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

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