Earth's orbit is not a perfect circle. It's elliptical. Earth's average distance from the sun is 150 million km. On January 3, Earth is at perihelion and is 3 million km closer to the sun than average. On July 4, Earth is at aphelion and is 2 million km farther away from the sun than average. The result of this is that Earth receives 6% more solar energy in January than in July.

If you live in the Northern Hemisphere the last sentence should not make sense to you. In January it is colder than in July! So what gives? Well, Earth's axis is tilted 23.5° relative to the plane that contains the sun and Earth (see figure 1). The tilt doesn't really make one pole that much closer to the sun than the other pole, as the figure suggests. You could calculate this distance knowing the radius of Earth and the fact it is tilted 23.5°--try it. The real significance of the tilt is the distance through the atmosphere that the sun's rays must travel (see figure 2).

A ray entering the atmosphere directly overhead, or parallel to a vertical line from Earth's surface, passes through a distance of 1 atmosphere (roughly 100 km). In comparison, a ray entering at an angle of 30° to that vertical line would traverse an atmospheric thickness about 15% thicker on its way to the ground. A ray which makes an angle of 80° with the vertical (this would be up near the North Pole in figure 2) would traverse an atmospheric thickness nearly six times that of the parallel ray. The farther through the atmosphere a ray must travel, the more likely it is to be absorbed, reflected, or scattered by molecules in the atmosphere, which would reduce the intensity of energy received at the surface.

The Seasons Referring to figure 1, we see that on June 21-22 the Northern Hemisphere is tilted towards the sun, and the sun's vertical rays are directly above 23.5° N. latitude, the Tropic of Cancer. The sun is high in the sky in the Northern hemisphere and hence more solar energy reaches the ground than in the Southern Hemisphere. Just the opposite occurs on December 21-22 when the Northern Hemisphere is tilted away from the sun. The sun's vertical rays are directly above 23.5° S. latitude, the Tropic of Capricorn. Now it is summer in the Southern Hemisphere.

Where do the sun's vertical rays strike during the two equinoxes? Note that Earth's axis is always tilted in the same direction. It is because Earth is on opposite sides of its orbit that the Northern Hemisphere is towards or away from the sun. If Earth's axis were not tilted, there really wouldn't be any seasons. The 6% variation in solar energy due to the orbit would not produce much seasonal variation.

Ice Ages Earth's temperature really does not have to decrease much for an ice age: it doesn't have to be really cold. There is one condition that is necessary. There must be substantial land masses in the high latitudes where ice sheets and glaciers can form. This is the case now with North America, Europe, and Northern Asia. If the land masses are available, the easiest way to have an ice age is to decrease the contrast between winter and summer. Warmer winters means that it snows more, because warm air can hold more moisture. Cooler summers means less snow melts, and hence the glaciers can grow.

Milankovitch Cycle Earth's orbit is not constant. Over a period of 90,000 to 100,000 years, Earth's orbit stretches into a longer ellipse and then returns to a more circular shape. At maximum eccentricity, Earth at perihelion would receive 20 to 30% more solar radiation than at aphelion.

The tilt of Earth's axis is also not constant. It varies between 22.1° and 24.5° during a cycle, which takes 41,000 years. The smaller the tilt, the smaller the difference between winter and summer. Earth's axis also wobbles or precesses, much like a top as it runs down. This means that the North Pole is not always tilted to the right in figure 1, but will eventually be tilted to the left. The cycle from tilted right to left and back to right takes 26,000 years. When tilted in the opposite direction, the North Star will not be Polaris, and the seasons will flip.

All three of these effects cause little change in the annual amount of solar energy reaching Earth. Their effect is to change the contrast between the seasons. Milutin Milankovitch, a Yugoslavian astronomer, plotted out these three changes to Earth's orbital geometry. He found that these changes were closely associated with changes in climate. In other words, over periods of tens of thousands of years Earth's climate is controlled by these variations in orbital geometry. Based on the Milankovitch cycle, the trend for the next 20,000 years is for a cooler climate with extensive glaciation in the Northern Hemisphere.

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