Dating the Earth - How old is it?
If someone decided to bake a cake to celebrate the earth's birthday, how many candles should they put on it? Some people contend that you'd need 4.5 billion, because they believe the earth is that old.
However, many are surprised to discover that this figure did not come from dating earth rocks, but rather rocks from outer space. A famous study published in 1956, dated a group of meteorites at 4.5 billion years and they assumed that this was the age of the earth.
Like all radioactive dating studies, these methods rely on a number of questionable assumptions but unlike dating earth rocks, this study also relied on the speculation that meteorites are leftover junk from earth's formation. Contemporary geologist, Arthur Holmes, remarked that this speculation was "unsound in principal."
But for ideas to become firmly entrenched it doesn't necessarily require solid data, it just needs sufficient repetition.