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09 Dec 15
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20 Apr 08
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heliocentric system
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29 Jan 08
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Giordano Bruno (lived 1548--1600 C.E.) revived Democritus' (a contemporary of Socrates) view that the Sun was one of an infinite number of stars.
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- The superior light-gathering power of his telescope over the naked eye enabled him to see many, many new fainter stars that were never seen before. This made Bruno's argument more plausible.
- The superior resolution and magnification over the naked eye enabled him to see pits and craters on the Moon and spots on the Sun. This meant that the Earth is not only place of change and decay!
Galileo Galilei (1564--1642 C.E.) was the first person we know of that used the telescope for astronomical observations (starting in 1609). The telescope was originally used as a naval tool to assess the strength of the opponent's fleet from a great distance. He found many new things when he looked through his telescope:
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- With his superior ``eye'' he discovered four moons orbiting Jupiter. These four moons (Io, Europa, Ganymede, and Callisto) are called the Galilean satellites in his honor. In this system Galileo saw a mini-model of the heliocentric system. The moons are not moving around the Earth but are centered on Jupiter. Perhaps other objects, including the planets, do not move around the Earth.
- He also made the important discovery that Venus goes through a complete set of phases. The gibbous and full phases of Venus are impossible in the Ptolemaic model but possible in Copernican model (and Tychonic model too!). In the Ptolemaic model Venus was always approximately between us and the Sun and was never found further away from the Earth than the Sun. Because of this geometry, Venus should always be in a crescent, new, or quarter phase. The only way to arrange Venus to make a gibbous or full phase is to have it orbiting the Sun so that, with respect to our viewpoint, Venus could get on the other side of, or behind, the Sun further away from us than the Sun. This was possible only if Venus orbited the Sun (see the figure at the end of the planetary motions section of the previous chapter).
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A scientific model cannot be proven correct, only disproven. A model that survives repeated tests is one that is consistent with the available data. His observations were consistent with the heliocentric model, but could also be explained with a geocentric model like Tycho's. But for Galileo, the observations were enough---he was convinced of the heliocentric system before he used his telescope and his observations confirmed his belief.
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More convincing evidence of the Earth's motion around the Sun would have to wait until 1729 when James Bradley (lived 1693--1762 C.E.) discovered that a telescope has to be slightly tilted because of the Earth's motion, just as you must tilt an umbrella in front of you when walking quickly in the rain to keep the rain from hitting your face. The direction the telescope must be tilted constantly changes as the Earth orbits the Sun. Over a century later, Friedrich W. Bessel (lived 1784--1846 C.E.) provided further evidence for the Earth's motion by measuring the parallax of a nearby star in the late 1830s.
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Evidence of the Earth's rotation (from west to east) is seen with the deflection of objects moving in north-south direction caused by the differences in the linear speed of the rotation at different latitudes. All parts of the Earth take 23 hours 56 minutes to turn once, but the higher latitudes are closer to the Earth's rotation axis, so they do not need to rotate as fast as regions nearer the equator. A moving object's west-east speed will stay at the original value it had at the start of its motion (unless some force changes it). If the object is also changing latitudes, then its west-east speed will be different than that for the part of the Earth it is over. Therefore, moving objects appear to be deflected to the right in the northern hemisphere and to the left in the southern hemisphere. This is called the coriolis effect after Gustave-Gaspard Coriolis (lived 1792--1843 C.E.) who deduced the effect in 1835 to explain why cannonballs shot long distances kept missing their target if the cannon was aimed directly at its target. See energy flow section for applications (and illustrations) of the coriolis effect to planet atmospheres.
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The coriolis effect and the Foucault pendulum are both based on Galileo's discovery that an object's motion (speed and/or direction) are changed only if there is a force acting on it.
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