What the scales record
How steady the atmosphere was, at a place and a time. Layers of air at different temperatures move across the line of sight and bend the light continuously; the result is an image that shimmers, swells and shifts.
The five-point scale is the shorter and the more commonly written in observing notes. It runs in Roman numerals from I to V, and it runs downward in quality: I is perfect seeing without a quiver, II slight quivering with calm moments, III moderate tremors, IV troublesome undulation, and V so unsteady that a rough sketch is barely possible.
The ten-point scale is finer, runs 1 to 10 upward — 1 very poor, 10 calm and sharp — and is defined by what the diffraction pattern of a bright star looks like at each step: whether the rings are complete, broken, or absent entirely.
Both are judgements made at the eyepiece, and both belong in a note beside what was observed, because an observation without its seeing is not comparable to any other.
What they do not promise
Nothing about transparency, which is a separate property of the same sky and frequently moves the other way. A perfectly steady night can be hazy, and a crystal-clear one can be unusable at high power.
Nothing about the instrument. On a poor night a large aperture and a small one converge, which is why a comparison between two telescopes made on a single evening proves much less than it appears to.
And nothing that persists. Seeing changes within an hour, so a figure recorded at the start of a session does not describe its end.
What they work with
Anything whose limit is fine detail: planetary and lunar observing, close double stars, high-magnification eyepieces, Barlow lenses and planetary cameras — where the whole lucky-imaging method exists precisely because seeing is intermittent.
On this site the scales appear wherever a page would otherwise imply that an instrument’s resolving power is available on demand.
The questions that keep arriving
Which way does each scale run?
They run opposite ways, which is the reason to state which one you are using. On the five-point scale, written in Roman numerals, I is perfect and V is barely usable. On the ten-point scale, 1 is very poor and 10 is calm and sharp. A note saying only “seeing 2” is ambiguous between excellent and dreadful.
How is seeing judged?
By looking at a bright star at high magnification and watching the diffraction pattern. A steady disc with visible rings is excellent; a boiling blob is poor. The ten-point scale describes the pattern at each step.
Does it matter for deep-sky work?
Far less than for planets. A faint galaxy is a soft extended object to begin with, so a little blurring costs little. Seeing decides planetary and lunar detail, and close double stars.
Last reviewed 17 September 2026