From Derek Bolton
In relation to Stuart Clark’s story on the Lagrangian points (21 February, p 30), the gravitational fields of the Earth and sun do not cancel each other at the Lagrange points. They add up to a net acceleration towards the sun that is of the same magnitude as that experienced by the Earth. This results in an orbital period exactly the same as the Earth’s, so an object at that point maintains its position in relation to the Earth-sun pair.
There is one point where the two gravitational fields truly cancel out, 260,000 kilometres from Earth in the direction of the sun – much closer than any of the five Lagrangian points. At this true cancellation point, the trajectory of any orbiting body would briefly become a straight line.
A Lagrangian or cancellation point does not necessarily trap anything that falls into it: the L1, L2, L3 Lagrangian points and the true cancellation point are not flat but saddle-shaped, so cannot trap material. The L4 and L5 regions are cup-shaped and so, as you point out, they can.
Birchgrove, New South Wales, Australia
