We know the Earth is about 4.5 billion years old. But when do we start the clock? When we became roughly spherical? When we were, say, 90% of our current size? When the surface cooled and hardened?
None of the links I found really say when to start counting; they all give the age and the method for determining it. Not even the USGS says much, but they do have a cool picture. I’m tempted to just give up and say we’re 10k years old.
Death from the Skies puts the activation of the sun at some “millions of years” after 4.6 billion years. By the time the sun lights up, most of the rest of the material in the new solar system is already formed into planets, so I’d guess that 4.5 billion years ago, the earth was very new, but contained most of the material it has now (which makes it spherical) and likely hadn’t cooled down at all.
Not sure about the moon, though.
Since the dating method involves the age of crystals found, the clock must start at the point that a significant portion of the earth was solid. Considering the precision/accuracy of 4.6 billion years, I’d guess +/- a hundred million years would include most reasonable starting points for the age of the Earth. IANAG so I don’t really know the accuracy or timescales.
So perhaps we go with the IAU definition of planet? (You can follow the links to the IAU site, but Wikipedia’s version is easier to read and more convenient.) That pretty much boils down to when we were round-ish and we’re the only big thing left in our orbit. By that definition, were we not a planet until after the Theia collision creating the moon?
BTW, I’m waiting for someone to return Death from the Skies to the library, is it any good? Bad Astronomy is a great site, so I’m assuming Plait’s book is just as enjoyable.
The oldest crystals are 4.4 billion years or so while the oldest meteorites are 4.6 billion. If we say that most of the material had clumped together by 4.6, that gives 100 million years for Earth to clean up the neighborhood until ‘birth’ and another 100 million for things to cool down enough to form lasting samples. It seems a little fuzzy drawing the line and saying, “Earth is a planet now” compared to something like the Big Bang, but I can live with that.
Did we perhaps puzzle that out ourselves? If so, take that science establishment! And if not, I ask the geologists to be gentle in their rebuking. 
Perhaps we should demand to see a birth certificate?
Well, I did find this Kenyan birth certificate, but it could be a fake. I’ll demand the Supreme Court to take a look at it anyways.
What type of precision does this type of dating have? I’ve never seen anything more precise than one decimal place so I assume +/- 100 million years.
At the bottom of this section, the dating of the meteorites seems to be precise within a few tens of millions of years. That’s a lot more accurate than I would’ve thought possible. Its pretty impressive.
Awesome!
Meteorites are easier to date because they don’t have weather and water like rocks on Earth do. To get good dates on Earth, you have to work to find solid crystals or other formations that help preserve the evidence. Otherwise, you can triple the age of a rock by soaking it in water overnight (since uranium, lead and many other radioactive elements are partly soluble).
I’d say we were a planet, but we weren’t this planet until then. But reading on the iron catastrophe seem to indicate thatsome people seem to date the start from 500 million yrs before that, so I think they’re going by a size/end of major impacts kind of definition.
AIUI, when Theia hit, it melted all the surface rocks that had previously formed. That’s why it’s impossible to find Earth rocks dating from the very beginning.
This would be the ur-ripoff of the Kenyan BC, after which all further ripoffs will be compared and found wanting.
It’s written!
Genesis, Chapter 1 Verse 1
Thanks for asking;)
In a slightly unrelated vein, Spitzer found evidence of a massive collission 100 light years away. Apparently a Moon-sized object hit a Mercury-sized one orbiting HD172555.