Earth is pushing the moon away faster now than it has for most of the past 50 million years, mostly a result of tides, a U.S. researcher says.

Matthew Huber of Purdue University says his models of the influence of tides on the moon's orbit help solve a longstanding mystery concerning the moon's age, NewScientists.com reported Wednesday.

The moon's gravity creates a daily cycle of low and high tides that dissipates energy between it and Earth, slowing the planet's spin on its axis and causing the moon's orbit to move farther away by about an inch and a half a year.

If that rate has always been the same, given where the moon's orbit is now the moon should be 1.5 billion years old, Huber said, yet some lunar rocks are 4.5 billion years old.

Huber and his colleagues gathered data on ocean depths and continental contours existing 50 million years ago to create a model of ancient tides. The model suggests the energy dissipation then was only half what it is today, so the moon was pushed away at a slower rate, Huber said.

The key is the North Atlantic Ocean, which is much wider today than it was 50 million years ago -- wide enough for water to slosh across once per 12-hour cycle, creating larger waves and very high tides that result in shoving the moon faster, he said.