Astronomy Magazine
Glossary · Telescopes & Instruments
Lagrange point
Five places where two bodies' gravity lets a third sit still relative to both.
For any two orbiting bodies there are five points where a small third object can keep station with them. L1, between the Sun and Earth, is where solar observatories sit — an uninterrupted view of the Sun and about an hour's warning of what is coming. L2, 1.5 million kilometres out on the night side, is where Webb and Gaia are: Earth and Sun stay in the same direction, so a single shield blocks both, and the telescope's cold side stays cold forever. Neither is stable — a spacecraft there is balanced on a ridge and must nudge itself back every few weeks — which is precisely what limits Webb's lifetime.
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The night a new telescope is pointed at the sky and actually works.
Seeing
How much the atmosphere is blurring the stars tonight, measured in arcseconds.
Questions
What is Lagrange point?
Five places where two bodies' gravity lets a third sit still relative to both.
Why does it matter?
L2, 1.5 million kilometres out on the night side, is where Webb and Gaia are: Earth and Sun stay in the same direction, so a single shield blocks both, and the telescope's cold side stays cold forever. Neither is stable — a spacecraft there is balanced on a ridge and must nudge itself back every few weeks — which is precisely what limits Webb's lifetime.