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The Arago Medallions, Paris, France

48° 50′ 4.23″ N, 2° 20′ 11.41″ E

François Arago and the Paris Meridian

François Arago was a French mathematician, physicist, and astronomer (and briefly the Prime Minister of France in 1848) who studied optics, magnetism, electricity, and astronomy. He strongly supported the then-controversial theory that light was made up of waves; he demonstrated that a rotating metal disc affects the motion of a magnetic needle suspended above it (for more on magnetism, see Chapter 75); and he showed that light moves more slowly in dense media. He was also a popular orator, and gave public lectures on astronomy for over 30 years.

But Arago is best known today as one of the directors of l’Observatoire de Paris (the Paris Observatory), where he lived and worked. The Observatory was established in 1667 with the support of King Louis XIV of France. Initially, the Observatory’s mission was to improve the instruments and maps needed for marine navigation. On Midsummer’s Day in 1667, the outline of the Observatory was traced on the ground, with measurements being made to determine the location for the Paris meridian. The Paris meridian (and hence the French definition of 0º of longitude) runs through the middle of the Observatory, and the entire building and site are aligned north-south along it.

One of Arago’s early tasks (while he was just secretary of the Observatory) was to help extend the Paris meridian. The length of the meridian was also used to calculate the length of a meter. The French Academy of Sciences had earlier decreed that one meter was to be one ten-millionth of the length of the Paris meridian from the North Pole to the Equator (one quarter of the way around the Earth). Arago was sent on a mission to continue the meridian line southwards into Spain and all the way to Formentera (the smallest and most southerly of the Balearic Islands). Measurement of the distance from Dunkerque and Barcelona (both along the Paris meridian) was used to determine the length of the meter to within 5 millimeters.

The Paris meridian was replaced by the Prime Meridian (which runs through Greenwich, UK; see Chapter 49) at the International Meridian Conference in 1884, but the French continued to use their own meridian as a reference point until 1911. To find the Paris meridian today, look for 2º 20’ 14” E on your GPS. Even better than looking at your GPS, you can follow a set of 135 bronze medallions set into streets, pavements, and courtyards throughout Paris.

These Arago medallions were the work of a Dutch artist, Jan Dibbets, who was commissioned to create a memorial to François Arago in 1994. The medallions run along the Paris meridian in central Paris and take in some of the city’s best-known monuments.

A trip along the path of the Arago medallions should begin near the Observatory itself. Just to the south of the Observatory garden on Place de l’Ile de Sein is a monument to Arago, and a single Arago medallion can be found at its base. Crossing the road toward the Observatory reveals another medallion set into the pavement. If the Observatory garden is open, it’s possible to walk inside and see the meridian line set into the ground; if not, skirt around the side following Rue du Faubourg Saint-Jacques and turn left onto Rue Cassini. This brings you to the entrance to the Observatory, where you’ll find more medallions and an atomic clock to set your watch to very accurate French time.

A gentle walk up Avenue de l’Observatoire approximately follows the meridian north and affords a wonderful view of the Basilique du Sacré-Cœur in the distance. You’ll reach le Jardin de Luxembourg (a large park containing the French Senate), where you’ll find more medallions and a view westward to the Eiffel Tower (see Chapter 18). To the east of the park is le Panthéon (see Chapter 13).

Exit the park on the north side and walk up Rue Garancière to the church Saint-Sulpice: the meridian passes close to it. Carry on up Rue Mabillon to Boulevard Saint Germain, where another Arago medallion awaits.

You can follow the Arago medallions north until you reach the River Seine at the Quai de Conti. Then cross the river via the romantic Pont des Arts and you are at le Musée du Louvre. The meridian line crosses the museum’s courtyard (with more medallions to be found outside and inside the museum) and continues on to Le Palais Royal. Further north, the line comes close to l’Opera, the medallions become more spaced out, and a little further on is the Place Pigalle and le Moulin Rouge.

Some of the medallions are missing (most notably the medallion closest to the pyramid inside the Louvre) and some have been replaced with a different design commemorating la Meridienne Verte (The Green Meridian). The Green Meridian was a project in the year 2000 to plant a line of trees (and hold a picnic) along the line of the Paris meridian throughout France. At least emotionally, the French haven’t yet ceded to Greenwich.

Practical Information

The Paris Observatory has an excellent site with details of its work; for the English version, see http://www.obspm.fr/presentation.en.shtml.

The Observatory itself is open on the first Saturday of the month, and guided tours are available on Tuesdays and Thursdays. The guided tour takes in the Cassini Room, which runs along the Paris meridian, and a map of France on the floor shows the meridian as it crosses the country.

True North and Longitude

Meridians are imaginary lines that follow the Earth’s surface north-south, cutting through the North and South Poles. Longitude is measured as the angle from the Prime Meridian (see Chapter 49), but any north-south line is a meridian of the Earth. The direction of the North Pole (which runs along any meridian line) is known as True North. It’s possible to find the meridian line for any point on Earth, and thus the direction of True North, with patience, a long stick, and a piece of string.

On a sunny day, place a stick vertically in flat ground and follow the shadow it casts. Start in the morning and mark the position of the end of the shadow at regular intervals (Figure 8-1). The shadow will move as the Earth rotates and the Sun’s position in the sky changes.

Figure 8-1. Following the path of the Sun with a stick

When the shadow is at its shortest, it is pointing to True North (if you’re in the northern hemisphere; in the southern hemisphere it will point to True South) and the shadow is tracing out the meridian line.

Since it’s hard to tell exactly when the shortest shadow occurs (it happens at noon local time, but you’d need a clock calibrated to your exact longitude to know when precisely that is), you might want to take two measurements to find the meridian line.

Once the Sun’s path has been traced out, use a piece of string attached to the stick to draw an arc of a circle that cuts through the path of the Sun in two places (see Figure 8-2). Try to make these as far apart as possible, using the biggest circle you can.

Figure 8-2. A circle cutting through the path of the Sun

The line from A to B (the two points where the circle cuts through the path of the Sun) is perpendicular to the meridian, and the meridian is at its center. So to find the meridian and True North, bisect the line AB by drawing a pair of circles: one centered on A and the other centered on B (see Figure 8-3). For simplicity, use the same string with the same length: the two circles will intersect at the stick and at another point, labeled C in the figure.

Figure 8-3. Creating the perpendicular bisector to the line AB

The line from the stick to point C is the meridian, with True North in the direction from the stick to C.