Warm enough for you?
On January 3rd we reached perihelion in our orbit around the Sun. Perihelion marks the point in Earth’s orbit where our planet is nearest to the Sun. Every year we reach perihelion during the first days of January. Our perihelion distance is about 2 million miles closer to the Sun than our average distance of 93 million miles.
Because Earth’s orbit around the Sun is not circular but elliptical, we are closer or farther away from the Sun at the extreme points of the ellipse. Perihelion’s opposite is known as aphelion and happens each year around the Fourth of July. Because Earth’s elliptical orbit is not as out-of-round, or eccentric, as the orbit of Mars, the effect on us is very subtle.
Our seasons are driven by an axial tilt of about 23.5 degrees from vertical. Scientists have measured an average increase of about seven percent in solar heating at perihelion. Although detectable, it is still not enough to generate panic sales of sunblock or cold drinks.
According to Kepler’s second law of planetary motion, planets move faster at perihelion than they do at aphelion. This difference in speed means that seasons last a little longer at aphelion. On Earth, this translates to a northern summer that is about five days longer than northern winter. Bad news for ski bums, but wonderful for beach nuts.
The climate story on Mars is quite different due to a combination of factors. Presently, Mars’ axial tilt is 25 degrees from vertical. And because Mars lacks a large moon, its tilt is very unstable. Over its history, Mars has experienced extreme fluctuations in climate related to shifting axial tilt.
Another influence on Martian climate is the eccentricity of its orbit. At aphelion, northern summer lasts about 3 weeks longer than northern winter. The added warmth of a longer summer season sublimates enough of the northern polar cap into gas that atmospheric pressure is increased by almost thirty percent.
During the next couple of weeks, Mars will be at peak visibility as it moves toward opposition with Earth. Already a noticeable presence in the evening sky, its ruddy glow dominates its celestial neighborhood in Leo. It rises above the east-northeastern horizon around 7PM.
