What are "Seeing Conditions"

Version 1

"Seeing" is the term astronomers use to describe the sky's atmospheric conditions. The atmosphere is in continual motion with changing temperatures, air currents, weather fronts and dust particles. These factors cause the star images to twinkle. If the stars are twinkling considerably we have "poor" seeing conditions and when the star images are steady we have "good" seeing conditions. Poor seeing is most noticeable when observing planets and the moon, whereas deep sky objects such as nebulae and galaxies are less affected by poor seeing conditions. On deep sky objects, the most important factor is the transparency of the atmosphere (a measure of how dark the sky is on a given night-determined by clouds, dust, haze and light pollution). Seeing conditions and transparency will vary widely from site to site, from season to season and from night to night.

Version 2

Adapted from Nortons Star Atlas and Reference Handbook Seventeenth Edition © Gall & Inglis Ltd Edinburgh

The term "seeing" is used to indicate the quality of the observing conditions at the time of observation: it is an attempt by the observer to evaluate the effects of atmospheric turbulence and impurities on the results of the observing. An indication of the seeing is invaluable in comparing observations with others made on other occasions. The effects of poor seeing upon visual observation are, principally, small and erratic movements of the object and diffusion of its image (i.e. a star will have a point image in good seeing, with the diffraction pattern of the telescope if it is a bright star, but will dissolve into a large fuzzy blob in moments of poor seeing). In the case of a bright object having a visible disk, such as the sun, moon or planets, poor seeing results in the limbs appearing to 'boil'; they appear really steady only in the most perfect conditions.

The observer should record the seeing by the use of carefully chosen phrases, in the light of the observer's experience and comparison of good and bad seeing in the past - e.g. 'images unsteady and rather diffuse', 'limbs boiling', etc. As an additional record of the conditions it is useful in many types of observations, especially planetary work, to use a numerical scale. Numerous such scales have been devised over many years, but they are rather subjective and unreliable in use; it is therefore recommended that observers use the simplest - the well-tried scale devised by the great planetary observer E. M. Antoniadi. Most organised groups of planetary observers have adopted the Antoniadi Scale.

Antoniadi Scale

The observer indicates with a roman numeral the quality of the seeing according to the following scale:

I - Perfect seeing, without a quiver.
II - Slight undulations, with moments of calm lasting several seconds.
III - Moderate seeing, with larger air tremors.
IV - Poor seeing, with constant troublesome undulations.
V - Very bad seeing, scarcely allowing the making of a rough sketch.

Click here to download sample observer's log