It's summer in the Northern Hemisphere, and while you're enjoying the long, balmy days at the beach or in nature, you might be surprised to know that the Earth is slowly approaching its farthest point from the Sun, known as aphelion.
Here's what you need to know about this celestial phenomenon that occurs every summer.
What causes aphelion and when does it occur?
Earth reaches aphelion every July, but this year it will occur at 1:06 a.m. Eastern time on Friday.
Earth has an aphelion because its orbit is elliptical, not circular. All the planets in our solar system move around the sun in long, thin circles, not perfect circles, says geologist Kirby Runyon of the Planetary Science Institute. And that's likely true for worlds around other stars, too.
Gravity is the cause of all these elliptical orbits.
“All the planets have a tendency to bump into each other, causing their orbits to deviate from a perfect circle,” says Dr Runyon. “It's literally a chaotic tug of war due to the tiny gravitational influences that the planets have on each other.”
Jupiter has the greatest influence because it is the most massive planet in the solar system, he added.
The degree to which an orbit deviates from a perfect circle is measured by its eccentricity. The higher the eccentricity, the more elliptical the orbit. Some objects in the solar system are more pronounced in this regard. Mars, with an eccentricity of 0.094, is between 129 and 155 million miles from the Sun. Pluto, whose distance from the Sun varies from 2.8 to 4.5 billion miles, has an even larger eccentricity of 0.244.
Earth's orbit, by contrast, has an eccentricity of just 0.017. “Earth's orbit is nearly circular,” says astronomer Larry Wasserman of the Lowell Observatory in Flagstaff, Arizona. “If you drew it to scale on a piece of paper, you wouldn't notice that it's slightly flattened.”
How far away from the Sun is it at aphelion?
At aphelion, the Earth is approximately 94.5 million miles from the Sun. Six months later, in the winter at the beginning of January, the Earth will be closest to the Sun at 91.5 million miles. This position is known as perihelion.
From Earth's perspective, 3 million miles may seem like a lot, but it's not much on an astronomical scale: The sun's size in the sky appears about 4 percent smaller at aphelion than at perihelion, but the effect is so small it would be hard to notice without sophisticated instruments, Wasserman said.
Does aphelion affect Earth's temperature?
A common misconception is that the seasons are caused by changes in the Earth's distance from the Sun, and yes, this does have a small effect: at aphelion, there is 7 percent less sunlight than at perihelion, making northern hemisphere summers and winters a little milder.
But that effect is offset by the Earth's axial tilt, causing the hemisphere to tilt toward or away from the Sun at different points in its orbit.
At aphelion, which occurs just a few weeks after the summer solstice, the northern half of the Earth tilts toward the Sun, making summer days longer and hotter, even though Earth is farther away.
And at perihelion in January, the Northern Hemisphere moves away from the Sun, causing shorter days and cooler temperatures.
In the Southern Hemisphere, the effect is reversed: When Earth is at aphelion, the hemisphere is moving away from the Sun, making Southern winters a little cooler than they would be if Earth's orbit was perfectly circular. Then, when Earth approaches perihelion in January, the hemisphere is moving closer to the Sun, making Southern summers a little warmer.
For planets with more extreme eccentricities, changes in distance can have a much larger effect: Mars' sunlight can vary by up to 31 percent along its orbit, for example.
It is by chance that the Earth reaches aphelion when its tilt to the Sun is greatest, and this will eventually change in the future as other planets in the Solar System pull or compress the Earth's orbit with their gravitational forces. Currently, the Earth's eccentricity is decreasing, making its orbit around the Sun more circular.
What would happen if there was no aphelion?
If the Earth's orbit were a perfect circle, the seasons would all be exactly the same length — currently, spring and summer are a few days longer than autumn and winter in the Northern Hemisphere, but little else would change. “If, somehow, we could snap our magic fingers and the Earth's orbit became more circular, we'd probably be fine,” Dr Runyon said.
But if something were to cause the Earth's orbit to become even more eccentric, the consequences could be disastrous. The seasons in the Southern Hemisphere would become more extreme, with unbearably hot summers and unbearably cold winters, which could result in crop failures and freeze-ups.
“If it gets really bad, advanced civilisations won't be possible,” Dr Runyon said.
For now, let’s be thankful that our planet is in good conditions.