The thickness of atmosphere the light passes through, as a multiple of the amount at the zenith (airmass 1). It is 2 at 30° altitude and nearly 6 at 10°: the image there is dimmer, redder and more turbulent. Hence the value of observing objects near culmination.
Related terms
- Zenith
The point in the sky directly overhead, at 90° altitude. That is where you look through the least air, so images are at their sharpest and brightest. Its opposite, below your feet, is the nadir.
- Culmination
An object’s crossing of the meridian, at the highest point of its path; the best time to observe it. For an object south of the zenith, its altitude then is 90° − latitude + declination. From London (51.5°), the Orion Nebula (−5.4°) culminates at about 33°.
- Seeing
The restlessness of the atmosphere that makes images dance and boil; it is also what makes stars twinkle. It is measured in arcseconds (the size of a star in a photo) or on scales from 1 to 10 (Pickering) or I to V (Antoniadi). It limits high magnification far more often than the instrument does.
- Atmospheric refraction
The atmosphere bends starlight and makes objects look higher than they are: about 0.5° on the horizon, barely 1′ at 45° altitude. When the Sun seems to touch the horizon it has in fact already set. Celestial navigation corrects for it.
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