Summer has arrived in the Northern Hemisphere. But while you’re enjoying long, pleasant days at the beach or in nature, you might be surprised to learn that our planet has crept closer to its maximum distance from the Sun, a phenomenon known as aphelion.
Here’s what you need to know regarding this celestial event that occurs every year at the beginning of summer.
What causes aphelion and when does it occur?
Earth reaches aphelion every July, and this year that happened on Friday at 1:06 a.m. Eastern Time.
Earth’s aphelion is due to its elliptical orbit, not circular. According to Kirby Runyon, a geologist at the Planetary Science Institute, all the planets in the solar system travel in elongated circles around the Sun, rather than perfect ones. And the same is likely true for worlds around other stars.
Gravity is responsible for all these elliptical orbits.
“All the planets tend to push each other,” pushing their orbits away from perfect circles, Runyon explains. “It’s literally a chaotic struggle between tiny amounts of gravitational influence that the planets exert on each other.”
Jupiter exerts the greatest influence because it is the largest planet in our solar system, he added.
The degree to which an orbit deviates from a perfect circle is measured by its eccentricity. The greater the eccentricity, the more elliptical the orbit. For some solar system bodies, the eccentricity is quite pronounced: Mars, with an eccentricity of 0.094, ranges from regarding 207 to 249 million kilometers (127 to 155 million miles) away from the Sun. Pluto, whose distance from the Sun ranges from regarding 4.5 to 7.2 billion kilometers (2.7 to 4.5 billion miles), is even more eccentric, at 0.244.
Our planet, by contrast, has an eccentricity of just 0.017. “Earth’s orbit is pretty circular,” says Larry Wasserman, an astronomer at Lowell Observatory in Flagstaff, Arizona. “If you were to draw it on paper to scale, you probably wouldn’t notice that it’s slightly flattened.”
How far are we from the Sun at aphelion?
At aphelion, the distance from the Earth to the Sun is regarding 152 million kilometres. Six months later, in early January, in winter, the Earth is at its closest point to the Sun, at 147 million kilometres. This point is known as perihelion.
From our surface, 5 million kilometers may seem like a lot, but it’s not that big on an astronomical scale. The Sun’s size in the sky appears 4 percent smaller at aphelion than at perihelion, an effect too small to notice without precise instruments, according to Wasserman.
Does aphelion affect Earth’s temperatures?
A common misconception is that the Earth’s varying distance from the Sun is what causes the seasons. However, it has a small impact: at aphelion we receive 7 percent less sunlight than at perihelion, resulting in slightly milder summers and winters in the Northern Hemisphere.
But this effect is offset by the tilt of the Earth’s axis, which means that at different points in its orbit the hemispheres are tilted toward or away from the Sun.
At aphelion, which occurs a few weeks following the solstice, the northern half of the planet is tilted toward the Sun, resulting in the longest and hottest days of summer, even though Earth is farther away.
And at perihelion in January, the Northern Hemisphere is tilted toward the sun, making days shorter and temperatures colder.
In the Southern Hemisphere, this impact is reversed. Because the hemisphere is tilted away from the Sun when Earth is at aphelion, southern winters are slightly colder than they would be if our orbit were perfectly circular. As the planet approaches perihelion in January, the hemisphere is tilted toward the Sun, so southern summers are slightly warmer.
For planets with larger eccentricities, the change in distance can have a bigger impact. For example, on Mars, sunlight can vary by as much as 31 percent along its orbit.
It is a coincidence that Earth reaches aphelion near the time when its tilt towards the Sun is greatest. And this will change over time, as other planets in the solar system gravitationally influence Earth’s orbit in the future. At present, its eccentricity is decreasing, meaning that its path around the Sun is becoming more and more circular.
What would happen if there was no aphelion?
If our planetary orbit were a perfect circle, the length of the seasons would be exactly the same (right now, spring and summer are a few days longer than fall and winter in the Northern Hemisphere), but not much else would change. “If we might somehow snap our fingers and Earth’s orbit became more circular, everything would probably be fine,” Runyon says.
But if something were to cause the Earth’s orbit to become more eccentric, the consequences might be catastrophic. In the southern hemisphere, the seasons would become too extreme: summers would be unbearably hot and winters intolerably cold. This might lead to crop failures and frosts.
“If things got really bad,” Runyon said, “advanced civilization wouldn’t be possible.”
For now, let’s be thankful that our planet is in a good place.
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