The Lagrange Points

The ring is Earth's orbit, and the five marks are the Lagrange points: places where the Sun's and Earth's pull together hold a small body in step with us, going round once a year. L1 and L2 sit on the line through the Earth, drawn far from it here; really they are 1.5 million km out.

Run half a year. All five go round with the Earth, holding their places: L3 always hidden behind the Sun, L4 always 60° ahead, L5 60° behind. They are fixed to the Earth's orbit, not to the stars.

Closer in. L1, on the sunward line, is where solar observatories watch the Sun with the Earth never in the way. L2, on the far end of the same line, keeps the Sun, Earth and Moon all behind a telescope.

Back on the ground: the far end of that line is the point straight away from the Sun, high at midnight. The Webb telescope works out there at L2, so it is in your sky tonight, too faint to see, on the ecliptic in Gemini's feet.

Webb is not parked on the point. L2 is a saddle, and a craft left there drifts off within weeks, so Webb circles it on a loop hundreds of thousands of km wide, about the size of the ring, firing its thrusters every few weeks to stay on it.

L1 to L3 are saddles too, but L4 and L5, the far corners of two equilateral triangles on the Sun–Earth line, are stable: what drifts in stays. L4 has a resident, the asteroid 2010 TK7, and Jupiter's L4 and L5 hold thousands of Trojans.