Friday 4 July, as the Earth continues its annual trek around the Sun, the Earth reaches a point in its orbit that is called aphelion. Aphelion is the greatest distance that separates the Earth from the Sun, and we are the furthest from the Sun for the year at this point in the orbit. So, at 0 UT on Friday 4 July (7 pm CDT Thursday 3 July) the Earth is 1.01682 AU (94,519,324 miles; 152,114,107 km) from the Sun. Approximately one-half year or one-half revolution later, on 4 January, the Earth is at perihelion and is 0.98330 AU (91,403,445 miles; 147,099,586 km), its minimum distance from the Sun for the year. This difference in distances is due to the shape of the Earth’s orbit being elliptical rather than circular. However the Earth has a mildly elliptically shaped orbit that is closer to being slightly out-of-round than the incorrect, very elliptical orbit that is often shown – like the illustration used here.
In Astronomy the shape of a planet’s orbit is called eccentricity, with 0 being a circle and 1 a straight line. Any value between 0 and 1 represents an ellipse. The shape of the Earth’s orbit is so close to being circular that the apparent size of the Sun does not appear to change as this animated graphic shows. The difference between perihelion and aphelion is about 3%.
Eccentricity for each planet is listed below for comparison.
Planet Eccentricity Mercury 0.2056 Venus 0.0068 Earth 0.0167 Mars 0.0934 Jupiter 0.0484 Saturn 0.0542 Uranus 0.0472 Neptune 0.0086 Pluto 0.2488
To read more about the Earth’s orbit and get some teaching ideas click here to download a PDF copy of my January 2011 Scope on the Skies column “Solar Explorations“.
Click here to go to the Qué tal in the Current Skies web site for more observing information.