Antoniadi scale

The Antoniadi scale is a system used by amateur astronomers to categorise the weather conditions when viewing the stars at night.

The Antoniadi scale was invented by Eugène Antoniadi, a Greek astronomer, who lived from 1870 to 1944. Living most of his life in France, he spent his time viewing Mars from Flammarion's observatory. He was very prestigious and was eventually given access to the Meudon Observatory, the largest of the time.

Now the scale is seen as the metric system of astronomy, being used as a default measurement all over the world.

The scale is a five-point system, with 1 being the best seeing conditions and 5 being the worst. The actual definitions are as follows:

Note that the scale is usually indicated by use of a Roman numeral or an ordinary number.

Astronomers have devised several methods for defining the quality of seeing apart from the Antoniadi scale, including:

...
Wikipedia