Understanding the age of the Earth was more difficult than early scientists imagined. The answer couldn't be found by counting generations of humans. The answer couldn't be found by measuring sedimentary layers. It wasn't until radioactive decay processes started to be understood in the 1890s that people could begin to get an accurate measure of the age of both the Earth and the Moon.
How did scientists pin down the ages of the Earth and the Moon? Using radioactive dating of rocks, we can measure the time since the rock was last melted. When rocks form from molten material, parent (and forming daughter) isotopes get locked into a single location. Prior to the rock's formation, the products of radioactive decays could escape (for instance in the mixing of liquid lava). Once the rock solidifies, decays happen in situ. By knowing the rate of decay processes, and measuring the ratio of parent and daughter isotopes, it's possible to place constraints on the age of a rock.
Armed with the technique of radioactive dating, geologists set out to find the oldest rocks on Earth. These oldest rocks are very rare because, after billions of years, most have been destroyed by the Earth's active geology. The oldest regions of unaltered crust are in Greenland, Western Australia, and South Africa, and they are about 3.9 to 4.0 billion years old. In one remarkable discovery from the Jack Hills region of Western Australia, a small zircon crystal was found with a radiometric age of 4.404 billion years. Think of it — a mineral smaller than the head of a nail that tells us about billions of years of history! We have to add to this age an uncertain number representing the time the Earth was molten and the surface was being heavily bombarded by Solar System debris. An upper bound to the age of the Earth comes from the most primitive meteorite materials, which are inclusions rich in calcium and aluminum. This measured age of the Solar System is 4.567 billion years. The best estimate for the total age of the Earth is 4.54 billion years with an uncertainty of only 1%.
To verify this number, scientists also wanted to date lunar rocks to see if they were comparable in age to the Earth. Each Apollo mission brought back a precious cargo of lunar rocks, soil samples, and drill cores. Together with some smaller samples brought back by earlier, unmanned Soviet probes, scientists gathered several hundred kilograms of rocks from nine different lunar sites. Lunar scientists eagerly tested these samples and began to unravel the history of the Moon. Many of the rocks were in the range of 3 to 4 billion years old, with a few chips dating back to 4.4 or 4.5 billion years ago. Apparently, most rocks from the first few hundred million years of the Moon's history were pulverized and destroyed by the intense bombardment that occurred at that time, which is why the earliest rocks and chips are rare. Scientists add about 100 million years to this age for the time it took the molten Moon to solidify. The best estimate of the age of the Moon from radiometric dating is 4.527 billion years, slightly younger than the Earth.
Absolute ages from radiometric dating were used to calibrate the relative ages of different parts of the lunar surface from crater-counting methods (see related article). Radioactive dating confirmed what the lunar cratering record indicated: the smooth dark plains, the maria, are considerably younger than the heavily cratered highlands. Once relative ages from the cratering record were calibrated, absolute ages could also be found for other planetary surfaces, such as Mercury and Mars by assuming cratering rates were consistent throughout the inner solar system.
The methods of radiometric dating have been refined over sixty years, and dozens of different radioactive species can be used for this research. When we apply the radioactive dating technique to all sorts of Solar System samples — terrestrial rocks, lunar rocks, and meteorites from deep space — they all yield an age of approximately 4.6 billion years. The technique is based on the well-understood physics of radioactivity, which can be tested in the laboratory. Scientists consider this proof of the age of the Earth, and it also led to the chronology of the entire Solar System. Consider this vast number for a moment. Compared to this, all the generations of humanity are only the blink of a cosmic eye. We've been around for less than a tenth of one percent of the history of our planet!