Happy New Anomalistic Year!

Time to pop the champagne again if you like to celebrate extremes. Today the Earth is at the point in its orbit closest to the Sun. Astronomers call it "perihelion," a description derived from the generic term for the point of greatest or least distance of a body from one of the foci in its elliptical orbit: apsis.

Johannes Kepler was the first to realize that the planets orbited the Sun in elliptical, rather than circular, orbits. This was radical at the time, because the heavens were considered divine and flawless; for the planets to orbit the Sun in non-circular orbits was tantamount to heresy. Yet, the detailed observations of Tycho Brahe that Kepler based his work on showed this to be the case.

Instead of a single center, like a circle, an ellipse has two centers, known as foci (plural for focus). When two celestial bodies are orbiting each other, they both move in elliptical orbits about a common center of mass where the gravitation force between them is balanced. This occurs at one of the foci. You can think of it like a seesaw where the center of mass is the fulcrum point where the seesaw balances. If two people of equal mass sat on a seesaw together, the balance point would be halfway between them. However, if one person greatly outweighed the other, the balance point would have to be moved much closer to the heavier individual. The center of mass between two celestial bodies is just the same. The mass of the Sun is about a million times more than the mass of the Earth, therefore the center of mass between the Sun and the Earth is greatly skewed towards the Sun.

Earth's elliptical orbit is characterized by an eccentricity of approximately 0.018. Eccentricity is defined as deviation from a perfect circle, where 0 is a perfectly circular orbit and 1 is a parabola, essentially a broken circle that is no longer a closed loop. Earth's orbital eccentricity actually varies from more circular (0.005) to somewhat elliptical (0.058) due to the gravitational interactions in the Solar System, but that's a post for another day.

(Figure so not drawn to scale)

With Earth's current eccentricity, it's orbit can be visualized as an ellipse with a major axis passing through both foci and connecting the two points of greatest and least distance from the Sun (the apses) This line is also formally called the "line of apsides" and the two apses as periapsis and apoapsis from the Greek "peri" meaning around and "apo" meaning from. To be specific to the Sun-Earth system, we can substitute "helios" for the more general apsis and therefore we get the slightly more familiar terms, at least to astronomers, of perihelion and aphelion which represent the two extremes of Earth's orbit. The distance between the Earth and Sun ranges from 98% to 102% of the average distance of approximately 93 million miles, at perihelion and aphelion, respectively - a difference of roughly 4 million miles. Not enough to make you take notice, unless you design spacecraft, study solar physics, and/or like to geek out on this stuff, like me.

Earlier today, at approximately 1900 GMT (~7pm in London), we swung through that point of closest approach, marking one anomalistic year since we last passed through perihelion at midnight on January 3, 2010. Daylight may be returning, but the Sun is now receding. So continue your new year(s) celebration and wave to the Sun as it sets today, as it will rise a little farther way tomorrow.