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Venus

After discovering the difficulties of Mercury, you will be glad to know that the second planet from the Sun, Venus, is - after the Sun and the Moon – the brightest object in the sky. A typical apparent magnitude for Venus is –4 and you probably have already seen it many times, although possibly identifying it as simply a very bright star.

Much about the rising and setting of Mercury also applies to Venus with the exception of one important statistic: distance from the Sun. Venus is on the average 0.72 a.u. from the Sun which puts it close to the Earth's orbit. Venus' maximum elongation from the Sun is about 46 degrees. Since its orbit is entirely within than of the Earth's, don’t expect to see Venus as midnight, but at various times it will set up to three hours after the Sun sets or rise three hours before sunrise. When Venus is at maximum elongation, it rises or set at a time of complete darkness and is very easy to see. You don't have even wait for maximum elongation to observe Venus because its inherent brightness which enables it to be seen even during the day except when it is close to the Sun. Venus can sometimes provide enough light to create shadows. My favorite time to look at Venus is just around sunset. The sky is beginning to darken to deep blue while along the ecliptic I may see a first quarter Moon midway between the horizons and toward the west the steady, shining light of Venus.

The main reason for Venus' brightness is its atmosphere. I didn't mention Mercury’s atmosphere because it doesn't have one. Venus, on the other hand, has a surfeit of atmosphere, mostly of carbon dioxide with generous doses of water and sulfuric acid. When we look towards Venus, we’re not seeing its surface but a thick layer of carbon dioxide clouds, starting at twenty miles from the planet’s surface and going up to thirty five miles. The clouds effectively reflect light from the Sun toward our eyes here on Earth, while at the same time prevent even close-by space probes from learning much about the surface is like.

Venus has a synodic period of 584 days so it takes over a year and a half to overtake the Earth. Once during every synodic period, Venus goes through a complete set of phases similar to those of the Moon. Mercury also goes through phases but since they are more difficult to see, only those of Venus will be discussed here. We may start Venus' synodic period at the moment of superior conjunction during which Venus, the Sun and then the Earth are on a straight line with each other. The face of Venus, directed towards the Earth (and the Sun), is completely lit up. We then have a full Venus. Besides being completely lit up Venus is then at its maximum distance from the Earth. After a quarter of a synodic period rolls by, Venus has moved a quarter of a turn in respect to the Earth. Actually, it has moved much more than a quarter revolution around the Sun, but the Earth has also moved, albeit more slowly during that period. After a quarter of a synodic period, Venus is not at is maximum elongation. It also has moved considerably closer to the Earth and will now appear larger to us. But now the Sun’s rays are striking it from the side as viewed from the Earth, just as if it were a quarter Moon. During the next quarter of a synodic period, Venus will gradually catch up with the Earth. Venus will appear larger and larger from the Earth as it approaches the Earth and less and less of its area will be lit. Just as it overtakes the Earth at the inferior conjunction, Venus will appear as a large, thin sliver.

Through your eyes Venus will appear as bright and steady star, while through binoculars, it will appear as a small white disk and you will be able to see its phases, particularly as it approaches to a "new" Venus phase. When Venus is at its inferior conjunction, it is closer to the Earth than any other planet. On very rare occasions, it makes a transit across the Sun's disk, the next one being on June 7, 2004.

During the first half of its synodic period, Venus is an evening star, gradually growing larger while at the same time, becoming a thinner sliver. During the second half of the synodic period, it is a morning star and the process is reversed. The disk of Venus grows smaller as the distance between the two planet increases, but it also changes from a crescent-like sliver back to a full circle. Sometimes it is possible to trace the entire cycle of disk to sliver and back to disk again, although the halfway points between superior and inferior conjunction when Venus' elongation is at its maximum are still the ideal time to see Venus.

Thanks to the thick cloud layer of Venus only about 80 features have been identified on its surface compared to over three times as many for its neighbor Mercury, none of which can be seen from the Earth. We are able only to see the upper stratum of Venus’ overcast skies as it passes through its phases.


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