Transcript Round Earth / Flat Earth - Tuslaw Local School District

Planetary Science
Round Earth / Flat Earth
Planetary Science Vocabulary
• horizon - where sky and earth appear
to meet
• line of sight - the straight unimpeded
path taken by light from an object to an
eye
• terrestrial - pertaining to the earth
• extraterrestrial - outside the earth
Historical Observations of a
Round Earth
• Ships apparently rising up out of the sea or sinking
into it
• Lunar eclipses - the leading and trailing edge of the
earth’s shadow were curved causing us to assume
the object casting the shadow was round
• As an observer increased altitude he could see
farther over the earth’s curvature
Eratosthenes - Greek mathematician, 3rd century BC
• Read of the city of Syene on the Nile River, 1 day
each year on summer solstice the sun shone directly
down an abandoned well illuminating the bottom
• He assumed that the sun was very far away and
therefore all of its light struck the earth in parallel
beams
• He reasoned that if the earth were flat, vertical poles
on the earth’s surface would cast no shadow at noon
on that day
• But he found that poles outside the city did cast
shadows, and the farther north they were placed, the
longer the shadows
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Page 4 Cont.
He reasoned therefore that the earth must be round
He measured that the sun shone at an angle of 7.2º in
the city of Alexandria
7.2º is about 1/50 of a 360º circle therefore the
circumference of the earth = 50 times the distance
between Alexandria and Syene
Our best guess is that he was within 6000 km of todays
measured circumference
Latitude
• In the northern hemisphere, the latitude of a
place is equal to the altitude of the north star
(Polaris)
• Christopher Columbus followed the 25th
parallel to try to find a shortcut to Persia but
ran into the Carribean Islands
• He had no idea how far he was around the
planet
Longitude
• There’s no “east star”
• You need an accurate clock that would run for
weeks at a time
• While at sea you observe the exact time the
sun reaches its zenith ( highest point = local
noon)
• Compare this to the exact time at home
• 360º / 24 hrs = 15º / hour
• 4 minutes = 1º
• Chronometers guided sailors for nearly 200
years
• Today sailors and pilots use GPS