Climate and the Seasons Study Guide
The earth is a giant spherical ball whose shape is called an oblate spheroid, in space, lit by the sun. This fundamental, grand fact sets the environmental context for much of our lives, including the length of the day, the length and diversity of the seasons, and climate.
Latitude and Longitude
Human beings have wrapped the earth in a coordinate system that is partly based on the sphere of the planet and is partly arbitrary. Why does latitude go from 0° at the equator to 90° at the poles? That is obviously rooted in the geometry of the circle, because ever since the Babylonians, the circle has been divided into 360 degrees. To go around the earth from the equator, up past the north pole (from 0° to 90° N), then back down to the equator on the other side of the planet (from 90° N to 0°), then south to the south pole (from 0° to 90° S), and then back up to your starting point on the equator (from 90° S to 0°), you will go through 360 total degrees of latitude.
It's the same with longitude: 360° total. The 0° line of longitude passes through Greenwich, England. (Guess what country established the system of longitude?) Rather than north and south, like latitude, longitude is measured east and west, with the numbers going to 180° at the line on the opposite side of the earth from the 0° line through England. Thus, longitude is measured from 0° to 180° W and from 0° to 180° E.
Subdivisions of the degrees of latitude and longitude are the minutes and the seconds. (These are not minutes and seconds of time, but obviously, the names in our geographical system of degrees are related to the names for time.) There are 60 minutes of latitude and longitude in each degree. The symbol for minute is a single straight quote, or single prime ('). There are 60 seconds of latitude or longitude in each minute. The symbol for second is the double straight quote, or primes (").
Here's another difference between latitude and longitude. Lines of latitude are all parallel to each other. Lines of longitude all intersect through the north and south poles. Please study Figure 3.1.
The fact that the earth is a giant ball set in the bath of sunlight, which comes in as nearly parallel rays of light, has tremendous effects on the amount of solar energy received at different latitudes. A unit parcel of ground, say a square meter, becomes tipped more and more, relative to the direction of the rays of the sun, as one moves into higher and higher latitudes. You can think of it as fewer and fewer rays of sun striking the parcel of ground as the latitude becomes higher. See Figure 3.2 for an illustration of how this works.
Assume that the time of year is when the sun is overhead at the equator and that one knows the amount of solar energy falling at the equator (Seq) per square meter (also assume no clouds). Then, with the following equation, one can calculate the amount of solar energy falling at any latitude (Slat) on a square meter of ground, by knowing the latitude (II) and computing its cosine of that latitude cos(II) (in a right triangle, the cosine of either of the other two angles is their respective adjacent sides divided by the hypotenuse):
|Slat = COS Φ Seq|
If the latitude is 60° N, then cos (60) = 0.5. That means that the amount of sunlight falling on a square meter of ground at 60° N is, on average over the year, only half of the amount of solar energy that falls at the equator on a square meter of ground.
Finally, here's the equation for the circumference C of Earth at the equator in terms of the diameter and the constant π (which is approximately 3.14):
|C = πD|
Because the diameter D is twice the radius R of a circle, we also have the equation for Earth's circumference in terms of R:
|C = 2πR|
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