Expert: Courtney Seligman - 9/20/2011

Question
Greetings!
Basically my question is if a real month (the time between let's say 2 consecutive full moons) has changed with something more than a day or so from the moment the moon got stable on Earth's orbit. Also, is the future of the lunation duration known? As in, will it increase? Will it decrease? A lunar month in 3011 AD will be longer or shorter? But one in 102,011 AD?
(Sorry for the many questions, just wanting to make things clear. A simple answer and/or further reference would be extremely helpful.)
Thank you!

Answer
The tidal interaction between the Earth and Moon is gradually slowing the rotation of the Earth (by one or two thousandths of a second per century), and gradually increasing the size of the Moon's orbit (by about an inch or two per year). As a result, the period of one lunar orbit is gradually getting longer. In just a few thousand (or even hundred thousand) years, the length of a lunation (from any one phase of the Moon to the next identical phase) will not change by more than a few seconds. But over the billions of years since the Moon was formed, the change in its orbit has been substantial, so it used to take only a week or so to go around us, whereas now it takes more like a month, and by the time the Sun swells up and (probably) swallows the Earth-Moon system, the lunar orbit will take about 35 or so (current) Earth days. In other words, most of the change in the period has already taken place, but there will be an additional change of about a day per billion years, even in the future.