Christmas opening times

Over the Christmas period the Royal Observatory, National Maritime Museum, Cutty Sark and Queen's House will be open as usual. We are closed to the public from 24-26 December. Plan your next visit

Today it seems appropriate to highlight the millennia-long connection between astronomy - so central to the story of the longitude at sea - and Christianity. This arose in large part thanks to the debate over how to set an annual date for the celebration of Easter, since the precise day of the resurrection had not been recorded. Some early Christians followed lunar precedents from the Old Testament related to Passover, which were based on the Hebrew calendar.

In 325, the First Council of Nicaea convened by the Roman Emperor Constantine I sought to separate the timing of the festival from the Jewish calendar, and it was ultimately decreed that it would always take place on the first Sunday after the full moon following the March equinox (the vernal equinox in the Northern Hemisphere). However, it was centuries before the different means of determining this date were better consolidated. After the introduction of the Gregorian calendar in 1582, Catholics and Protestants in the West used a different method of date determination than did most of the Eastern Orthodox churches, which still based their calculations on the Julian calendar.

It has always seemed to me that one of the more interesting results of this intersection of astronomy and religion in search of the 'true' date of Easter was the embedding of large astronomical instruments in Catholic cathedrals and duomos during the sixteenth through eighteenth centuries - since annual and predictive calculations required knowledge of the equinoxes and the length of the solar year. This was discussed in John Heilbron's The Sun in the Church: Cathedrals as Solar Observatories (2001) and was revisited by Alistair Kwan in his Ph.D. dissertation Architectures of astronomical observation: from Sternwarte Kassel (circa 1560) to the Radcliffe Observatory (1772) (Yale University, 2010).

The brass meridian line and obelisk gnomon of St. Sulpice in Paris, requested in 1727 by priest Jean-Baptiste Languet de Gergy. Its utility reportedly protected it from destruction during the French Revolution.

In effect, these religious buildings were turned into solar observatories for calendrical purposes. A graduated north-south meridian line (often of brass) would be run across the floor of the church and then up a gnomon facing the position of the sun at noon, such as the sculptural obelisk in the distance in the photo above of St. Sulpice in Paris. Sunlight was focused through a glassed oculus on a south wall. As Dr. Kwan points out, there were many difficulties in trying to turn a cathedral into such a solar observatory:

'While not everyone who wants to build a meridional camera obscura needs to make it both very long and very tall, the opportunities to build one are constrained by both the existence of existing architecture and the possibility of modifying it. For example, north-south naves are relatively rare; obviously only a subset of these will be both long and tall enough for high-precision equinoctial and winter meridian line measurements. Next, the astronomer has to be in the area or able to go there. Third, he needs access privileges for interventions to the fabric, and also for interruptions to liturgical activities spanning months, if not years.'

However, as the late Professor Curtis Wilson pointed out, it was also true that 'for a century and more, a carefully constructed meridiana in a cathedral could outdo other instruments in the precise determination of angles. Telescopic sights were first applied to graduated arcs in the 1660s, but the available lenses blurred the image, a defect overcome only with the introduction of achromatic lenses around 1760. Graduating arcs accurately was another problem. Only in the 1780s, with the advent of the circle-dividing machine, did divided-arc instruments win secure primacy over meridiane.'

The meridian which the astronomer Cassini had built in the Basilica San Petronio in Bologna during the mid-seventeenth century was a model for many others and is reportedly the longest in the world. (Giovanni Domenico Cassini was also one of the early pioneers of the method of finding longitude by observations of the moons of Jupiter.) Below are photographs of the extant working meridian at Santa Maria degli Angeli e dei Martiri in Rome, which Pope Clement XI commissioned from Francesco Bianchini in the early eighteenth century in part to match Cassini's accomplishment.

The meridian in Rome was also intended to help predict the date of Easter and check the accuracy of Gregorian calendar reform. Bianchini added more holes in the ceiling through which he could make telescopic observations to determine the right ascensions and declinations of the stars Polaris, Arcturus and Sirius as well. And since the church was housed in the former baths of Diocletian, these instrumental installations could further represent Christianity's calendrical victory over the pagans...!

Francesco Bianchini's meridian

Sunlight on the meridian just before solar noon

Images: Wikimedia.