Even though Earth is more than 3 million miles (5 million kilometers) closer to the sun today (Jan. 5) than it will be in July, you can hardly tell. I took a walk today and can confirm that it’s still the dead of winter — snow, gray skies and leafless trees as far as the eye can see.
If the Earth orbited the sun in a perfect circle its distance would never vary during the year. But like every other planet — and for that matter every comet and asteroid — our blue world orbits in an ellipse. An ellipse resembles an oval with the sun off-center to one side. In early January, the Earth occupies the closer side of its orbit, and in July the more distant side.
On January 5 at precisely 1:48 a.m. CST, Earth reached its closest point to the sun called perihelion. The word comes from the Greek “peri” for close and “helios” for sun. At that moment we stood 91,398,199 miles (147,091,144 km) from the great fire responsible for all good things.
Six months from now on July 4, the planet will reach aphelion, its farthest distance from the sun, at 94,507,635 miles (152,095,295 km).
Eccentricity, denoted by the letter ‘e’, refers to how stretched out a planet’s orbit is compared to a perfect circle, defined as e=0. Elliptical orbits range from a fraction greater than “0” to a fraction less than “1”. The closer to zero, the more circular; the closer to “1”, the narrower the ellipse and the further the sun lies to one side the ellipse. Earth’s current eccentricity is 0.0167, very nearly a circle. But not quite.
The difference between perihelion and aphelion amounts to about 3.3 percent of Earth’s average distance from the sun of 93 million miles. That’s a substantial fraction but not enough to offset the much more dramatic temperature changes brought on by the seasons, which are caused by the tilt of Earth’s axis. Because the northern hemisphere has been cooling down since fall, any potential extra warmth from perihelion is lost on us.
Earth’s changing distance also affects the planet’s orbital speed. When closest to the sun, Earth moves faster; when farther away it slows down. The planet’s average speed through space is 66,600 mph (106,000 km), but today we’re zipping along 2,160 mph faster than we will on July 4. This additional velocity hurls us toward summer faster than the return “trip” to winter. The result? Winter lasts only 89 days compared to summer’s 93! If you live in the southern hemisphere, where the seasons are opposite those in the north, summers are shorter than winters.
Given that the southern hemisphere is closer to the sun in summer and farther from it in winter, does that mean it experiences more extreme seasons? It might were it not for all the water “down under” compared to the northern hemisphere. Large bodies of water moderate temperatures and even out seasonal differences between the hemispheres.
Changing distance also means changing size. Celestial objects’ sizes are measured in degrees and fractions of degrees called arc minutes and arc seconds. Both the sun and moon have the same apparent size even though they differ vastly. Each spans approximately 0.5° across, equal to 30 arc minutes. The difference between the sun’s diameter at perihelion vs. aphelion comes to just 1 arc minute or 1/30th of its diameter. Pretty tough to see with your eyes — and you should never look at the sun anyway — but easy to record with a camera. Take a picture sometime in the next few days and do the same in July the place them side by side, and you’ll see for yourself.
Mercury has the greatest eccentricity of all the planets with e=.2056 or 12 times greater than Earth’s orbit. The difference between perihelion (29 million miles) and aphelion (43 million miles) causes the sun’s apparent size to vary dramatically from 1.6° wide (three full moons) at perihelion down to just 1.1° degrees (two full moons) at aphelion. And get this. It’s twice as bright when closest!