Feel the Heat?
Wednesday January 3rd at 5:35 UT (11:35 pm CST Tuesday January 2nd), as the Earth continues its annual trek around the Sun, the Earth reaches a point in its orbit that is called perihelion. Perihelion is the minimum distance that separates the Earth from the Sun, and we are the closest to the Sun for the year at this point in the orbit. So this year, 2018, the Earth is 0.98333 AU (147,099,586 km; 91,403,445 miles) (compared with last year 2017: 0.98331 AU (147,101,082 km; 91,404,374 miles) from the Sun. Approximately one-half year or one-half revolution later, on July 6th, the Earth is at aphelion and is 1.0167 AU (94,508,169 miles; 152,096,155 km), its maximum distance from the Sun for 2017. 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“.
Here is a good classroom activity about the Earth’s orbit and its effect on the apparent size of the Sun: Why Does the Size of the Sun Appear to Change? A Year of the Sun.