As Chandrayaan-2 is well on its course, the Moon’s orbit is shifting away from the Earth at 3.8 centimetres a year. This is not a new development and it does not pose any problem for the lunar probe or communication with it. A signal from the Earth will still reach the Moon at the speed of light viz., 1.27 seconds, as the average distance to be covered would be 384, 000km or about 30 Earth diameters. It has become possible to observe the small increase in the distance of the Moon’s orbit accurately because the astronauts of Apollo 11, 14 and 15 missions had left retro- reflectors on the lunar surface.

    The reflectors are still in good shape, as there is no air or pollution there. The panels reflect the incoming laser light sent from the Earth in the same direction they came from. Each of these panels has 100 corner cubes placed in a 46cm square aluminium base. It is a key to measure the distance to the Moon, as the time taken....