PICKERING SCALE:

1. Image usually about twice the diameter of the third ring.

2. Image occasionally twice the diameter of the third ring.

3. Image about the same diameter as the third ring, and brighter at the center.

4. Disk often visible, arcs (of rings) sometimes seen on brighter stars.

5. Disk always visible arcs frequently seen on brighter stars.

6. Disk always visible, short arcs constantly seen.

7. Disk sometimes sharply defined. (a) Rings seen as long arcs. (b) Rings complete.

8. Disk always sharply defined. (a) Rings seen as long arcs. (b) Rings complete, all in motion.

9. (a) Inner ring stationary. (b) Outer rings momentarily stationary.

10. Rings all stationary. (a) Detail between the rings, sometimes moving. (b) No detail between the rings.

Users of refractors may use this scale of classification, since American astronomer W.H. Pickering, devised this scale on the basis of his observations, using a 5-inch refractor. Seeing is generally consider Very Bad: 1-3, Poor: 4-5, Good: 6-7, Excellent: 8-10. Pickering scale was based on the appearance of star disks, and is not very suitable for planetary work. The Greek Astronomer E. M. Antoniadi who is famous for his work with the planets Mercury and Mars, produced the following scale for planetary work.

ANTONIADI SCALE:

I. Perfect seeing, without a quiver.

II. Slight undulations, with moments of calm lasting several seconds.

III. Moderate seeing, with large tremors.

IV. Poor seeing, with constant troublesome undulations.

V. Very bad seeing, scarcely allowing the making of a rough sketch.

Personally I prefer to use the Antoniadi scale due to its direct application to a planetary body such as the Moon. Other wise the observers needs to turn his scope onto a nearby star to make the determination using Pickering method.