that the Moon was created as a result of a catastrophic impact between Earth and another planet, resulting in a re-accumulation of fragments at an initial distance of (24,000 km or 15,000 mi). In this theory, the initial impact is assumed to have occurred 4.5 billion years ago. History of measurement Until the late 1950s all measurements of lunar distance were based on optical angular measurements: the earliest accurate measurement was by Hipparchus in the 2nd century BC. The space age marked a turning point when the precision of this value was much improved. During the 1950s and 1960s, there were experiments using radar, lasers, and spacecraft, conducted with the benefit of computer processing and modeling. This section is intended to illustrate some of the historically significant or otherwise interesting methods of determining the lunar distance, and is not intended to be an exhaustive or all-encompassing list. Parallax The oldest method of determining the lunar distance involved measuring the angle between the Moon and a chosen reference point from multiple locations, simultaneously. The synchronization can be coordinated by making measurements at a pre-determined time, or during an event which is observable to all parties. Before accurate mechanical chronometers, the synchronization event was typically a lunar eclipse, or the moment when the Moon crossed the meridian (if the observers shared the same longitude). This measurement technique is known as lunar parallax. For increased accuracy, certain adjustments must be made, such as adjusting the measured angle to account for refraction and distortion of light passing through the atmosphere. Lunar eclipse Early attempts to measure the distance to the Moon exploited observations of a lunar eclipse combined with knowledge of Earth's radius and an understanding that the Sun is much further than the Moon. By observing the geometry of a lunar eclipse, the lunar distance can be calculated using trigonometry. The earliest accounts of attempts to measure the lunar distance using this technique were by Greek astronomer and mathematician Aristarchus of Samos in the 4th century BC and later by Hipparchus, whose calculations produced a result of ( or ). This method later found its way into the work of Ptolemy, who produced a result of ( or ) at its farthest point. Meridian crossing An expedition by French astronomer A.C.D. Crommelin observed lunar meridian transits on the same night from two different locations. Careful measurements from 1905 to 1910 measured the angle