Since ancient times, the Moon's mean rate of motion relative to the stars had been treated as being constant, but in 1695, Edmond Halley had suggested that this mean rate was gradually increasing. Later, during the eighteenth century, Richard Dunthorne estimated the rate as +10" (arcseconds/century2) in terms of the resulting difference in lunar longitude,-- also given in Philosophical Transactions (abridgements) (1809), vol.9 (for 1744–49), pp. 669–675 as "On the Acceleration of the Moon, by the Rev. Richard Dunthorne".</ref> an effect that became known as the "secular acceleration of the Moon". Pierre-Simon Laplace had given an explanation in 1787 in terms of changes in the eccentricity of the Earth's orbit. He considered only the radial gravitational force on the Moon from the Sun and Earth but obtained close agreement with the historical record of observations.