Venus Transits and Saturn Rings

Timelines of Events

  • c.150 CE
    • Ptolemy calculated the distance between the Earth and the sun to be 1,210 times the radius of the Earth, or roughly 5 million miles (8 million km).
  • 1619
    • Although the absolute values are unknown, Kepler's third law provides the ratio of planetary orbital sizes.
  • 1610
    • Galileo announces the discovery of four moons orbiting Jupiter.
  • 1631
    • The first known planetary transit is seen by French astronomer Pierre Gassendi as Mercury passes across the solar disk.
  • 1655
    • Titan, a moon orbiting Saturn that is 50% larger than the moon of Earth, is found by Christiaan Huygens.
  • 1716
    • According to Edmond Halley, a precise timing of Venus's transit could result in a precise measurement of the Earth's distance from the sun.
  • 1801
    • The first asteroid is located between Mars and Jupiter in its orbit.
  • 1859
    • James Clerk Maxwell, a Scottish physicist demonstrates that Saturn's rings cannot be solid because they would disintegrate under the pull of gravity.
  • 1960s onward
    • Spacecraft have recently entered orbits around Jupiter and Saturn.
    • Voyager 2 passed by Neptune and Uranus.
    • There have been numerous moon discoveries.
  • 2012
    • The most recent Venus transit occurred; the next two will occur in 2117 and 2125.

Venus Transit

  • In 1639, Jeremiah Horrocks, a 20-year-old English astronomer, discovered inaccuracies in Kepler's tables, which led him to forecast Venus would cross the face of the sun.
    • Horrocks wrote to his associate William Crabtree, urging him to observe the transit because it was only four weeks away.
  • Using helioscopes, which focused the sun's image from a telescope onto a plane, Horrocks and Crabtree independently set up their systems on December 4, 1639.
    • They were the first people to witness a Venus transit.
  • Horrocks attempted to estimate Venus' size and distance as it crossed the solar disk.
  • He pointed out that it subtended, or covered, an angle of 76 arcseconds at Earth, which was less than Kepler's estimated value.
  • Horrocks determined that the disk of Venus subtended an angle of approximately 28 arcseconds as seen from the sun using the ratios of planetary distances known from Kepler's third law.
  • Horrocks determined that Mercury subtended the same angle as Venus using information from a Mercury transit that had occurred in 1631.
  • He calculated that the distance between Earth and the sun is 59 million miles (95 million km), assuming that all planets follow the same angle toward the sun.
  • The Earth subtends 17.8 arcseconds at the sun, which is located 93 million miles (150 million km) away, refuting Horrocks's incorrect assumption.
    • But he was the first to estimate the size of the solar system with some degree of accuracy.

Saturn's Rings

  • Giovanni Cassini, an Italian astronomer working at the Panzano Observatory near Bologna, was given a cutting-edge refracting telescope by Rome's Guiseppe Campini in 1664.
    • He used it to observe the orbits of Jupiter's four known moons, measure the spin period and polar flattening of the planet, and discover the bands and spots on Jupiter.
  • Cassini was asked to supervise the construction of the new Paris observatory because of his reputation as a brilliant observer.
    • He focused his telescope there on Saturn, whose largest moon, Titan, had been found by Christiaan Huygens in 1655.
    • In 1671 and 1672, Cassini found two more moons: Iapetus and Rhea.
    • In 1675, he correctly deduced that there was a significant gap in the Saturnian rings and that the rings were composed of many tiny orbiting bodies rather than being a solid object.
    • In 1684, two more ephemeral satellites, Tethys and Dione, were found by him.
  • The number of known satellites in the solar system has nearly doubled thanks to Cassini's observations.
    • Since then, the number has sharply increased.
    • There are more than 60 known satellites for each of Jupiter and Saturn.
  • The moons of the gas giants in the outer solar system come in two sizes: large moons that formed along with the planet, and smaller moons that were plucked from the asteroid belt.
  • While Mercury and Venus lack moons in the inner solar system, Mars has two small asteroidal moons that have been captured.
  • Astronomers are still unsure of how the enormous moon that orbits the Earth, which has 181 its mass, formed.