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Longitude, Latitude and Sunshine

© Wesley Colley

Most people are taught the seasons during primary education. Remarkably, however, a recent survey of Harvard graduates revealed that more than half could not properly explain the cause of the seasons. With this in mind, I would like to say a bit about seasons, time and sunshine.

First of all, the Earth rotates West-to-East on its rotational axis once per day; the rotation axis defines the north and south poles. The rotation causes the sun to rise in the east, and causes the sun to rise earlier the farther east you go. We need time zones to keep the sun at maximum elevation at noon everywhere on the earth. Every 15 degrees of east longitude introduces an hour addition in local time, but our lack of agreement on local time causes the need for an international date line: If you started off in New York and travelled east very quickly, you would need to move your watch forward an hour every time you travelled 15 degrees of longitude. By the time you got back to New York, you would find that you had set your watch ahead one full day, which is clearly a contradiction, since it's still today. Therefore, we agree than when you cross 180 degrees longitude (in the Pacific) you set your watch back one day, so that when you get back to New York, the time and date are correct, and the sun is highest at noon.

The Earth orbits the sun once a year, but it does so in a plane that is not aligned with our rotational equator (the Equator). The Equator is inclined 23 degrees to our orbital plane (the Ecliptic). This causes different latitudes to see the sun at different angles (at a given time, say noon) during different months of the year. The intersection of the plane of the Equator and the Ecliptic plane is a line, called the line of nodes, that always points in the same direction. Twice a year, at the Equinoctes (or Equinoxes), this line of nodes points at the sun, so that the sun is shining perfectly perpendicularly onto the earth's equator; that is to say, the point at which noontime sun is directly overhead on the Equator is called an equinox. There is always, in fact, a point on the earth at which the sun is shining directly overhead. At the Vernal Equinox (about March 21), this point is moving from the Southern Hemisphere to the Northern; at the Autumnal Equniox (about September 21), the opposite is happening. The maximum latitude at which the sun can be viewed directly