When Jupiter is near opposition digital photographs of it can be useful for some interesting activities. One of the visual characteristics of Jupiter is the apparent difference between its polar and equatorial diameters. This seeming flattening of the poles could be an optical illusion due to the vivid horizontal banding of clouds or it could be a true physical difference between the polar and equatorial dimensions. We can determine which by photographing the planet and measuring its equatorial and polar diameters.
Proceed as described in the previous sections. Take several pictures, download them and save the best. Use a metric scale to measure the polar and equatorial diameters of the planet on your best print. They can also be measured on a computer monitor using the linear scale described in Chapter 7. The equatorial diameter of Jupiter is 142,800 km. You can use that value to establish a print scale km/mm and then calculate the polar diameter. These measurements can also be made, with the linear scale from Chapter 7, on a computer monitor. Figure 8.12 is an illustration of the scale overlaying the image of Jupiter.
In addition to sketching the appearance of the Jovian cloud belts, changes in their angular width and the latitudes of their borders can be measured by overlaying the planet's image with the coordinate grid from Figure 7.6. The procedure is the same as that used for measuring lunar libration.
Drag the grid to overlay Jupiter's image on the monitor. Then zoom in to sufficiently enlarge the image. Stretch the grid to fit the image and rotate it until the latitude lines on the grid are parallel to the cloud belts. Finally, measure the latitudes of the cloud belts and the angular widths of belts and zones. The image will be small but measurable.
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