Are radiometric dating methods reliable
There are many radiometric clocks and when applied to appropriate materials, the dating can be very accurate.
As one example, the first minerals to crystallize (condense) from the hot cloud of gasses that surrounded the Sun as it first became a star have been dated to 4568 plus or minus 2 million years....!! Other events on earth can be dated equally well given the right minerals.
We have also obtained a very similar age by measuring Pb isotopes in materials from earth.
I should mention that the decay constants (basically a value that indicates how fast a certain radioactive isotope will decay) for some of these isotope systems were calculated by assuming that the age of the earth is 4.56 billion years, meaning that we will also calculate an age of 4.56 billion years if we use that decay constant.
Because geochronologists want to measure isotopes with different masses, a mass spectrometer works really well for dating things.
I do think that radiometric dating is an accurate way to date the earth, although I am a geochronologist so I have my biases.
Uranium eventually decays into lead, and lead does not normally occur in zircon, except as the radioactive decay product of uranium.
Therefore, by measuring the ratio of lead to uranium in a crystal of zircon, you can tell how much uranium there originally was in the crystal, which, combined with knowing the radioactive half-life of uranium, tells you how old the crystal is.
Most estimates of the age of the earth come from dating meteorites that have fallen to Earth (because we think that they formed in our solar nebula very close to the time that the earth formed).Hope that helps, and please ask if you'd like more details! I think that I will start by answering the second part of your question, just because I think that will make the answer to the first question clearer.