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Two children on a beach in the Gaza Strip during a heat wave.

Two children bathe on a beach in the Gaza Strip during a heat wave.

Photograph by Ali Ali, European Pressphoto Agency

Ker Than

for National Geographic

Published July 5, 2013

With the United States in the clutches of a heat wave that is spurring deadly wildfires across the Southwest, it might feel like the sun is punishingly close right now. Actually the exact opposite is true: Earth will be farther from the sun on Friday, July 5, than on any other day this year.

That's because the orbits of all the planets in our solar system—including Earth's—are elliptical, as opposed to circular. The phenomenon was first explained in mathematical detail by the 17th-century German astronomer Johannes Kepler.

Earth's elliptical orbit means there will be a point each year when the planet is closest to the sun, called perihelion, and a point when it is farthest away, known as aphelion.

(Related: "Sun Headed Into Hibernation, Solar Studies Predict.")

On Friday at 10:46 a.m. ET our planet will be at aphelion—94,508,959 miles (152,097,426 kilometers) from the sun. This year's perihelion was on January 1 at 11:39 p.m. ET, when Earth was 91,402,559 miles (147,098,161 kilometers) from the sun.

The timing and distances change slightly from one year to the next due in part to small variations in the Earth's orbital elements, said Mark Hammergren, an astronomer at the Adler Planetarium in Chicago, Illinois.

"The difference in dates is due more to the fact that the time it takes the Earth to complete one full orbit of the Sun"—one year—"is not an integer number of days, so the exact times of perihelion and aphelion vary from one year to the next," he added.

On average, Earth is about three million miles—or about 3 percent—farther from the sun at aphelion than at perihelion. As a result, the sun in the sky will look about 3 percent smaller, but you may not notice the difference without a telescope.

"This is not like the moon, where it can vary as much as 12 percent ... There's no 'supersun,'" said Joe Rao, a meteorologist and a guest lecturer at New York's Hayden Planetarium.

(Related: "Solstice Supermoon Fills Sky This Weekend.")

Distance Won't Dampen Fourth of July Heat

So if Earth is at its maximum distance from the sun, why are those of us in the Northern Hemisphere sweltering in the heat of summer?

Because it's the tilt of the Earth, not our distance from the sun, that determines the seasons.

Earth's north-south axis is tilted by about 23.4 degrees, so during its orbit the Poles point in different directions from the sun.

By coincidence, Earth reaches aphelion when the North Pole is tilted more toward the sun than the South Pole is.

"Because the Earth has a tilt, it means that in the summer months [the Northern Hemisphere] receives a longer duration of sunshine—so the day is longer and the night is shorter—but also the sunlight hits the ground more vertically," Hammergren said.

(Related: "Summer Solstice 2013: Why It's the First Day of Summer.")

In fact, the Northern Hemisphere receives nearly twice as much sunlight during this time of year than in the winter. Because the sun takes a high angular path across the sky in the summer, it is in the sky for about 15 hours during this time of the year.

"In the wintertime the sun takes a low path ... and is in the sky for just nine hours," Rao said. "You lose six hours of daylight."

Because of the way the Earth absorbs the sun's heat, the worst of summer still lies ahead. "The hottest time of the year doesn't coincide with the solstice, and it doesn't coincide with early July," Rao said.

"There's actually an atmospheric lag—it takes the better part of four to five weeks before we reach the hottest point of the year, which will come in late July or August."

Our planet's oceans take even longer to absorb heat than the atmosphere does, which explains why the hurricane season doesn't peak until September.

"The hurricane season reaches its peak on the 10th of September," Rao said. "That's almost three months past the summer solstice. That's because hurricanes feed on warm ocean water and the waters reach their temperature peak in the early to middle part of September."

5 comments
Alec Sevins
Alec Sevins

The tired canard that the Sun is "getting hotter" (distance notwithstanding) is also false, as it has been cooling slightly since at least 1978.

jim adams
jim adams

As i understand it, (from William Rudiman, a climate scientist's wonderful little book, Plows, Plagues and Petroleum), the three orbital characteristics of our planet are: the axis tilts back and forth from about 22 degrees to 24 degrees; the axis wobbles like a spinning top, and Earth orbit varies from almost a circle to an elongate ellipse. These three cause climate variations from glacial to almost tropical and sea levels vary as much as 120 feet. Also, during the last 2.5 million years, we've had about 55 mild to severe glacial cycles.

We have various ways of measuring the climate cycles of our climate's past, and these cycles coincide with the cycles i mention above --- until now.

Our planet should be cooling, according to orbital characteristics, but instead we are warming. Rudiman documents how about 8000 years ago, we started clearing forests for agriculture. We know this from pollen analysis in lakes and ponds, and satellite views that show ancient field shapes (in the same way that shining a light along sheet rock shows minute changes in elevation of the spackle) and CO2 measurements from ice cores. Our ancestors burgeoning agriculture was enough to shift the albedo of our planet and start a minute upward trend of warming.


Fast forward almost 8000 years and we can measure a long slow warming trend -- until the Industrial Revolution, when -- on a graph -- the warming trend angled upward much more steeply -- coincidentally at about the same angle as a hockey stick on it's side.

We've increased atmospheric CO2 from 280ppm (parts per million) to 400ppm since the Industrial Revolution. That's a miniscule, but significant amount, and the graph shows our CO2 is increasing at an increasing rate, and we should be at 500, then 600ppm in a decade or three. Unless we take emergency measures starting now.

There is fine balance between the amount of heat coming in from the sun and the amount which reflects back into space. Too much heat reflecting away and our planet cools; too much being kept in the atmosphere and our planet heats. CO2 acts like a blanket around the planet, keeping heat in. More CO2 acts like a thicker blanket. Add methane to the atmospheric mix and it's like adding a super blanket.

This article describes perihelion and sort of relates it to summer heat increase. So yes, perihelion, in general means it will be a little cooler, but add CO2 into the mix, and perihelion doesn't mean much.

Welcome to Global Warming which is based on all the things our ancestors did to increase their security and comfort without knowing the consequences of their actions. Today, we know the consequences. And tho the climate deniers will scream about it, the science of this site is unfortunately valid.



Check out: http://www.planetextinction.com/planet_extinction_permafrost.htm and let's do something about it. Starting now.

Alec Sevins
Alec Sevins

@jim adams

"...and let's do something about it. Starting now."

Finding a way to get people to stop voting for the GOP (Greedy Obstructionist Polluters) would be a good start.

Don Barr
Don Barr

@Alec Sevins @jim adams  

Yeah because I can see how Al Gore is donating all of his hundreds of millions of dollars in profits to combating climate change...   /sarc off

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