In his later career, Herschel discovered two moons of Saturn, Mimas and Enceladus; as well as two moons of Uranus, Titania and Oberon. He did not give these moons their names; they were named by his son John in 1847 and 1852, respectively, after his death. Herschel measured the axial tilt of Mars and discovered that the Martian ice caps, first observed by Giovanni Domenico Cassini (1666) and Christiaan Huygens (1672), changed size with that planet's seasons. It has been suggested that Herschel discovered rings around Uranus. Herschel introduced but did not create the word "asteroid", meaning star-like (from the Greek asteroeides, aster "star" + -eidos "form, shape"), in 1802 (shortly after Olbers discovered the second minor planet, 2 Pallas, in late March), to describe the star-like appearance of the small moons of the giant planets and of the minor planets; the planets all show discs, by comparison. By the 1850s 'asteroid' became a standard term for describing certain minor planets. From studying the proper motion of stars, the nature and extent of the solar motion was first demonstrated by Herschel in 1783, along with first determining the direction for the solar apex to Lambda Herculis, only 10° away from today's accepted position. Herschel also studied the structure of the Milky Way and was the first to propose a model of the galaxy based on observation and measurement. He concluded that it was in the shape of a disk, but incorrectly assumed that the Sun was in the centre of the disk. This heliocentric view was eventually replaced by galactocentrism due to the work of Harlow Shapley, Heber Doust Curtis and Edwin Hubble in the 20th century. All three men used significantly more far-reaching and accurate telescopes than Herschel's. Discovery of infrared radiation in sunlight In early 1800, Herschel was testing different filters to pass sunlight through, and noticed that filters of different colors seemed to generate varying amounts of heat. He decided to pass the light through a prism to measure the different colors of light using a thermometer, and in the process, took a measurement just beyond the red end of the visible spectrum. He detected a temperature one degree higher than that of red light. Further experimentation led to Herschel's conclusion that there must be an invisible form of light beyond the visible spectrum. He published these results in April 1800. Biology Herschel used a microscope to