The significance of lunar eclipses

 

          By observing eclipses of the Moon – or, more precisely the shape of the shadow cast by the Earth upon the lunar surface – it can be deduced that the Earth is a sphere. In the third century BC, Aristarchus of Samos calculated the diameter of the Moon by measuring how long it took to pass through the Earth’s shadow. He arrived at a reasonable estimate of 4,600 km, the actual value being 3,476km. Looking back at Eratosthenes’ calculation of the size of the Earth, Aristarchus applied a trigonometric method to his result and calculated the distance from the Earth to the Moon. Hipparchus refined these estimates in about 150 BC, and assigned values of 4,200 km for the diameter of the Moon, and 425,000 km for its distance. It was left to Ptolemy, in the second century AD, to calculate values of astonishing accuracy: 3,700 km for the lunar diameter, and 376,000 km for the distance.

          In the seventeenth century, a partial solution to the urgent problem of the determination of longitude involved eclipses of the Moon, which could be observed from many places simultaneously; and because of these measurements, in 1634 the length of Mediterranean Sea was found to be 1,000 km less than had previously been thought.

          By studying light falling upon the Moon during eclipses, Daniel Barbier and Daniel Chalonge of the Institut d’Astrophysique in Paris discovered that part of the Earth’s ozone layer – so vital for the existence of life on our planet – is confined to a level between 50 and 80 km in altitude. Another modern development, which takes advantage of the absence of reflected sunlight during eclipses, is the use of lasers, pointed at mirrors left on the Moon during the Apollo and Lunakhod missions, to make precise measurements of the Moon’s secular acceleration and the slowing of the Earth’s rotation. Astronomers at the Lick Observatory first made such measurements in 1969, but not without difficulty, due to a 545-m error in the position of the observatory. Lunakhod 2 left behind a laser reflector, the TL2, with 14 cataphotes, and the Apollo 11 and Apollo 14 missions deposited two 100-element reflectors in the Mare Tranquillitatis and at Fra Mauro. The Apollo 15 mission set up a laser reflection panel with 300 elements near the Hadley Rille site, and laser measurements were carried out by the McDonald Observatory in Texas, in conjunction with instruments at Haleakala in Hawaii, and at Cerga, near Grasse in France. Since we know that the speed of light in a vacuum is 299,792,458 m/s – a value used as the basis for the standard metre – light-travel time to and from the Moon can be determined to an accuracy of a few tens of picoseconds by the laser method, using a caesium clock. This interval, of about 2.5 seconds, defines the distance to the Moon to within 10 mm. As a result of such experiments, we now know that the Moon is escaping the Earth’s gravitational attraction by one or two metres per century. What is measured here is the segment between a point on the Earth and a point on the Moon, and the value for this distance is determined not only by the distance to the Moon as it pursues it eccentric orbit; it is also dependant on lunar libration, the plasticity of the Moon, the influence of the Sun and the rotation and tides of the Earth. Thus it is that the compilation of data has led to a rich harvest of astronomical information.