What is L2 and why does Herschel orbit around L2?
L2 is a special point which exists in any gravitational two-body system. The ‘L’ refers to the mathematician Joseph-Louis Lagrange (http://www-groups.dcs.st-and.ac.uk/%7ehistory//Mathematicians/Lagrange.html) Lagrange showed that there are five special points in a system like Sun/Earth where the Earth rotates around the sun. The first Lagrange point L1 is the point inbetween Earth and Sun where the Earth’s and the Sun’s gravity cancel out exactly. As a result, a celestial object would experience no pull at all either towards the sun or towards Earth. L2 is the second Lagrange point ‘behind the earth’, from the sun’s point of view. It is 1.5 million km away from the Earth. At this spot, sun and Earth pull in exactly the same direction and keep L2 orbiting around the sun at a constant distance from the Earth. However, unlike the Moon, which orbits the earth at a constant distant while the Earth orbits the sun, L2 will never cross the Earth’s orbit and get between sun and Earth. L2 will a
