1202.1071 (Costantino Sigismondi)
Costantino Sigismondi
The Clementine gnomon has been built in 1702 to measure the Earth's obliquity
variation. For this reason the pinhole was located in the walls of Diocletian's
times (305 a. D.) in order to remain stable along the centuries, but its
original form and position have been modified. We used an astrometric method to
recover the original position of the pinhole: reshaping the pinhole to a circle
of 1.5 cm of diameter, the positions of the Northern and Southern limbs have
been compared with the ephemerides. A sistematic shift of 4.5 mm Southward of
the whole solar image shows that the original pinhole was 4.5 mm North of the
actual position, as the images in the Bianchini's book (1703) suggest. The oval
shape of the actual pinhole is also wrong. Using a circle the larger solar
spots are clearly visible. Some reference stars of the catalogue of Philippe de
la Hire (1702), used originally for measuring the ecliptic latitude of the Sun,
are written next to the meridian line, but after the last restauration (2000),
four of them are wrongly located. Finally the deviation from the true North, of
the meridian line's azimuth confirms the value recovered in 1750. This, with
the local deviations of a true line, will remain as systematic error, like for
all these historical instruments.
View original:
http://arxiv.org/abs/1202.1071
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