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ASTR 1P01 Lecture Notes - Angular Diameter

Course Code
Bozidar Mitrovic

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First Lecture after Test 1
October 12
Aristarchos of Samos (310-230 BC) proposed a theory that the earth is rotating on its
axis and revolving around the sun which was based on observations
1) he concluded that the sun must be at much greater distance from the Earth than the
moon. He argued that if these distances were comparable one would observe the
And if the moon is revolving at a constant speed ( a reasonable assumption) the internal
of time from third quarter moon to first quarter moon would be shorter than the interval
from first to third quarter. But these intervals are nearly the same (about 2 weeks)
By measuring the times between the quarter phases he determined angle A to be 87
degrees (the actual value is now 89 degrees and 57 minutes of the arc)
- the sun must be at greater distance than the moon
- he used geometry to find out the ratio of distances, comparison of distance from sun to
earth and moon to earth
- measure time it takes from third to first quarter and from first to third quarter
sun 20 times more distant than the moon, sun has 20 time bigger diameter than the
moon because have the same angular size
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