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Lecture 2

Course Code
Eric Shaver

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ASTB03 – Lecture 2
Greek Astronomy
Tried to prove that Earth was stationary – was not in motion
Evidence: Objects would be left behind by rotating or revolving Earth
Moving Earth would produce parallaz shift in stars' positions
Astronomical Measurements
Relative measurements: comparing 2 things in nature (for example, the moon and the sun)
Aristarchus (310 BC)
How far the sun and moon are from the Earth in relative form
Based on observations of timing of Moon's phases
Found that sun is 19x further away than the moon was
Sun is actually 392x further away
Found that Sun is 7x diameter of Earth
Sun is actually 109x the diameter of Earth
Eratosthenes (276 BC)
Sun is never directly overhead in Toronto
In a particular place in Egypt (Syene), Sun was actually directly overhead
Position of Sun was also observed from Alexandria
They found the angle between Alexandria & Syene = 70 degrees
Measurement unit they used was “stade”
Distance all around the Earth was 250 000 stade (40 000 km)
Appolonius (220 BC)
Introduced two ways of describing the motion of things
Introduced eccentric circles, deferents & epicycles
When planet is father from the earth it would appear to go slower than when it is closer to the
earth (car analogy), even though it's in constant speed
This guy tried to explain retrograde motion
The planet would go in its own mini circle (epicycle), as it went around the earth in a circle
This theory about epicycles seemed to explain retrograde motion well even though it was wrong
Hipparchus (190 BC)
Looked at the position of stars
Discovered the “precession of the celestial poles”
The point that stars circled around a long time ago is no longer the same
Magnitude Scale for Brightness
Looked at the importance of the brightness of stars
Brightest stars were called Magnitude 1
And the dimmer ones were Magnitude 2
Had no way of describing a star that was Magnitude 1
He was using eyes to judge brightness
These magnitudes are now called apparent magnitude (judging by looking)
The more negative something is, the brighter it is!
Sun: m= - 26.8
Moon: m = - 12.5
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