At Paris he was employed to do some geodesic measurements by using eclipse times of Jupiter's moons to determine longitudes, and obtained a longitude difference between Greenwich and Paris of 9m 23s (modern value: 9m 20.93s). Maraldi observed several comets (the first comet of 1742 (C/1742 C1, 1742), the great comet of 1743-4, discovered by Klinkenberg and De Chéseaux (C/1743 X1, 1744), comet 1746 De Chéseaux (C/1746 P1, 1747), comet Halley 1759, comet 1762 Klinkenberg (C/1762 K1, 1762) and comet 1769 Messier (C/1769 P1, 1769), and calculated cometary orbits. He also observed transits of Mercury and Venus.
Maraldi helped in the publication of 25 volumes of the Connoissance des Temps, and published Lacaille's catalog of southern stars, Coelum Australe Stelliferum.
In August and September 1746, he observed Comet De Chéseaux of that year with Jacques Cassini. While doing so, he discovered two "nebulous stars" which are actually globular clusters: M15 in Pegasus on September 7, 1746 and M2 in Aquarius on September 11, 1746 (Maraldi 1746). He continued to trace the comet until the evening of December 5.
J.-D. Maraldi retired in 1772 to Perinaldo, Italy where he died on November 14, 1788.
11 years after he left, in June, 1783, his house on the site of the Paris Observatory became the home of another famous astronomer, Pierre Méchain.
He was honored, together with his uncle, by naming Moon crater Maraldi (19.4N, 34.9E, 39 km diameter, in 1935). He is not separately mentioned in the naming of Mars crater Maraldi (62.0S, 32.0 W, 124km diameter, in 1973), which may thus honor his uncle only.
Delambre (1827) mentions a third Maraldi (Jaques-Philippe, Maraldi III, 1746-1797) who did some observations of planetary satellites at Perinaldo.
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