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Orbits and Light

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The earth-sun orbital plane – the ecliptic.

Orbital Plane – the Ecliptic

The earth orbits the sun. The plane of that orbit is called the ecliptic. This plane is not coplanar with the earth's equator or celestial equator. That is, the earth rotates in a direction tilted to the plane of its orbit around the sun. That tilt from the orbital plane is called obliquity. All of the planets in our solar system have a non-zero obliquity. Earth's obliquity is 23.5°.

Sun at noon

Sun in the sky

Because the earth is tilted with respect to the earth-sun orbital plane, sometimes the earth is tilted towards the sun, sometimes it is tilted away from the sun, sometimes the earth is neither tilted towards or away from the sun. At “noon” the maximum height the sun can reach in the sky (and hence how many hours of daylight) changes from day to day. The animation to the right shows this variation for someone standing at a mid-northern latitude.

Light Rays

Certain youth find that a magnifying glass can do fun things with sunlight. One can take a beam of light (with area equal to the size of the magnifying glass lens and shrink it to a small spot. All the energy of the initial beam is focused into a smaller area which makes the light in that small area more “intense”. This same effect plays a role in seasons. But instead of being focused, light rays from the sun are “spread out”.

The angle the light rays from the sun make with the surface of the earth is called the angle of incidence. When there is a large angle of incidence, the light rays are spread out over a large area. The strength of the incoming beam is weakened as far as a small portion of the surface is concerned. When the light bears straight down on a surface, it isn't spread out at all and the surface feels the full intensity of the beam. Such rays are called the most direct rays.

The simulator below shows how a beam is spread out when the angle of the light coming down to the surface is changed.

Light Rays and Angle of Incidence