In a natural irony, the northern hemisphere's hottest season occurs when Earth is farthest from the sun. On July 5 at 5:27 p.m. Central Daylight Time, the planet will stand 94,510,886 miles (152,100,527 kilometers) from our star at its aphelion point. That's a little more than 3 million miles farther away than when we were closest — called perihelion — on Jan. 2 this year. The Earth's average distance from the sun is 93 million miles (149.7 million kilometers).
The planet's pendulum-like swings from near to far occur because we zip around the sun on an elliptical path, with one end of the orbit closer to the sun and the other farther away. Not only does Earth's distance from the sun vary across the year, but so does its speed — faster when closer and slower when farther away.
At the moment, we're "plodding" along some 2,200 miles per hour slower compared to last January. Being farther away also means the sun appears about 3.6% smaller now than during the winter.
You can convert any distance into light-time. Moving at 186,000 miles per second (300,000 kps), a light beam takes light 1.3 seconds to travel the 239,000-mile distance to the moon. That means the moon's just 1.3 light-seconds away. The sun is a little more than 8 light-minutes away.
On July 5th, light from the sun will take 8.5 minutes to reach Earth compared to 8.3 minutes at perihelion in January. That's an extra 18 seconds. On the next sunny day, count out 18 seconds, and you'll get some sense of what 3 million miles (4.8 million km) feels like traveling at the speed of light.
Looking at that first diagram above, you might think Earth's orbit is really stretched out, but the truth is, it's very close to a circle. If you drew it on a sheet of paper it would look like a circle to your eye.
While the 3-million-mile difference between perihelion and aphelion sounds like a lot, that's only about a 3% difference. Not much, right? But it does have consequences. Sunlight falling on the globe is about 7% less intense now than during January, when Earth is closest. And yet the average temperature of the planet right now is about 4 degrees warmer than at perihelion. Wait a second. We're warmer when we're farthest from the sun?
Yes! Continents and oceans aren't distributed evenly around the globe. There's a lot more land mass concentrated in the northern hemisphere. Land absorbs and releases heat much more readily than water. This increases Earth's average temperature in July. The watery southern hemisphere absorbs heat more slowly and spreads it out across the oceans.
Yet the contribution that Earth's varying distance from the sun makes is small potatoes compared to effects caused by the 23.5° tilt of its axis. The yearly change in orientation of the axis toward and away from the sun radically alters the number of daylight hours and the height of the sun above the horizon (high in summer, low in winter). In turn, these cause the huge temperature swings from winter to summer and back again.
While its seasonal effects are minor, Earth’s varying distance from the sun significantly affects the length of the seasons. Because the planet orbits more slowly in summer, that season lasts longest — 94 days. In contrast, winter has just 89 days. The opposite situation is true in the southern hemisphere, where it's currently winter, their longest season.
You might wonder why the winter and summer solstices are closely aligned with perihelion and aphelion, respectively. It's just coincidence. In fact, in 1246, not too many years after the Magna Carta was signed, perihelion and the December solstice coincided. They've been drifting apart since then at the rate of one day every 58 years.
Four thousand years from now, perihelion will occur on the first day of spring. In about 10,000 years it will coincide with the summer solstice. As you can guess, that will further increase heating in the northern hemisphere in the summer months while also making for slightly colder winters.
The reason for this perihelion drift is precession, which I described in this earlier post, combined with apsidal precession. The first is a slow wobble of the Earth's axis that causes it trace a circle in the sky over a period of 26,000 years. While the tilt remains the same, the axis can point away from the sun at perihelion (as it does right now) or toward it, as it will roughly 10,000 years from now.
Apsidal precession is the slow turning of Earth's entire elliptical orbit due to the gravitational influences of the planets, primarily Jupiter and Saturn. These two forms of precession combine so that it takes between 20,800 and 29,000 years (on average 23,000 years) for perihelion to return to the same date.
That's not all. As we expand the time frame, small changes accumulate to make big differences. The shape of the Earth’s orbit slowly shifts from more elliptical to nearly circular in a cycle that takes between 90,000 and 100,000 years. When the orbit is highly elliptical, the perihelion distance is smaller, and amount of heat we receive can increase by 20%-30% compared to aphelion.
A close perihelion also means a distant, colder aphelion. The changing shape of Earth's orbit, called its eccentricity, may have been responsible for waves of glaciation in the past and undoubtedly will again in the future. In the short term, however, it looks like we'll be sweating it out for a while.
Try to stay cool.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.