Is Earth Round and Why?
I decided to write a series of astronomy articles for people to educate themselves. I think it’s cool to read or watch something and get some knowledge which will stay with you for the rest of your life and probably will help you one day. The problem with most textbooks that they are boring and difficult to comprehend sometimes. One of the reasons I started this blog is to show people that science is not that difficult, and can be fun if you cook it right (and I am not talking about popular “blow some stuff up” science). Let’s start with our home planet which we all love the most — Earth. I will start from very simple things and gradually move to more advanced stuff.
If you go to the field and look around you might think that Earth is flat. That’s what ancient people thought, just go to the field and look around, it’s pretty obvious that the Earth is flat. What things can make you think otherwise? Yes we can fly to space and look at it, but it’s easier said than done. Imagine you will travel to the past, how can you prove people that Earth is round? You will go to the harbour and watch for the departing ships of course :-) You know that Earth is round, so if you look at the departing ship sooner or later it will disappear under horizon. It wouldn’t happen if Earth is flat:
Many of us will recognize this old picture :-) Image from: bitsandpieces.us
OK, now you probably believe me that Earth is round, but curious mind does not stop here. Why the heck Earth is round? Why not cube or asteroid like shapeless piece of rock. Apparently any object will become round if it is heavy enough. Space objects form differently and consist of different stuff, but big enough objects are always round. If object’s mass is big enough gravity becomes a dominant force comparing to all the others which shape the space body. For example let’s say something happened (more on this later) and a mountain grew up. Eventually if will be smoothed, because rocks will fall from the mountain top to the bottom and guess what form should object have for anything be impossible to fall? Gravitation makes things fall down, so if gravitation is strong enough it will shape any irregular object to sphere. I have some proof for you on this one:
This is probably what pops up in your mind when you hear "asteroid". Irregular shaped piece of rock. Image from: stsci.edu
But have you seen pictures of the biggest known asteroid (pay attention — biggest asteroid):
Ceres is the biggest known asteroid in the main asteroid belt. You can see with your own eyes that it is quite round. Image credit: Hubble Space Telescope
How Old is Earth and Why Do We Think So?
We say that Earth has formed about 4.6 billion years ago. Well, that’s simple you say, just take the oldest rock we know and that’s the Earth age. Oldest rock we found so far is about 3.9 billion years old. The problem with rocks is that we think they were melted on the early stages of Earth formation and we can measure age of a solid thing only (because melted stuff mixes all over with everything). Do you know by the way, how to they know how old is the rock? When rock solidifies it has some amount of special elements which start to decay over time. We can pick some magma sample (just head to the nearest active volcano ;-) and measure how many those special elements in the magma. We also know how fast those elements decay, so we measure how much of those decaying elements is in the rock and we can compare that amount to the one we measured in magma sample and see how much of them decayed. The more stuff has decayed the older the rock.
We assume that there is the same amount of decaying elements in the current magma as in the magma 4 billion years ago. Image from: pelelani.com
Thus old rocks don’t give us a good answer about Earth age, because all planet was melted in the very beginning. Where do we get our 4.6 billion figure from then? We study other objects in Solar system like Moon, planets, asteroids and meteorites. Many of them are much smaller than Earth, it means they cooled way faster, so we can get their age and they say: “Hey, Earth and asteroids are formed in the same time for sure, so Earth should be as old as the meteorites we get”. That’s how Earth got it’s age.
What’s Inside The Earth?
Did you ever wonder? Probably you already saw some kind of picture which shows the thin Earth crust and the magma and the core. How do we know all that? Sure enough we drilled a super deep hole and measured everything, right? The deepest research hole we ever made in Earth is about 12km (~7.5 miles), and Earth radius is about 6,370km (~3,950 miles). Thus we drilled about 0.1% of the way to the Earth center and just 99.9% left to go. The way they look inside the Earth is a bit different — blast a dynamite (or wait for the earthquake) and measure sound waves. Sound has different speed in different materials and partly reflects when there is a border between two different materials (like crust and magma for example). Same technology is used on the ships to map the sea floor (without dynamite part of course).
Melted rock was always a wow for me. Image from wikimedia commons. Click for bigger version.
We also know that the deeper we dig the warmer (if you want to learn more about Earth internal heat theories, read my why Earth is so hot inside post ) it is and of course we can see what comes from volcanoes. All these observations allow us to conclude what’s inside the Earth, but you right, we never actually saw what’s in there.
We Walk On Floating Rocks
While magma is kind of liquid and rocks we walk on are solid magma is still more dense than rocks, and rocks are floating on it, just like ice is floating on water. Earth crust is not one big piece of rock, but rather split in several big pieces. If you live in the middle of such piece — you’re safe, but if you live on the edge, you can experience an earthquake. What’s an earthquake? “Earth is shaking” is a good answer, but why? When two huge floating rocks collide the pressure between them builds up. Rocks are hard enough to sustain some pressure, but sooner or later it cracks or one floating rock goes under another or they both just crush and squeeze. In any case they will be moving and that’s what we call an earthquake. Unfortunately with our current technology we can not predict an earthquake, we just can say whether a particular region dangerous or not, that’s why it is always such a big disaster.
Japan is unlucky to be located on the edge of two tectonic plates, so earthquakes are rather often there. Image from: digitalworldtokyo.com
What Was in the Beginning?
You know already that we live on big floating rocks and those rocks don’t stand still, they are moving. Because of that movement earthquakes happen. One curious guy discovered, that east coast line of South America and west coast line of Africa seems to fit very well. He suggested that continents move and some time ago there was one big continent which split into several continents we know today.
Once there was one big continent -- Pangaea, which later split into several ones we know today. Image from : wikipedia.org
For sure this theory makes sense, when God was creating the Earth it was much easier for him to create one big piece of land than to worry about different continents.
–> Read part 2 about the Moon.
Astronomy Articles, News and Pictures 