01 January 2013

Closest Approach

This week the Earth comes closest to the Sun on its annual journey along the elliptical path. The distance between the two bodies drops to a mere 91.4 million miles or about 147 million kilometers. Around Independence Day the Earth will be at its furthest point: 94.5 million miles or about 152 million kilometers. Stick two pins in a piece of cardboard and tie a loop of string between them. Using a pencil, trace out a shape by stretching the string all the way out. What you get is an ellipse:

The two pins represent points called the foci (plural of focus). If the two foci were one point, your ellipse would be a circle. Imagine the Sun is one focus. The Earth's orbit is the ellipse. When the Earth is furthest from the Sun, it is called aphelion. When it is closest, it is called perihelion. "Helios" is Greek for "sun" and "ap-" and "peri-" mean "far" and "close." You can see that the ellipse illustrated is highly exaggerated. The Earth-Sun distance only varies by about three million miles, so if we could view the orbit from space it would look very much like a circle. Most people have a hard time with the notion that we residents of the Northern Hemisphere are closer to the Sun in winter and further in summer. It is below freezing where I live today, for example, and will likely be in the high 80s or low 90s (Fahrenheit) on the Fourth of July. We should conclude, then, that the distance from the Earth to the Sun is not important when talking about the seasons. Three million miles may seem like a lot, but not when you are 90+ million miles away! Every schoolkid ought to know that is is the axial tilt of the Earth that gives us seasons. If you have ever spent time in the Tropics, you know that the day length varies little throughout the year. Whereas we folks who live in the Temperate Zones experience long summer days (and short nights) and short winter days (and long nights). The Earth's axis tilts 23.5º from the plane of its orbit, which is why the Tropic of Cancer is 23.5º north of the Equator and the Tropic of Capricorn 23.5º south. This axial tilt means that solar radiation is spread out over a larger area during the Northern winter and thus we get colder weather:

Try this with a globe and flashlight. You'll see the beam will cover more ground when the Northern Hemisphere is tilted away (winter) and be more concentrated when tilted toward the light source (summer). This is a simple concept, and it gives us an easy scheme for describing seasonal change. After you get a handle on it, ask your friends what they know. You might be surprised how many of them have no grasp of grade school science. Be a good citizen and enlighten them.


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