Columbus concluded that Earth is round while watching a distant ship sink beneath the horizon. You too can observe the curvature of Earth on the San Francisco Bay (and other places); not only that: you can determine Earth’s radius from such observations. I present here a method to use the vertical shift between two bridges over the Bay due to Earth’s curvature as seen in a photograph, where the more distant bridge is apparently lower than the proximate one (as shown in Fig. 1). Starting with each bridge’s known dimensions and location and applying atmospheric refraction corrections and then measuring the vertical shift between the two bridges on the image allows a calculation of Earth’s radius within a surprising 1% of the accepted value.

1.
https://en.wikipedia.org/wiki/Christopher_Columbus#Geographical_considerations. A round Earth was well accepted by the educated of the era.
2.
For example, Earth’s curvature is clearly visible, without calculation, in the eastward view from the top of a hill next to Interstate 80 near West Wendover, Nevada. https://www.roadsideamerica.com/tip/51347 or https://flatearth.ws/bonneville.
3.
The USGS map locator: https://store.usgs.gov/map-locator, https://maps.usgs.gov/map/, enter “San Francisco Bay” in the Search field, or navigate to the coast of California.
4.
The NOAA PDF Chart 18650 scale 1:20,000 locator: https://www.charts.noaa.gov/InteractiveCatalog/nrnc.shtml.
6.
Greek geographer Eratosthenes: https://en.wikipedia.org/wiki/Eratosthenes.
7.
Bedford Level experiment with water in a long ditch. https://en.wikipedia.org/wiki/Bedford_Level_experiment.
9.
Experiment with measuring shadows on a tall building:
J.
Carroll
and
S.
Hughes
, “
Using a video camera to measure the radius of the Earth
,”
Phys. Edu.
48
,
731
735
(
2013
).
10.
C. O.
Oakley
,
Analytic Geometry
(
Barnes & Noble
,
New York
,
1961
), section 34, “Circle determined by three conditions.”
11.
Readers can access the supplemental materials at TPT Online at https://doi.org/10.1119/10.0014307, under the Supplemental tab.

Supplementary Material

AAPT members receive access to The Physics Teacher and the American Journal of Physics as a member benefit. To learn more about this member benefit and becoming an AAPT member, visit the Joining AAPT page.