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Meridian Matters

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Ken Tapping, June 10, 2009

In the sky this week...

> Mars and Venus are close together in the eastern sky before dawn.

> Jupiter rises in the early hours and Saturn is in the southern sky most of the night.

> The Moon will reach Last Quarter on June 15.

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A short while ago I was in Paris, and it turned out the hotel was only a short walk from St. Sulpice, a church featured prominently in a popular recent book and film. Even when one ignores the strange things supposed in the book and film to have gone on at the church, the story mentions something in St. Sulpice of high astronomical interest: a combined meridian and precise sun-driven calendar. The purpose is to indicate the dates on which religious festivals should be celebrated.

Traditionally, churches are built in a cross configuration, with the nave running west to east, and crossed by the transept, which runs north-south. High on the wall of the southern end of the transept at the St. Sulpice church is a large stained-glass window, about 25 metres above the floor, in which there is a metal plate with a round hole in it. When the Sun is in the south, a spot of light is cast on the floor. On the floor below the window, 11 metres from the wall, is a square marble plaque, bearing a Latin inscription and the date 1745.

Across that plaque, running northward, is a brass strip about a centimetre wide. It then continues northwards between marble strips, some 50 metres to the northern end of the transept, where there is an obelisk almost 11 metres high. The strip continues up the centre line of the obelisk. Part way up is an arrow and two wavy lines. Scattered along the line across the floor are little brass indicators. This is how it works. During the year the elevation angle of the Sun at noon changes. On the summer solstice it is at its highest, and on the winter solstice it is at its lowest. At the spring and autumn equinoxes it is halfway between the extremes. On the summer solstice the sunlight comes through the hole at a high angle, putting a spot of light on the brass strip at the plaque. On the winter solstice it comes through at a low angle and falls on the strip at the arrow and wavy lines on the obelisk. On the strip crossing the floor the indication points identify where the spot of sunlight would fall on the strip on the equinoxes and various religious festivals that occur at fixed dates. 

The information sheet about the device at the church gave the precise dimensions, so it was possible to estimate how accurately it would indicate the date. It turned out that with the instrument being that large, one could read off the daily date without any difficulty. Of course this simplicity of use comes from a lot of detailed surveying measurement and precise construction.

A meridian is a line drawn from one of the Earth's poles to the other, running precisely north-south. This church meridian is not related to the Paris meridian, which was set up by the Paris observatory for navigation. That runs about 100 metrers east of the church, through the Paris Observatory and the Louvre. A similar meridian was set up through the Royal Greenwich Observatory, in the UK, also for navigation, and after a lot of international discussion, it was finally agreed the UK line would be the one used, which is why the line of zero degrees longitude passes through Greenwich rather than Paris.


Ken TappingKen Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,
E-mail: ken.tapping@nrc-cnrc.gc.ca