The oldest rocks and soils from the moon are about the same age--4.6 billion years old.Scientists assume that meteorites and moon rocks were not subjected to the extensive alteration that Earth rocks have undergone.Recently, rocks over 3.96 billion years old have been dated from northern Canada, Wyoming, and China.The ages of these oldest rocks still don't tell us how old the Earth is, but they do establish a minimum age.
The grains of sand in the top half of the hourglass are the radioactive parents, and those falling to the bottom are the stable daughters.
The rates of decay of various radioactive isotopes have been accurately measured in the laboratory and have been shown to be constant, even in extreme temperatures and pressures.
These rates are usually expressed as the isotope's half-life--that is, the time it takes for one-half of the parent isotopes to decay.
Therefore, their ages indicate when they were formed.
Because all parts of the solar system are thought to have formed at the same time (based on the solar nebula theory), the Earth must be the same age as the moon and meteorites--that is, about 4.6 billion years old.
The discovery gave scientists a tool for dating rocks that contain radioactive elements.