Perihelion Paradox? Closer Sun, Colder Days

Earth’s oval or elliptical orbit causes our distance from the sun and orbital speed to vary during a year. This week we’re both closest and moving fastest. Illustration: Bob King
Earth’s elliptical orbit causes our distance from the sun and orbital speed to vary during a year. This weeked we’re both closest and moving fastest. Illustration: Bob King

Yesterday, while preparing dinner, Earth reached its closest point to the sun for the year. This annual milestone, called perihelion, from the Greek ‘peri’ (close) and ‘helios’ (sun), happens paradoxically every January. Shouldn’t it be warmer if we’re closer to the sun?

Earth’s distance from the sun varies over the course of a year because we orbit around it in an ellipse, similar to an oval with the sun off to one side. In early January, we’re on the side of our orbit closest to the sun; on July 4th we’ll be on the opposite end and farthest from our star at a point called aphelion (uh-FEE-lee-on). If Earth’s orbit were a perfect circle with the sun at its center, the distance would be the same 365 days a year.

Eccentricity or ‘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 likes to one side. Earth’s current eccentricity is 0.0167 — very close to but not quite a circle.

The apparent motion of the sun around the sky is really a reflection of Earth’s yearly cycle around the sun. The seesaw-like up-and-down movement of the sun from season to season is caused by Earth’s titled axis and our planet’s changing orientation to the sun during the year. Credit: Thomas G. Andrews / NOAA
The apparent motion of the sun around the sky is really a reflection of Earth’s yearly cycle around the sun. The seesaw-like up-and-down movement of the sun from season to season is caused by Earth’s titled axis and our planet’s changing orientation to the sun during the year. It and not Earth’s varying distance from the sun causes the seasons. Credit: Thomas G. Andrews / NOAA

As it is, the difference between perihelion and aphelion amounts to about 3 million miles or 3.3% of Earth’s average distance of 93 million miles from the sun. Not insubstantial. While the change in distance does affect the amount of heat we receive from the sun, it’s not nearly enough to offset the much more dramatic temperature changes brought on by the seasons, which are caused by the 23.5° tilt of our planet’s axis.

The northern hemisphere tilts toward the sun in summer, lifting it high in the sky from our perspective. Long days and more intense heating from the nearly-overhead sun make it feel like a sauna out there.  Six months later, when the hemisphere is oriented away from the sun, short days make us shiver.

The orbits of Mercury (red), Earth (blue) and Mars (black). The solid lines indicate each planet's elliptical path around the Sun. The dotted lines show circular paths with the same mean separation from the center. Earth is almost exactly the same distance from the Sun at aphelion and perihelion, but the orbits of Mars and Mercury depart significantly from a circle. For more information, please visit Bridgewater College's
The orbits of Mercury (red), Earth (blue) and Mars (black). The solid lines show each planet’s elliptical path around the Sun. The dotted lines show circular paths centered on the sun. The orbits of Mars and Mercury depart significantly from a circle. Credit: Bridgewater College

Because our distance from the sun varies, so does the sun’s size and the planet’s orbital speed. When closest to the sun, Earth moves faster than when farther away, the same way sun-hugging Mercury orbits faster than distant Jupiter. Our average speed is 18.5 miles per second (66,600 mph) through space, but today we’re zipping along 2,160 mph faster than we will on July 4th. Speed also affects the length of the seasons.

At perihelion (northern winter), Earth travels a little faster than when farthest in July, making winter 89 days long vs. 93 for summer. Exactly the opposite situation occurs in the southern hemisphere, where the (short) summer season has been underway for the past two weeks. Naturally, no one in Namibia or Australia thinks there’s anything odd about the sun being closest to Earth and the season.

Which brings up the question — does the southern hemisphere experience more extreme seasons? While it gets more solar energy in summer and less in winter compared to the northern hemisphere, there’s much more water there compared to the northern hemisphere. Large bodies of water moderate temperatures and even out any differences between the hemispheres.

Difference in the size of the sun when Earth was at aphelion (top) last July and this week at perihelion. It’s very obvious in side-by-side photos but extremely difficult to discern with the naked eye. The difference amounts to just 1.1 minute of arc or 1/30 the diameter of the full moon. Credit: Giorgio Rizzarelli
Difference in the size of the sun at aphelion (top) in July and perihelion in January. The difference amounts to just 1.1 minute of arc or 1/30 the diameter of the full moon. Credit: Giorgio Rizzarelli

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 to appear twice as bright in the planet’s sky when closest. Its sizes varies dramatically, too — 1.6° at perihelion, shrinking to 1.1° degrees at aphelion. For reference, the sun from Earth appears 0.5° across, equal to 30 arc minutes (30′) and varies by just 1′ minute (1/30th) from perihelion to aphelion.

Wishing you a sunny, perihelic kind of day!