SIZES AND DISTANCES
IN THE SUN-EARTH-MOON SYSTEM:
An Introductory Astronomy Lab
Caveat Corner: Shadows of Obelisks
You may have noticed that the angle of the final point of an obelisk is in
fact shallower than the angle that the Sun's rays are hitting the Earth in
Alexandria at noon on the summer solstice. This actually means that the
shadow you see on the ground would not even show a point and would actually
be a shadow of the side of the obelisk. If you think about this a bit
you realize that this complicates the measurement considerably. It seems
unlikely, in light of this, that Eratosthenes would have actually used an
obelisk for his measurement. It is much more likely that he constructed his
own plumb line (a string with a bob attached to the bottom which is used for
determining the vertical) and measured the shadow cast by this. Perhaps the
story about the obelisk is a bit of a myth, but it certainly adds to the
drama of this measurement.