The sun unleashed a record solar flare in November 2003.

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The earth’s orbit around the sun is not a circle. The earth’s orbit around the sun is slightly elliptical. Therefore, the distance between the earth and the sun varies throughout the year.

At its nearest point on the ellipse that is the earth’s orbit around the sun, the earth is 91,445,000 miles (147,166,462 km) from the sun. This point in the earth’s orbit is known as perihelion and it occurs on January 3.

The earth is farthest away from the sun on July 4 when it is 94,555,000 miles (152,171,522 km) from the sun. This point in the earth’s orbit is called aphelion.

The slight ellipse in the earth’s orbit does have a slight impact on the amount of solar energy being received by the earth. This 3.3% difference in distance does not impact the earth as much as the seasonal variations, however.

Scientists utilize the average distance from the earth to the sun as the standard for one astronomical unit (1 AU). This average distance from the earth to the sun is 92,955,807 miles (149,597,870.691 km). It takes light from the sun about 8.317 minutes to reach the earth.

The earth takes 365 days, 5 hours, 48 minutes, and 46 seconds (365.242199 days) to make a full revolution around the sun.

The earth’s wobble and change to its axis and orbit around the sun all change on a cyclical basis. These account for changes to the earth-sun interaction and are known as Milankovich cycles.

Eccentricity

Eccentricity is the change in the shape of the earth’s orbit around the sun. Currently, our planet’s orbit is almost a perfect circle. There is only about a 3% difference in distance between the time when we’re closest to the sun (perihelion) and the time when we’re farthest from the sun (aphelion). Perihelion occurs on January 3 and at that point, the earth is 91.4 million miles away from the sun. At aphelion, July 4, the earth is 94.5 million miles from the sun.

Over a 95,000 year cycle, the earth’s orbit around the sun changes from a thin ellipse (oval) to a circle and back again. When the orbit around the sun is most elliptical, there is larger difference in the distance between the earth and sun at perihelion and aphelion. Though the current three million mile difference in distance doesn’t change the amount of solar energy we receive much, a larger difference would modify the amount of solar energy received and would make perihelion a much warmer time of the year than aphelion.

Obliquity

On a 42,000 year cycle, the earth wobbles and the angle of the axis, with respect to the plane of revolution around the sun, varies between 22.1° and 24.5° . Less of an angle than our current 23.45° means less seasonal differences between the Northern and Southern Hemispheres while a greater angle means greater seasonal differences (i.e. a warmer summer and cooler winter).

Precession

12,000 years from now the Northern Hemisphere will experience summer in December and winter in June because the axis of the earth will be pointing at the star Vega instead of it’s current alignment with the North Star or Polaris. This seasonal reversal won’t happen suddenly but the seasons will gradually shift over thousands of years.

Milankovitch Cycles

Astronomer Milutin Milankovitch developed the mathematical formulas upon which these orbital variations are based. He hypothesized that when some parts of the cyclic variations are combined and occur at the same time, they are responsible for major changes to the earth’s climate (even ice ages). Milankovitch estimated climatic fluctuations over the last 450,000 years and described cold and warm periods. Though he did his work in the first half of the 20th century, Milankovich’s results weren’t proven until the 1970s.

A 1976 study, published in the journal Science examined deep-sea sediment cores and found that Milankovich’s theory corresponded to periods of climate change. Indeed, ice ages had occurred when the earth was going through different stages of orbital variation.

For More Information

Read related articles in the Climate section of my site.

Hays, J.D. John Imbrie, and N.J. Shackleton. “Variations in the Earth’s Orbit: Pacemaker of the Ice Ages.” Science. Volume 194, Number 4270 (1976). 1121-1132.

Lutgens, Frederick K. and Edward J. Tarbuck. The Atmosphere: An Introduction to Meteorology.