Besides, Flammarion observed the Moon and planet Mars. In 1873 and 1885, he brought up the hypothesis that Mars' color might be attributed to vegetation. He published several popular books (L'astronomie Populaire in 1879, of which over 100,000 copies were sold and an English translation by J.E. Gore appeared in 1894, as well as a book on Mars, La Planète Mars, supporting the existence of "canals", built by an advanced civilisation, Vol. 1 in 1892 and Vol. 2 in 1909), and encouraged amateur astronomy. In 1877, Flammarion founded the Astronomical Society of France. In 1882, he was donated a private observatory and estate by a M. Meret who admired his work.
In 1922, he was made a Commander of the Legion of Honor for his astronomical life-work.
Camille Flammarion passed away on June 3, 1925 in Juvisy-sur-Orge (Essonne, France).
Flammarion was honored by the naming of a Moon Crater (3.4S, 3.7W, 74.0 km diameter, in 1935) and a Mars Crater (25.4N, 311.8W, 173.0 km, in 1973). Asteroid (1021) Flammario has been discovered by Max Wolf on March 11, 1924 and provisionally known as 1924 RG; also as A910 CE and 1977 UM from independent findings.
In 1877, Camille Flammarion had found and acquired Messier's personal copy and notes of the Messier Catalog from an old book store. He used this as reference for various works including a revised version of the catalog, and between 1917 and 1921, published a series of profound articles on his systematic observations, and the contemporary knowledge about these objects. Evaluating Messier's handwritten notes, he tentatively identified M102 with NGC 5866 before 1917 (Flammarion 1917, 1921a). In 1921, based on another handwriting by Messier, he appended the first additional object to Messier's Catalog, when he added M104 to the Messier Catalog, which he identified with William Herschel's H I.43 or NGC 4594 (Flammarion 1921b, 1921c). This was the first of a number of additions to Messier's catalog. After finishing the Messier Catalog, he continued by observing other, or additional "Principal Star Clusters and Nebulae" visible from France with medium-sized instruments which he numbers consecutively starting with No. 105; we have compiled a list of Flammarion's additional objects.