The time-lapse animation which is shown was carried out on March 15, 2012 from inside the zenith well of the Cassini room. At that time, the ephemeris (This is a table of values that gives the positions of astronomical objects in the sky at a given time) of the Sun gives the moment of transit of the center of the sunspot on the meridian line: 11h59mn26s UTC+1 (Paris civil time). The recorded time as can be seen from the animation shows a slight delay of about a couple of seconds at most. This delay is real and was also found for other times of the year. It results from a very slight positioning error of the meridian line in the plane of the meridian but also, and more crucially, as a result of significant movements of the building itself that occurred since the mid-eighteenth century onwards. The Observatory risked becoming a ruin and so significant work of consolidation and restoration had to be undertaken between 1786 and 1793, work which has greatly affected the integrity and accuracy of the meridian line.

However, even if the line is still close enough to the true meridian, to within a few millimeters, it is quite different for the graduation in meridian altitude above horizon. Because of the tilt of the building towards the south of the southern facade, many intervals have widened between the copper rulers. Moreover, the original horizontal plate, where the hole was drilled for the gnomon, has been replaced - presumably during the restoration work of the late eighteenth century - by a lens whose vertical position above the ground is too high by about 5 cm. The result of all these negative factors is, large errors can be made in reading the altitudes, in the range of as much as 3 arcmin (one arcminute is equal to one sixtieth (1/60) of one degree).