Earth at 2017 Aphelion

Earth at Perihelion   Monday July 3rd, 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 20 UT (3 pm CDT) on Monday July 3rd the Earth is 1.01668 AU (94,506,310 miles; 152,093,163 km) from the Sun.
   Approximately one-half year or one-half revolution earlier, on January 4th, the Earth was at perihelion, its minimum distance from the Sun for this year (0.98331 AU (91,404,375 miles; 147,101,082 km). This difference, about 3%, 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.
sun2014-ani   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 monthly observing information, or here to go to bobs-spaces.

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