On Saturday, January 2nd, 2010, the Earth was only 147,098,000 kilometers away from the Sun. But don’t worry. Earth is not spiraling down to the Sun for an eventual collision. This is an annually recurring event known as perihelion.
Earth perihelion occurs every year in early January, and marks the minimum distance between Sun and Earth for the year. There are two related concepts worthy of discussion around this time.
1) Earth does not orbit the Sun in a perfectly circular orbit. In fact, no planet does. Planets (and moons, comets, asteroids, artificial satellites and anything else) orbit in ellipses. This was theorized by Johannes Kepler 400 years ago, and is now known as Kepler’s first law of orbital dynamics. An elliptical orbit has two focal points. The Sun is one of the focal points for planetary orbits; the other is a mathematically calculated point in space, but there is no physical object there.
The measurement of this non-circularity is called eccentricity, and varies between 0 (a perfect circle) and 1 (an infinitely elongated ellipse–essentially a flat line). Earth’s eccentricity is 0.0167–a very small number. Venus and Neptune has the most circular orbits in the solar system (e=0.0068 and 0.0095 respectively). Pluto’s eccentricity is 0.249–very high for a planet.
Pluto’s high eccentricity was a significant factor in the decades-long debate as to whether or not it should be considered a planet. Ironically, when the IAU (International Astronomical Union) decided on the definition of a major planet, back in 2006, eccentricity wasn’t part of the final definition, which reclassified Pluto as a ‘dwarf planet.’
2) Seasonal changes are not due to the changing distance between Sun and Earth. Seasons are the result of the tilt of the Earth’s axis relative to its orbital plane. Even though we are at perihelion in early January, there is a minimal effect on temperatures and weather systems.
Earth will be at aphelion, or its furthest distance from the Sun, 0n July 6th, 2010. The distance between the two at that time will be 152,096,500 km.