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= Jean Tarde = From Wikipedia, the free encyclopedia

Jean Tarde (1561-1636) was a French astronomer, cartographer, and geographer. He is mostly known for his works on sunspots and his map of the diocese. He adopted Copernican heliocentrism and was influenced by Galileo Galilei following their meetings in Italy. After studying the spots on the sun for five years, Tarde published his theory that the sunspots were planets revolving around the sun. In addition to his astronomical works, Tarde made many contributions to cartography and geography in France, including mapping the diocese using a quadrant and studying its geography.

Biography
Early life and education

Tarde was born in 1561 in La Roque-Gageac, France, to the le sieurs du Pont, a bourgeoisie family in the gentry. There is little information on his family, and the name of his father is unknown. He attended the University of Cahors and earned a degree in law, then continued his studies at the Sorbonne, or University of Paris.

Later years

Tarde became a priest for the parish of Carves, and later became the canon theologian of the Sarlat cathedral church. In 1594, he became the vicar-general for the diocese of Sarlat after designation by the bishop. Finally, Tarde became the Almoner, or military chaplain, of Henry IV in 1599, a position that granted him income.

At the request of the Bishop of Cahors in 1606, Tarde mapped the surrounding area of the diocese. He also studied the geography, locating the city of Gaul. In charting the diocese, Tarde used a quadrant, an instrument consisting of a compass needle and a sundial, about which he published the work Les usages du quadrant a l’esguille aymantee (1621).

In 1614, Tarde visited Italy with his bishop and detailed his travels in a diary. He stopped in Florence and paid a visit to Galileo Galilei, whose work he had likely been acquainted with through Robert Balfour. From Galileo, Tarde learned about the two stars around Saturn, phases of Venus, and spots on the sun. Galileo had promised to send Tarde a telescope lens, but it is unknown whether Tarde actually received it. In Rome, Tarde met Christopher Grienberger, who provided more information on the sunspots and taught Tarde methods of observing the sun.

Tarde returned to Sarlat in 1615 and began conducting his own observations of the sun and writing his theory of sunspots. He published his theory in the Latin work Borbonia sidera (1620), which was then translated into French in the work Le astres de Borbon (1622). He dedicated his work to Louis XIII in hopes of gaining patronage, much like Galileo with the Grand Duke of Tuscany from the Medici family.

Death

Tarde died in 1636 in La Roque-Gageac, France. He was 75 years old.

Astronomy
Sunspot theory

Tarde theorized that the spots on the sun were small planets, which he named the “Bourbon stars” after the French royal family. Tarde believed that the planets revolved the Sun within Mercury’s orbit. He claimed that there were as many as thirty planets, which was the most he observed at once. Tarde’s theory was consistent with Christoph Scheiner’s theory that the sunspots were stars which traveled between the earth and sun. To observe the sun, Tarde had used Scheiner’s method of projecting a telescopic image on a white surface in a dark room.

Refutation of Galileo’s objections

Though Tarde drew some of his inspiration from Galileo, there were many points of disagreement between their ideas. Tarde rejected Galileo’s idea that the sunspots were from the sun itself because that would violate the Aristotelian belief that the heavens were incorruptible. He believed religiously that the sun could not possess imperfect blemishes like the earth.

Galileo pointed out in Scheiner’s theory that if the sunspots were stars, they would emit light and therefore be seen outside of the face of the sun. Tarde responded with an argument that the Bourbon stars did not emit light of their own but rather reflected the light of the sun, and they orbited so close to the sun that they could only be seen when shading the solar face. Galileo also objected that all planets were spherical, so if the sunspots were planets, they should not be irregular. Tarde agreed that the spots were not perfect circles, but he attributed this to inaccuracy in imaging the sun due to its brightness.

Tarde explained that the Bourbon planets had phases similar to that of Venus in a cycle of crescent shapes and full spheres. However, only the dark side of the planets could be observed from earth. As a Bourbon planet revolved around the sun, its light side merged with the sun’s light while the dark side remained in the shape of a crescent. When a planet travels closer to the center of the sun, the dark portion increases in size and thus appears as a spot. Tarde maintained that this was why the spots were irregular in shape and varied in size.

There were also other critics of Tarde’s theory, such as Pierre Gassendi, who in a letter to Galileo in 1625 indicated that there was no identified sunspot that demonstrated the motion of true planets. In fact, Tarde himself was aware of this flaw.