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Cos Law - © Dr. A. Marsh, Square One, www.ecotect.com
The Earth's orbit around the Sun is not circular but elliptical, meaning that it is closest to the Sun in late December - early January and farthest away in late June - early July. However, this has only a slight effect on the intensity of solar radiation. Of more importance is the axial tilt of the Earth at 23.45°. This means that the path of the Sun through the sky changes significantly throughout the year.
This has an effect because the lower the Sun is in the sky, the more of the Earth's atmosphere the solar radiation has to pass through in order to reach the surface, thus the more scattering and absorption it is subjected to. When the Sun has a lower elevation angle, the solar energy is less intense because it is spread out over a larger area reducing its intensity and its heating capacity as per the Cos Law.
Simply translated do not expect any solar ONLY cooking device to obtain summer maximum cooking temperature during winter, spring and fall seasonal phases. However, seasonal variation in solar heating capacity is not a problem for our versatile solar electric cooking ovens with built-in thermostatically controlled energy efficient low wattage electrical back–up. Now that you know about Cos's Law would you ever want to solar cook without back up?