Happy Perihelion!

Happy Perihelion y’all! This year the blessed event occurs on January 4. Given all of the other celebrations that tend to happen around this time, you could be forgiven for not having noticed. In fact, you may have no idea what perihelion is at all.

Which is fine by me because it gives me, a huge orbit nerd (look, we all have our little foibles), an excuse to talk about perihelion, Earth’s orbit, and orbits in general, which is an absolute delight for me. So let’s take a look at what perihelion is and what it means.

Periwho?

Everything orbiting the Sun has a perihelion. In a nutshell, the term refers to the point in an orbit around the Sun when the object doing the orbiting is closest to the Sun. If you were following the news about the Parker Solar Probe’s epic solar flyby on December 24^th, you may have heard this close approach referred to as a perihelion. That’s because it was the point in Parker’s orbit where it was closest to the Sun (and closer than anything else manmade has ever gone, but that’s a subject for a different blog post).

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As a side note about vocab, perihelion can only refer to things in orbit around the Sun. That’s the “helion” part of the word. If you are referring to an object orbiting something else, the word you’d use changes. For instance, the Juno spacecraft orbits Jupiter in a highly elliptical orbit that periodically brings it close to the giant planet. This point in the orbit is called a perijove. Orbiting the Earth? Then you’d have a perigee. The Moon? A perilune. 

And that is the end of the terms I knew off the top of my head before starting this post, but I went and looked up a bunch of others just for you: perihermion for Mercury, pericythe for Venus, periareion for Mars (I feel I like I should have known that one), perichron for Saturn. The generic term for the closest point of a generic orbit of a generic object around another generic object is periapse.

Incidentally, while we’re on vocab, the opposite of perihelion (which means “near Sun”) is aphelion (“away from Sun”)…and apojove and apogee and apoapse, and all the rest.

Getting Some Sun

When we refer to Earth’s distance from the Sun, we usually say that it’s 93 million miles away (a distance that we sometimes some up as an astronomical unit or AU), but this isn’t strictly accurate. It’s an average. Earth’s orbit is not a perfect circle. It’s very close, but it’s slightly elliptical or, as astronomers call it, eccentric. If an orbit had an eccentricity of 0 we would consider it a perfect circle. Earth’s orbital eccentricity is about 0.017, so almost a circle.

But you know what they say about “almost”. In this case it means that Earth’s distance from the Sun varies throughout its orbit by about 3 million miles. The closest Earth gets to the Sun, the perihelion we will reach on January 4, 2025, is about 91.5 million miles. Aphelion, if you’re wondering, is about 94.5 million miles. It’s a total change of a whopping 3%.

Earth reaches perihelion right after New Year’s each year. The exact date changes from year to year due to the (already stated) fact that Earth’s orbit isn’t a perfect circle, the fact that the Moon tugs on Earth, and the fact that the Julian calendar doesn’t perfectly match up with Earth’s yearly movement around the Sun. Generally we hit perihelion somewhere between January 2-6. Aphelion comes six months later. This year it will be on July 3.

Winter Sun

“But wait!” I hear my Northern Hemisphere friends cry, “How can the frigid part of the year be when we’re closest to the Sun?” Easy, because Earth’s distance from the Sun does not cause seasons. That would be down to the tilt of Earth’s axis. 

Perihelion just happens to be the point in our planet’s orbit when the Northern Hemisphere is tilted away from the Sun and the Southern Hemisphere is tilted towards it and the Southern Hemisphere is getting more direct sunlight than the Northern. It’s how direct your sunlight is (aka how tilted toward the Sun your hemisphere is) that causes seasons, not distance from the Sun. 

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One thing the Earth’s orbit does affect, however, is the length of the seasons. Things move faster when they’re closer to what they orbit. This is why Mercury goes around the Sun faster than Earth does—it’s closer. So if Earth is a little closer to the Sun during January, it’s moving just a bit faster.

This means it moves through the quarter of its orbit that corresponds to Northern Hemisphere winter and Southern Hemisphere summer just a little quicker than the other quarters. As a result, Northern winter/Southern summer is the shortest season of the year. Only by a couple of days, but still, I’ll take a couple of extra days of summer (sorry Southern Hemisphere friends!).

Being Eccentric

The difference between Earth at perihelion and Earth at aphelion is fairly minimal, because Earth’s orbit is so close to circular. That’s not true of all things in our solar system. Of the planets the award for most eccentric orbit goes to Mercury, with an orbital eccentricity of 0.21 (again, the higher the number, the more elliptical the orbit, going up to an eccentricity of 1 at which point you’re technically no longer in an orbit, just swinging by). 

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There’s a nearly 15-million-mile difference in Mercury’s distance to the Sun at perihelion vs. aphelion. Of course, since even at its farthest Mercury is still very close to the Sun, it makes no practical difference to the way the planet functions. Mercury gets cooked at either point.

But that’s not necessarily true of something like, say, Pluto, with an eccentricity of nearly 0.25. Pluto’s distance from the Sun varies by nearly 2 billion miles over the course of its orbit! At perihelion, Pluto comes closer to the Sun than Neptune does.

That amount of change can have an impact that the tiny difference Earth experiences can’t. For one thing, Pluto’s atmosphere, such as it is, changes dramatically between perihelion and aphelion. At one point it was thought that closer to aphelion Pluto’s atmosphere froze out altogether and fell to the ground as snow. Newer data suggests that it might remain, at least a little bit, but it changes its density so much that calling it an atmosphere is stretching the point.

Being Even More Eccentric

Of course when it comes to highly eccentric orbits that result in huge differences between perihelion and aphelion, comets win everything. Earth’s old reliable friend Halley’s Comet has a highly eccentric orbit, around 0.97. With its relatively short (for a comet) orbital period, around 76 years, the difference between perihelion and aphelion for Halley’s Comet is about 3.1 billion miles. 

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But as comets go, that’s not that impressive. Let’s take a more extreme example. One of the few good things to come out of 2020 was C/2020 F3 NEOWISE, one of the brightest comets in decades. Its orbit has an eccentricity of 0.999. As it swung through our solar system it got about 27 million miles from the Sun at perihelion. When it finally reaches aphelion, about 3,400 years from now, it will be 33 billion miles from the Sun.

These crazy changes in distance to the Sun that comets experience is what makes them put on their incredible celestial fireworks displays. They get so much more heat and light near perihelion than they do at the other end of their orbits that they almost turn into different creatures. It’s also why they’re ephemeral—they whip through the perihelion parts of their orbits at hundreds of thousands of miles per hour, meaning their time in the inner solar system tends to be brief.

Woohoo, Perihelion!

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It’s a fun fact about things orbiting other things that having a perfectly circular orbit is nearly impossible. There are just too many little factors that pile up that can make an orbit slightly eccentric. Our planet actually has one of the lowest eccentricities of any object going around the Sun, though Venus (0.007) and Neptune (0.009) both beat us.

But just in case you were dying to know what the most circular orbit in the solar system is, it’s not one I’ve talked about yet because there’s no perihelion or aphelion—the object isn’t orbiting the Sun. The winner is Triton, the largest moon of Neptune (so apparently it has a “periposeideum”). It has an orbital eccentricity of 0.000016 which means, for all intents and purposes, Triton’s orbit is a perfect circle or as near to it as you’re likely to ever find. 

So as we swing through Earth’s perihelion and endure our Northern Hemisphere winter (it’s cold here in Boston! Thanks, polar vortex!), don’t forget to look ahead. After all, aphelion is only six months away!