By De Pree Ch., Axelrod A.
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We define a day as a period of 24 hours—but not just any 24 hours. Usually, by a “day,” most people, and certainly ancient cultures would have meant the period from one sunrise to the next. 32 04 1981 CH03 6/11/01 8:58 AM Page 33 Chapter 3 ➤ The Unexplained Motions of the Heavens But how long is a day, really? What do we mean when we say a day? It turns out that there are two different kinds of days: a day as measured by the rising of the sun (a solar day), and a day as measured by the rising of a star (a sidereal day).
In fact, you can use this same trick (if sufficiently distant) to crush cars, or planes flying overhead. All because of the fact that as things get more distant, they appear smaller— their angular size is reduced. The surface of the earth is real and solid. You can easily use absolute units such as feet and miles to measure the distance between objects. The celestial sphere, however, is an imaginary construct, and we do not know the distances between us and the objects. In fact, simply to locate objects in the sky, we don’t need to know their distances from us.
Venus is closer to the sun than we are, and that fact keeps it close to the sun in the sky (never more than 47 degrees, or about a quarter of the sky from horizon to horizon). The best times to observe Venus are at twilight, just before the sky becomes dark, or just before dawn. The planet will be full when it is on the far side of the sun from us and crescent when it is on the same side of the sun as we are. With a telescope on a dark night, you may be able to observe the “ashen light” phenomenon.
Astronomy by De Pree Ch., Axelrod A.