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Lecture 4 - LECTURE 4 By 250 B.C 2 theories of solar...

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LECTURE 4 (1/24/2008) By 250 B.C., 2 theories of solar systems: 1) Geocentric theory (later popularized by Ptolemy in 2 nd Century A.D. 2) Heliocentric theory (associated with Aristarchos) To choose between them make predictions 1) Heliocentric stars should appear to shift positions on the sky as the earth moves around the sun a. Parallax- “a change in the apparent direction of a distant object due to the observer’s change in vantage point” (hold finger our and blink each eye, distance changes) Isaac Newton – realized that the brightness is inversely proportional to 1/ (distance)^2. In 1830, first telescope was ready to make parallax detections 1 st detections of stellar parallax In Northern Hemisphere, it was Friedrich Bessel 1838, found faint star 61 Gyni In Southern Hemisphere, it was Thomas Henderson working at Cape of Good Hope (South Africa) measured the parallax of bright star Alpha Centauri Stellar Distances Parallax of Alpha Centauri – ¾ “ (arc second), or (1/4800) o ; This is about the size of golf ball
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