How old is the earth?
Short answer: 4,540,000,00/H30 Earth-years, plus or minus 1%.
Long answer: We don’t know exactly because direct dating of the earliest material on the surface of the Earth will only tell use a minimum age; Prior to that, the Earth’s surface was probably molten, and even after that, it may be that the earliest non-molten material has been recycled into the planet’s interior by tectonic processes. Also, the earth is a big round ball of stuff that condensed into this shape from part of a large disk-shaped blob of stuff known as the Solar Nebula. When exactly, given this, did the Earth become the Earth? Since the process took millions of years, we can’t pinpoint the age of the Earth more exactly than a certain range.
What are the oldest rocks?
The oldest rock formations on Earth are between about 3.8 and 3.9 billion years old., but there are older bits of more ancient rocks that were incorporated into these early rocks, and they date to something closer to 4.4 billion years old. These and other early materials are dated primarily using a variety of parent-daughter radiometric techniques, with the most effective for this time period being a lead-lead system.
Since rock from the time of the Earth’s formation isn’t available (because it didn’t really exist or was gobbled up in the fiery beginnings of the big round ball) the preferred method of dating the Earth is to calculate the age of meteorites. The earliest meteorites essentially date the condensation of materials in the solar system into the planets, and thus, the date of these meteorites indicates the date of the early Earth. (The Earth existed prior to this condensation in the form of whatever parts of the early solar nebula would eventually condense into this particular planet, of course.)
Meteorites from other planets?
Some meteorites are known to be fragments of Mars, so the oldest dates among these can also verify the date of accretion of material into planets in our solar system.
Rocks from the moon have not been remelted or otherwise messed up by tectonic processes and therefore would provide an excellent estimate of the age of the Earth as well. Also, since there is no real weathering of rocks on the moon, methods other than parent-daughter decay can be used, such as Fission Track dating (the older a rock, the more cosmic rays pass through it, blasting tiny little tracks in the otherwise homogeneous matrix).
Zeroing in on the age of the earth
There are hundreds of published dates of various older materials, but the following table gives a reasonable summary of some of the more important dates, culled from various sources (see list of references below):
If we chart this on a graph, we see one date that is much earlier than all the other dates, and a few that are younger.
The younger dates are simply of materials that we don’t think date the Earth’s formation, but that we know would post date it by not much. These dates verify the earlier cluster of dates that would correspond to the actual formation of the planet. The single earlier date is an obvious outlier.
Taking this series of dates, notice that the oldest (non-outlier) dates are about four and a half billion years old. As stated in the short answer.
Further information about the age of the Earth:
Dalrymple, G. Brent. 2001. The age of the Earth in the twentieth century: a problem (mostly) solved. Geological Society, London, Special Publications 2001, v. 190, p. 205-221. Click Here.
Dalrymple, G. Brent. 2006. How Old is the Earth: A Response to “Scientific” Creationism. The TalkOrigins Archive. Click Here.
Norman, M. D., Borg, L. E., Nyquist, L. E., and Bogard, D. D. (2003) Chronology, geochemistry, and petrology of a ferroan noritic anorthosite clast from Descartes breccia 67215: Clues to the age, origin, structure, and impact history of the lunar crust. Meteoritics and Planetary Science, vol 38, p. 645-661.
Stassen, Chris. 2005. The Age of the Earth. The TalkOrigins Archive. Click here.
Wikipedia, Teh. 2010. Age of the Earth. Click here.