24h40m , very close to the currently accepted value of 24h37m23s . After NASA ’ s Viking landings on Mars , University of Texas astronomer George Van Biesbroeck noted that he had spent 10 years observing Mars to pin down its rotation rate and that NASA ’ s Deep Space Network measurements of the Viking landers ’ signals had provided a better rotation period in a year ! Cassini changed his name to Jean Dominque Cassini after he moved to the Paris Observatory and became its director in 1671 . He then put Mars to a practical use : How big is the Solar System ? Cassini and other French astronomers observed Mars from Paris and from South America ( as did John Flamsteed , an English astronomer ). Their results came within about 10 percent of the currently accepted value . It wasn ’ t improved upon until more than two centuries later by American astronomer Simon Newcomb . The modern value used radar returns from Venus and is better yet . The way this worked was to measure the position of Mars against background stars just after it rose above the horizon after dusk and again before it set at dawn . These measurements of the diurnal parallax of Mars were made during the perihelic opposition of 1672 , when Mars was slightly closer to Earth than at its closest in 2018 . “ Diurnal ” refers to Earth ’ s day-night cycle for the dusk and dawn measurement . “ Parallax ” is the measurement of the angular difference in Mars ’ position in the sky , which changes slightly as Earth rotates during the night . ( Technically , the parallax is half the angular difference .) “ Perihelic ” refers to Mars being at the point in its orbit when it is closest to the Sun . “ Opposition means that Sun-Earth-Mars are ( almost ) on a straight line from one to the other . ( See the Observation Activity # 2 for observations you can make to demonstrate the ideas .) Aside : Huygens and Cassini were both also instrumental in advancing
28
Figure 3 : Maps of Mars . These two maps have been distorted in order to make the latitude and longitude scales match for easier comparison . In addition , the top map was reverted so both maps match the modern convention for east and west . South is up on both maps . ( Top ) Schiaparelli ’ s ( 1882 ) published map of Mars showing the enigmatic channels . The farthest right meridian marks longitude 0 °. ( Bottom ) Lowell ’ s 1896-7 map of Mars , published in 1905 but circulated earlier . Other examples of his maps can be found on the web . Make your own comparisons and judgments of features . Syrtis Major is prominent in the right halves of both maps centered near longitude 290 °.
knowledge of Saturn . Huygens observed the previously mysterious cycles of appearance-disappearance of Saturn ’ s ring [ observed as singular at the time ] and explained them , complete with illustrations , as due to the tilt of the planet ’ s rotation axis being observed from Earth during different times in Saturn ’ s orbit around the Sun . In short , the ring disappears during Saturn ’ s seasonal equinoxes . He also discovered Titan , Saturn ’ s largest satellite . Cassini discovered that Saturn had not one ring , but two , separated by what we now call the Cassini division . [ Now 8 rings have been identified .] He also discovered 3-1 / 2 satellites . He does get credit for discovering all of Iapetus , it ’ s just that Iapetus was only visible on one side of Saturn [ celestial
Sky ’ s Up