effects of various astronomical bodies – most significantly the Sun and less so Venus and Jupiter. Other forces responsible for minute perturbations are: gravitational attraction to other planets in the Solar System and to asteroids; tidal forces; and relativistic effects. The effect of radiation pressure from the Sun contributes an amount of ± to the lunar distance. Although the instantaneous uncertainty is a few millimeters, the measured lunar distance can change by more than from the mean value throughout a typical month. These perturbations are well understood and the lunar distance can be accurately modeled over thousands of years. Tidal dissipation Through the action of tidal forces, the angular momentum of Earth's rotation is slowly being transferred to the Moon's orbit. The result is that Earth's rate of spin is gradually decreasing (at a rate of ), and the lunar orbit is gradually expanding. The current rate of recession is . However, it is believed that this rate has recently increased, as a rate of would imply that the Moon is only 1.5 billion years old, whereas scientific consensus supports an age of about 4 billion years. It is also believed that this anomalously high rate of recession may continue to accelerate. It is predicted that the lunar distance will continue to increase until (in theory) the Earth and Moon become tidally locked, as are Pluto and Charon. This would occur when the duration of the lunar orbital period equals the rotational period of Earth, which is estimated to be 47 of our current days. The two bodies would then be at equilibrium, and no further rotational energy would be exchanged. However, models predict that 50 billion years would be required to achieve this configuration, which is significantly longer than the expected lifetime of the Solar System. Orbital history Laser measurements show that the average lunar distance is increasing, which implies that the Moon was closer in the past, and that Earth's days were shorter. Fossil studies of mollusk shells from the Campanian era (80 million years ago) show that there were 372 days (of 23 h 33 min) per year during that time, which implies that the lunar distance was about (383,000 km or 238,000 mi). There is geological evidence that the average lunar distance was about (332,000 km or 205,000 mi) during the Precambrian Era; 2500 million years BP. The giant impact hypothesis, a widely accepted theory, states