A creationist website makes this argument for the 6,000 year old earth. I'm embarrassed to say I don't know how to do the math to evaluate the claim myself. However, the time scales involved seems to lend some credence to this argument. "The Moon is slowly drifting away from the Earth. If it is getting further, at one time it was much closer. The Inverse Square Law dictates that if the Moon were half the distance from the Earth, its gravitational pull on our tides would be quadrupled. 1/3 the distance, 9 times the pull. Everything would drown twice a day. Approximately 1.2 billion years ago, the Moon would have been touching the Earth. Drowning would be the least of our concerns! – See more at: http://www.allaboutcreation.org/how-old-is-the-earth.htm#sthash.LSHK4GQk.dpuf"
[Physics] True or false: the Moon was touching the Earth 1.2 billion years ago
earthmoonorbital-motionsolar systemtidal-effect
Best Answer
Extrapolation is as likely to produce meaningless results when done into the past as when done into the future.
The most widely accepted theory for how the Earth-Moon system was formed is the Giant impact hypothesis: around 4.5 billion years ago, the (then itself still very new) Earth was nearly destroyed by a collision with another planet roughly the size of Mars. This created a huge cloud of debris orbiting the Earth, much of which coalesced into the moon.
So the moon first formed at a certain distance from the Earth, and that distance has since then grown due to tidal braking (and the same effect is steadily increasing the length of the day on Earth).
An interesting detail about your quote is that their extrapolation of 1.2 billion years is much shorter than the time since the impact, but without any indication of how that number was computed, it means nothing. It is not a linear extrapolation of the current distance (384,000km) and rate of increase (3.8cm/a) because that yields over 10 billion years. But the rate of increase really cannot be meaningfully extrapolated because it depends on the magnitude of friction caused by tidal forces, and that depends on what the Earth looks like. After the impact, the Earth was probably mostly liquid for some time, which would cause much more tidal friction and thus a much more rapid increase in distance.
But the creationists are correct about one thing: Drowning would, indeed, be the least of our concerns back then!