What is seeing in astronomy?

Seeing describes the turbulence of Earth's atmosphere and its effect on image sharpness in astronomical observation. Turbulent air layers refract and distort starlight, which leads to shimmering, blurred stars.

28.06.2026 6 min read

What is seeing? The invisible enemy of astrophotography

Q: How is seeing measured?

Seeing is measured in arcseconds ("). A value below 1 arcsecond is considered good, below 0.5 arcseconds excellent. Values above 3 arcseconds are very poor.

Q: Why do stars twinkle but planets do not?

Stars appear as point sources and react sensitively to atmospheric turbulence. Planets have a measurable disc whose light comes from different angles and partly averages out.

Q: What influences seeing?

Local heat sources such as asphalt and buildings, near-ground air layers and the jet stream at 8-12 km altitude. On calm, clear winter nights seeing is often better than in summer.

The night is clear, not a cloud in the sky, the mount tracks smoothly. And yet: the stars quiver, planetary detail smears, the image looks as if someone held cling film in front of the eyepiece. Welcome to the club. The culprit is so-called seeing.

Seeing describes the turbulence of Earth's atmosphere and its effect on starlight. Air is not a homogeneous medium but a constantly moving mixture of layers of different temperature and density. Every time light from a star falls through these turbulent layers it is refracted, deflected and distorted. In the eyepiece the result looks like shimmering, dancing star discs.

Why do stars twinkle but planets do not?

A good question that every non-astronomer will ask as soon as you bring up the topic. Stars twinkle because they are so far away that even in the largest telescope they appear as point-like light sources. A single ray of light, sent through turbulent air, dances merrily back and forth.

Planets, on the other hand, have a measurable disc. The light arrives from many slightly different angles at once, and the disturbances partly average out. That is why planets twinkle less, even though they shine through the very same atmosphere. Under poor seeing they still suffer, though: detail smears, the belt structures on Jupiter turn soft, Saturn's rings lose their sharpness.

How is seeing measured?

The most common unit is the arcsecond ("). It describes the apparent diameter of a star's blur disc caused by atmospheric turbulence. The smaller the value, the better the seeing.

Professional observatories at good sites such as Paranal in Chile regularly reach values below 0.5 arcseconds. In Central Europe 1 to 2 arcseconds is a realistic average, on good nights sometimes less. Values above 3 arcseconds turn planetary imaging into a test of patience.

What influences seeing?

Seeing arises wherever air masses of different temperature meet. This happens on several levels:

Local seeing

The most common and at the same time most easily influenced factor. Asphalt, rooftops and concrete slabs store heat during the day and release it at night. The rising warm air right above the observing site causes local turbulence. Anyone observing from a roof terrace is fighting their own building. Meadows, fields and forest clearings are therefore much better suited than courtyards or car parks.

Ground-level seeing

The near-ground air layer up to a few hundred metres can also be turbulent, especially when air warmed during the day cools and sinks in the evening. This effect is typical of sunny summer days, on which the night brings poor seeing despite a clear sky.

Jet stream

At 8 to 12 kilometres altitude the jet stream races through the atmosphere at up to 300 km/h. When it runs directly above the observing site it causes strong turbulence and poor seeing, even in completely clear and calm weather at the ground. If the jet stream is far away, seeing can be exceptionally good.

Seeing in SkyGrade

SkyGrade obtains seeing data from the service 7Timer, which evaluates atmospheric models and provides forecasts for astronomical seeing. The value feeds directly into the deep-sky score and the planetary score.

For deep-sky objects such as galaxies and nebulae, seeing is less critical than for planetary imaging, because these objects are extended sources. Cloud cover and transparency matter more there. For planetary imaging, on the other hand, seeing is often the single most important factor, deciding between crisp detail and mushy discs.

Practical tips

Observe from meadows or in nature if possible, not on terraces or paved surfaces
Let your telescope acclimatise for at least 30 to 60 minutes before observing, so the optics reach ambient temperature
Planetary imaging benefits most from good seeing, deep-sky imaging somewhat less
Check the jet stream: if it is not over your region, chances of good seeing are good
Summer nights often have worse seeing than winter nights, but the nights are shorter and warmer