is not completely round. From his data
Eratosthenes also determined another
accurate measurement: the Earth’s diam-
eter. He deduced the Earth was 7,850
miles (12,631 kilometers) in diameter,
which is close to the modern mean value
of 7,918 miles (12,740 kilometers).How do scientists measurethe
Earth’s rotational speed?
The Earth’s rotational speed is based on
the sidereal period of the Earth’s rotation,
but it differs depending on where the
observer is located. By dividing the distance traveled once around the Earth by the
time it takes to travel that distance, the speed can be determined.
For example, a person on the Earth’s equator will travel once around the Earth’s
circumference—or 24,900 miles (40,079 kilometers)—in one day. To get the speed,
divide the miles by the time it takes to get back to the same place (around 24 hours),
or just over 1,000 miles (1,609 kilometers) per hour. A person at one of the poles is
hardly moving at any speed. This is because there is so little distance traveled in a day
(a stick stuck vertically in the ice exactly at the North or South Pole will only travel
about 0.394 inch [1 centimeter] per day.)
What about other places on Earth? Traveling north or south from the equator
toward the poles decreases one’s tangential rotational speed. Thus, the rotational
speed at any point on the Earth can be calculated by multiplying the speed at the
equator by the cosine of the point’s latitude.
But remember that the rotation of the Earth is not always consistent from year to
year or even season to season. Scientists know there are other factors in the rotational
equation, including the differences caused by widespread climatic conditions. For exam-
ple, during El Niño years (the periodic upwelling of warmer waters around the equator
in the Pacific Ocean off South America), the rotation can slow down. This happened
between 1982 and 1983, when the Earth’s rotation slowed by 1/5,000th of a second.What is the geologic time scale?
The geologic time scale is a way of managing large amounts of time in a convenient
chart. The scale is actually a measurement encompassing the entire history of the
Earth—from its beginnings some 4.55 billion years ago to the present day. The largest
divisions include eons, eras, and periods; the smaller time divisions include epochs,
296 ages, and subages.
Eratosthenes brilliantly used his knowledge of
angles and mathematics to be the first to determine
the Earth’s circumference accurately.