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Glossary · Telescopes & Instruments
Photometry
Measuring precisely how bright something is, repeatedly, and watching for change.
Photometry is the least glamorous and most productive technique in astronomy. Exoplanet transits, variable stars, supernovae, asteroid rotation and gravitational microlensing are all brightness measured against time. The difficulty is calibration: the atmosphere, the instrument and the detector all vary, so the real skill is measuring a target against comparison stars in the same frame so the errors cancel. Space telescopes exist partly because the atmosphere makes precision photometry so hard. The precision now routinely reached from space is a few parts per million, which is what it takes to see an Earth-sized planet cross a Sun-sized star — a dip of about one part in ten thousand, lasting hours, against a star that is itself varying. Kepler found most of its planets by staring at the same field for four years and doing arithmetic.
Where it has come up lately
Related terms
Lagrange point
Five places where two bodies' gravity lets a third sit still relative to both.
Adaptive optics
Bending a mirror hundreds of times a second to cancel out the atmosphere.
Interferometry
Combining separate telescopes so they resolve detail like one enormous dish.
First light
The night a new telescope is pointed at the sky and actually works.
Questions
What is Photometry?
Measuring precisely how bright something is, repeatedly, and watching for change.
Why does it matter?
The precision now routinely reached from space is a few parts per million, which is what it takes to see an Earth-sized planet cross a Sun-sized star — a dip of about one part in ten thousand, lasting hours, against a star that is itself varying. Kepler found most of its planets by staring at the same field for four years and doing arithmetic.