Happy Earth/Sun Perihelion Day! Let’s Talk Seasons…

The Earth’s orbit around the Sun is elliptical, meaning that as it orbits the Sun, there are times that Earth is closer to, as well as, further away from the Sun. The distance is actually very minor, in the larger sense of things. Perihelion is the term for when the Earth is closest to the Sun and occurs today with the Earth being ~147,166,462 km (91,445,000 mi) from the Sun. At aphelion (when the Earth is furthest from the Sun), it’s ~152,171,522 km (94,555,000 mi) away from the Sun, a difference of 5,005,060 km (3,110,000 mi). Sure, this seems like a big change. When you consider that the distance is ~3.4% of the Earth’s total distance from the Sun, it’s not that vast. Wait… if today is the day that the Earth is closest to the Sun, shouldn’t it be warmer? It’s a common misnomer that the distance in orbital difference is the why the Earth has seasons. The Earth should be warmer when it’s orbit takes it closer to the Sun. The idea makes sense, even if its wrong. The reason for Earth’s seasons is actually due to its 23.5 degree axial tilt. How did that tilt happen? How does that tilt give us seasons?

As explained above, the idea that seasons exist here on Earth due to Earth’s elliptical orbit bringing Earth closer to the Sun at some points, is a fallacy. The tilt of the earth is the actual reason for Earth’s seasons. Well, interesting point of fact, it’s actually only winter right now for people in the Northern Hemisphere (from the North Pole to the Equator) of the Earth, it’s the opposite season in the  Southern Hemisphere. Have a look at the graphic to the right. This shows the Earth’s axial tilt in relation to the Earth’s position during its orbit around the Sun. You can see that in (what Northern Hemispherians would call) the winter, the Northern Hemisphere actually gets less of the Sun’s light, while the Southern Hemisphere is getting more. This causes it to be winter in the Northern Hemisphere while its summer in the Southern Hemisphere. Half a year and half an orbit later, the opposite is true.

So now you know that Earth has seasons due to it’s 23.5 degree axial tilt. It would be safe to say that Earth would be a perpetual tropical zone, without that tilt. To those of us in the Northern Hemisphere currently, that’s not looking so bad right about now. The Earth hasn’t always had that tilt, however. In its early stages, during the Late Heavy Bombardment, Earth’s axis was, essentially, straight up and down. So what caused that tilt? Most scientists think that it was the same thing that caused the formation of the Moon, the collision of two planets.

The Late Heavy Bombardment was a time (~4.5 million years ago) when the Earth was being battered by the debris left over from the formation of the four inner, rocky planets. Earth was rampant with vulcanism and the entire crust of the Earth was super hot and very fragile, much like a cooling lava flow.

There have been many studies done on rocks that have been returned to Earth from the Moon (by teams of geologists, physicists and astrophysicists) that show that the isotope ratios between lunar rock (rocks from the Moon) and terrestrial rock (rocks from the Earth) are identical. In other words, the Earth and the Moon have very similar, if not identical, origins. You see, the moon didn’t exist as celestial body meant to orbit the Earth and give us tides, it didn’t exist at all until the “Theia”, a planet that was, approximately, the size of Mars, slammed into the early Earth, striking it a glancing blow and tilting our axis to 23.5 degrees. This collision blasted the magma like mantle of both the Earth and Theia off of the planets, forming an accretion disc around the Earth. At this time, the entirety of the Earth would have been an ocean of magma that had an accretion disc around it that consisted of the combined mantles of Earth and Theia. Over time, the Earth cooled and began creating the cellular structures (amino acids) of early life on the Earth. While that was happening here, the Moon was forming right next door, ~64,373 meters (40,000 miles) away from the Earth.

There is plenty of evidence to show that, like Earth after  Theia’s impact, the Moon was also once an ocean of magma. There are a couple of theories about the coalescence of the Moon from an accretion disc to accommodate the molten origin of he moon. One states that the moon could have actually formed from this lava like debris in a mere ten years via constant collision between the smaller parts. Each collision would have the impactor imparting  its energy into the larger body in the form of heat, causing that larger piece to remain molten. Continue this process until the Moon exists. This is the prominent theory, as it is the explanation given on NASA’s website. Sounds good to me. I’ll let British space scientist Dr. Maggie Aderin-Pocock explain it with an animation from the BBC:

I tend to enjoy the second theory more, however, as it contains a night sky that I would LOVE to see. The second theory, put forth by David Smith of the Massachusetts Institute of Technology (MIT) and known as “The Big Splat”, hypothesizes that the accretion disc formed into two smaller moons that eventually collided together much the same way that Earth and Theia did. This large impact imparted enough energy into the moon to cause it to become molten. Think of looking up into the sky and seeing two moons (albeit, much smaller moons) crossing the night sky! That would be amazing! NASA is looking into this theory and recently sent up a pair of solar probes that will measure the gravity of the Moon, known as the GRAIL Mission. The mission was completed on December 17, 2012 and the information obtained is still being combed over. I will be putting up a post with that info as soon as it becomes more readily available. Here’s NASA’s description of the theory:

That’s it for this post. I know, that’s a very long and convoluted way to say, “Happy Sun/Earth Perihelion Day!”, but I know it interested me and thought it might interest you. Time for a question… Which theory of Moon formation do you favor? Feel free to leave your comments/suggestions/questions in the comments section below and don’t forget to share this post wherever you like to share things that you like (there’s a few buttons at the bottom to help with that). Should you be interested in more stuff like this, you can always subscribe to my RSS Feed, “Friend” me on Facebook, “Follow” me on Twitter or “Circle” me on Google+!

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