is positive in the northern hemisphere and negative in the southern hemisphere. The east longitude
is positive and the west longitude
Examples of time zones: Paris (France):
, New York:
Latitudes are limited below the Arctic circle and above the Antarctic circle:
Snapshot 1: The date is February 10, 2011.
Location 1 is Lafrançaise (France), latitude
, noon is at 13:9:16 local time,
Location 2 is Meerut (India), latitude
, noon is at 12:33:28 local time,
. The distance
between the parallel of Lafrançaise and the parallel of Meerut is
km, which gives 40030 km for the Earth's circumference. Reported measurements are
You can find details on Lafrançaise (France) at  and on Meerut (India) at .
Snapshot 2: This case reproduces Eratosthenes's original measurement of the Earth's circumference in Alexandria and Aswan on summer solstice. The date is June 21, 2012 (summer solstice).
Location 1 is Alexandria (Egypt), latitude
Location 2 is Aswan (Egypt), latitude
Actually, the latitude at Aswan is
, but we use the latitude of the Tropic of Cancer (
) to agree with Eratosthenes's knowledge that at Aswan, on the summer solstice (June 21) at noon, the sun shines straight down a well.
between the parallel of Alexandria and the parallel of Aswan is
km, which gives 40030 km for the Earth's circumference. Eratosthenes assumed that Alexandria and Aswan belong to the same meridian; the small difference in longitude explains the slightly different local noon times.
Eratosthenes found stadia
, which leads to the Earth's circumference being 39250 km, assuming a stadium
value of 157 m .
Snapshot 3: The date is August 13, 2012.
Location 1 is Paris (France), latitude
, noon at 13:55:31 local time,
Location 2 is Rio de Janeiro (Brazil), latitude
, noon at 11:57:343 local time,
. The distance
between the parallel of Paris and the parallel of Rio de Janeiro is
km, which gives 40030 km for the Earth's circumference.
Snapshot 4: If Eratosthenes had lived in Brazil, he could have measured the circumference of the Earth in São Paulo (almost on the tropic of Capricorn) and Porto Alegre on the summer solstice (December 21 in the southern hemisphere) and obtained the same result.