Ancient Greek astronomers observed the motions and positions of objects in the sky in an effort to explain the Earth's place in the cosmos. Most stars (and planets) rise in the east and set in the west. The apparent motions of these stars are known as <1> or daily motions. <2> stars are those that are always above the horizon and don't rise or set. You can watch these stars <3> the northern and southern celestial poles depending on whether you are located in the north or south hemisphere. Planets move <4> through the constellations in a narrow band of the sky known as the <5>. Mars, Jupiter, and Saturn undergo retrograde motion, the apparent backward motion of the planet. Retrograde motion occurs near <6>, when the planet traces out a loop or some version of a squiggle in the sky with it's movement. Mercury and Venus don't make retrograde loops, but always appear very close to <7> in the sky. One last important observation was that in ancient times stars showed no signs of parallax. Parallax is the apparent change in position of near objects relative to more distant objects when the <8> changes position.
The most influential figure in Greek Astronomy was Aristotle. His theories were based on three main principles. First, that the Earth was <9> -- determined by observing it's shadow on the moon during an eclipse. Second, that all solar, planetary and lunar motion was circular and uniform in speed. He believed each object was carried on a <10> crystalline sphere. Lastly, Aristotle thought the Earth to be <11> since parallax was unapparent and cloud motion was in the direction opposite the Earth's rotation. His views were widely accepted for many years and in 1200 AD he was named the Ultimate Authority on nature by St. Thomas Aquinas.
Many other Greeks made important contributions to astronomy. Aristarchus had a talent for geometry. This is evident in the calculations he made to estimate the relative sizes and separations of the Earth, sun and moon. He calculated the sun to be 20 times larger and also 20 times farther away than the moon. His methods were valid but he lacked the tools to make accurate angular measurements. Today we know the actual factor is about <12>. Eratosthenes used the sun at the zenith in Alexandria and the sun's angular distance from the zenith in Syene to measure the circumference of the Earth. The size he found was 250,000 stadia and by one estimate he came within 1% of the actual value. Hipparchus invented the magnitude system which is useful for classifying stars in terms of their apparent brightness -- the $firstmag stars in the sky are magnitude <13> and the $secondmag are magnitude <14>.
Ptolemy's theories were based on the <15> model of Aristotle. He developed these ideas further using a combination of $wrongAnswer and <16> to help explain retrograde motion. He connected Mercury and Venus to the Earth in a line with the sun to account for their close vicinity to the sun in their circular motions. Ptolemy's theories allowed reasonably accurate predictions of planetary positions because of the many <17> that could be varied to compensate for errors.
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