunately they had already noticed the differences and were so kind to let the scaffolds on our house for an extra week.

Not one month later I experienced new faults: in our situation the gnomon is not aligned with the celestial poles. Moreover, the sun changes its position on the celestial sphere, being at a positive declination in summer, at a negative declination in winter, and having exactly zero declination (i.e., being on the celestial equator) at the equinoxes. Because I put the marks in the month of July, we could already see four weeks later that the sundial was running behind. Another inconvenience: sundials indicate the local solar time. To obtain the official clock time, three other corrections had to to be made, which I overlooked of course, in my naïve, pre-Galilean, Ptolemaean world vision. First, the orbit of the earth is not perfectly circular and its rotational axis not perfectly perpendicular to its orbit. The sundial’s indicated solar time thus varies from clock time by small amounts that change throughout the year. This correction — which may be as great as 15 minutes — is described by the equation of time. A sophisticated sundial, with a curved style in the hour lines, may incorporate this correction, but of course not in our case. I choose for straight lines. Secondly, I didn’t reckon with the longitudinal correction; never before I questioned myself of the abnormal situation that Brussels and Budapest lie in the same time-zone, but not Bucharest…

All our hope now goes to the daylight saving time system. Who knows, perhaps in wintertime (end of October) the sundial will catch up with the real time again…

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