Fig. 9.7. Jupiter's Great Red Spot and surrounding area, captured in a strip sketch by Clay Sherrod. Note that Clay made transit timings of the preceding edge, center, and following edge of the GRS. (Credit: P. Clay Sherrod).

Z o to the disk provided for drawing the planet. Intensity generally refers to the relative darkness or brightness of a feature. The A.L.P.O. uses a numerical scale to indicate the relative intensity of features seen.

The A.L.P.O. intensity scale is a numbered scale from 0 to 10, with 0 indicating the darkest feature and 10 the brightest. Many years ago, Phillip Budine stated these intensity values as:

10 Unusually brilliant zone 9 Extremely brilliant zone 8 Very bright zone 7 Bright zone 6 Slightly shaded zone 5 Dull zone 4 Dusky polar belt 3 Dark belt 2 Very dark belt 1 Extremely dark belt 0 Black, shadow of a planet

With the use of this scale, you can estimate the relative brightness of belts and zones and individual features, such as condensations, festoons, and bright ovals (Fig. 9.3).

With experience, you can estimate the intensity of features with good accuracy. The ability to make intensity estimates certainly improves with practice but might appear intimidating to the beginner, who might have difficulty getting started. In

my own experience I have found, at least during recent apparitions, the general overall intensity of the North Equatorial Belt (NEB) to be 3, with the relative brightness of the North Tropical Zone to be 7. Condensations along the north edge of the NEB to be 2, and bright ovals imbedded in the NEB to be 7.5-8, and so on. You should not approach these intensity estimates with any preconceived notion, but my description here can provide an idea of the process.

The application of the scale is certainly subjective, and one observer might estimate the intensity of a feature to be slightly brighter than another observer. However, with practice, you can learn to be consistent in the application of the scale as you become familiar with the appearance of the planet. In spite of this subjectivity, the intensity scale is a much more quantitative and accurate method than simple word descriptions, which can be rather vague or ambiguous at best. This quantitative scale has another major advantage; it can be statistically analyzed over time. And, in spite of the subjectivity, the effect of personal bias can be minimized when intensity estimates by a large number of observers during the same apparition are included in the statistical analysis.

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