So, it is apparently a big mystery, the Earth. Especially its age.
There's no evidence to prove definitively how old the earth actually is. After all, none of us were here for its creation.
Except, well, there is uranium. Given any amount of uranium, half of it will turn into lead (by a series of decays) in a 4.5 billion year period. Also, if you put a ball of uranium together very quickly it becomes quite explody. Many Japanese people know this first hand, sadly.
It just so happens, right now, that we think that the oldest rocks we've found in the Earth-Moon system are about four and a half billion years old. We think this because uranium. It's like, in them. We guess that it's 4.5 billion years old because science. It's a lot to explain. But I'm pretty sure the Pentagon doesn't order uranium based on a guess. I think they do it because they know exactly how much they need to do the job.
Oh, also there is hydrogen. Hydrogen is the most abundant baryonic element in the universe. It also happens to be very explody. Not only can you light it on fire with a match, but if you squish it hard enough, it becomes even more explody.
It just so happens that if you know MOAR MATH, you can calculate how long it takes a ball of hydrogen and helium the size of our Sun to evolve to the state that it is in today. When hydrogen is compressed in the core of the Sun, it fuses into helium. That compression keeps things lively, and makes all the heat and light we see today. It also gradually changes the composition of the star, until it runs out of hydrogen. Then, it starts making heavier things until iron, after which the star dies.
Using that awful math, scientists have "guessed" that the Sun is about 4.5 billion years old, too.
Oh, and they "guessed" they could make a thing called Ivy Mike. Check it out. It's a big cloud shaped like a mushroom, and they used hydrogen and "guessed" they could make that explode, too. Now, I'm sure there are some people who think that's all a myth, and there's no such thing as weapons you have to "guess" about the properties of. But, like I said, there are a lot of Japanese people who don't think such things are a fucking guess.
Now, remember these scientists and their guesses and math. Why, one time a bunch of scientists guessed they could put a rocket into orbit, and their guesses were really accurate. Another time, they guessed they could put a bunch of other guys on the moon, and they guessed pretty close there too. Like, "sunrise in your space capsule will occur at 15:31 hours on Tuesday as you orbit the Moon," and wow, they guessed like real close.
Then there were those guys who guessed they could put a rover on Mars. They guessed so good they hit a target just a few miles across from over fifty million miles away. But hey, that was just "guessing," wasn't it?
Hey also, scientists guessed that if they put a bunch of satellites in orbit, you could measure your distance from them and guess your address, even if like you're moving. Believe it or not, those guessy little toys are now stuck to every dashboard in the nation. They guess real good, guys. Like, you should try it.
In closing, I'd like to re-iterate that anyone who thinks scientists are "guessing" is a fucking idiot.