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Giacomo Filippo Maraldi
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First of the Maraldi astronomers

J D Cassini

The name of the Maraldi family was originally Marvaldi. They were based in the village of Candeasco in Liguria, where their family was particularly associated with architecture of the Ligurian baroque style in the area of Imperia having five generations working in the seventeenth and eighteenth centuries. This photograph illustrates the head of the door to their house in Perinaldo.

Giacomo Filippo Maraldi was born on the 21st August 1665 in Perinaldo, at that time in the county of Nice. The oldest child of Giovanni Francesco Maraldi di Oneglia and the youngest sister of Giovanni Domenico Cassini – also known as Cassini I – he was invited by his uncle to Paris in 1687 to assist with the latter’s geodesic work at the Paris Observatory where he lived with other astronomers who filled the Observatory’s rooms. Giacomo Filippo Maraldi is also known as Jacques Phillipe Maraldi and, to distinguish him from other Maraldi astronomers, is referred to as Maraldi I.

It appears that he published little under his own name and his work is known mainly, if not solely, through the records of the Académie. Giacomo Filippo was apparently devoted to his uncle, perhaps naturally forming many of his own opinions on those Giovanni Domenico propounded.

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Work on astronomy

Giacomo Filippo is mainly known for his work on the planet, Mars, though he also worked on observations of the stars. He made careful observations of Mars at every opposition with particularly good results from the perihelic oppositions of 1704 and 1719. He determined the rotation period of Mars noting that the same region returns to be visible after 36 rotations of Mars (or 37 rotations of Earth), thus calculating a rotation period of about 24h 40m.

He discovered banded structures on the surface of Mars which are actually the Mare Sirenum and Mare Tyrrhenum abledo features. In addition he observed an hourglass sea, Syrtis Major, and studied the properties and changes of the polar caps on Mars.

In 1704 he observed white areas at the poles of Mars, but didn’t refer to them as being ice. He also noted that the southern cap, which was more easily observed due to its being tilted towards the Earth, wasn’t centred on the rotational pole of Mars. In 1719 he at last suggested that the white areas might be ice caps, noting that the southern cap changed its size and disappeared in August and September, returning later and, in doing so, was evidence of seasons on Mars.

Giacomo Filippo carried out observations of Jupiter’s moons, observed comets 1699 (C/1699 D1), 1702 (C/1702 H1), 1706 (C/1706 F1), 1707 (1707 W1), 1723 (C/1723 T1) and 1729 (C/1729 P1), and calculated various cometary orbits. He recognized that the corona visible in total solar eclipses belongs to the Sun and not to the Moon. In 1704, he discovered the variable star R Hydrae, a Mira type variable and, in 1704 and 1705, he created a catalogue of star positions, in an early but unsuccessful attempt to obtain stellar parallaxes.

On the 24th July 1721 Giacomo Filippo and Jacques Cassini accompanied two of the principal astronomers from the Observatoire to observe a partial eclipse of the sun. Louis XV took a keen interest in the event and went himself to the Observatoire where the chart of the moon was explained to him. In 1726, with the death of Guillaume Delisle, the King conferred on Giacomo Filippo the title of First Geographer.

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Work on cartography

Giacomo Filippo was also involved in other areas. He spent years working on measurements of the meridian between Dunkirk and Amiens. In 1700 and 1701, he assisted Jacques Cassini, J.M. de Chazelles, and Piere Couplet on the project to extend the Paris meridian to the southern border. In 1718 he returned to this work, assisting Jacques Cassini and G. de la Hire extend the survey of the Paris Meridian to the north. In 1701, he went to Rome to work on a calendar for Pope Clement XI. In addition he helped Bianchi to determine the meridian of the church of the Carthusians.

He was elected to the Académie des Sciences in 1694 as a student, an associate in 1699, and a pensioner or Associate in 1702.

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Work on mathematics

Giacomo Filippo is also known for his work in mathematics. In 1712 he stated that the internal angle of a dodecahedron, which came to be known as the Maraldi angle, was 109º 28´.

The problem of calculating this angle goes back to Aristotle’s noticing the form of bees’ honeycombs. This was investigated geometrically by Pappus, but it wasn’t until the seventeenth century that the angle between the planes created by four lines coming to a point at mutually equal angles was calculated accurately by Giacomo Filipo. He is understood to have done so experimentally, but it was soon understood that it was impossible to do so with the instruments available to him at the time. Koenig, realising this, used calculus to obtain a result of 109º 26´. However, in 1739 Colin Maclaurin confirmed that Giacomo Filipo had indeed been correct.

Giacomo Filippo Maraldi died on the 1st December 1729 in Paris at the age of 64. He was honoured in 1935, together with his son, by the naming of Moon crater Maraldi (19.4N, 34.9E, 39 km diameter) and, without an explicit mention of his son, Mars crater Maraldi (62.0S, 32.0 W, 124km diameter), in 1973.

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