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# Phases of Venus: Changes in Shape and Size (page 2)

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Author: Janice VanCleave

1. Find a way to determine where Venus is in its orbit. Observe the phases of Venus to see whether Venus is waxing or waning. Binoculars that magnify 10× are powerful enough to see Venus's phases when it is close to Earth (a few days before and after inferior conjunction). At other times, you will need a telescope of at least 50×. For best results, use a 100× telescope with a dark blue or green filter to cut the Sun's glare. Diagram the position of Venus in relation to the Sun and Earth as shown in Figure 17.2. Note that the greatest elongation for Venus is about 47°. For information about viewing Venus, see Richard Moeschl, Exploring the Sky (Chicago: Chicago Review Press, 1993), p. 272. CAUTION: Only make observations when the Sun is below the horizon. Never look directly at the Sun because it could permanently damage your eyes.
2. An arc is a segment of a circle, which measures 360°. Each degree can be further divided into 60 parts, called minutes of arc, and each minute of arc can be divided into 60 parts called seconds of arc. The width of Venus's lighted part ranges from 10 to 61 seconds of arc in angular size. The angular size is 10 seconds of arc when Venus is near superior conjunction and 61 seconds of arc when approaching inferior conjunction. This information as well as the elongation, magnitude, and percentage of illumination of the planet appears in Sky & Telescope magazine each month and on the website www.galaxies.com. Science Fair Hint: Use this information, along with your diagrams and observations, to prepare a table for your display.
3. Try measuring the angular size of Venus using the techniques described in Chapters 1 and 2 of this book. CAUTION: Only make observations when the Sun is below the horizon. Never look directly at the Sun because it could permanently damage your eyes.

### Get the Facts

Venus is bright partly because it is close to Earth. At its nearest, it lies about 100 times farther away than the Moon. But another reason is the reflectivity of its surface. Albedo is a measure of an object's reflective power. What makes Venus so reflective? What is its albedo and how does Venus's albedo compare to that of the Moon or Mercury? For information, see Patrick Moore, Exploring the Night Sky with Binoculars (New York: Cambridge University Press, 1986), p. 184.

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