Venus at its brightest, season at its merriest

8:11 AM, Dec 6, 2013   |    comments
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( - Venus shines its brightest tonight, as it reaches the point known as the greatest illuminated extent. Ignoring possible changes to cloud conditions, Venus is brightest when it appears to take up the most area in the telescope eyepiece.

Although Venus has peaked in brightness, the next week or so will still be a good time to look for Venus in the pre-twilight sky. Even before the Sun sets, Venus is easily visible. I saw it Wednesday night while the Sun was still up, shining through thin clouds. Look for Venus in the southwest about one-third of the way up the sky during twilight.

There are two types of conjunctions - times when a planet is aligned with the Sun. Inferior conjunctions occur when a planet passes bybetween the Earth and the Sun. Venus and Mercury are the only planets that have inferior conjunctions.

All of the planets pass by the Sun on the far side of their orbits. Since Mars, Jupiter and the other outer planets only have one kind of conjunction, it is simply called a conjunction. With Venus and Mercury, we call crossings on the far side of the Sun superior conjunctionsto distinguish it from the inferior conjunctions.

During the days between the superior conjunction of Venus and the following inferior conjunction, Venus may be seen as the Evening Star. This is called the evening apparition of Venus.

The current evening apparition began after March 26, 2013, when Venus was invisible on the far side of the Sun during its superior conjunction. Right afterward, Venus is too close to the Sun to be visible to the unaided eye. Eventually - sometime in April, 2013 - Venus became visible very low in the bright twilight after sunset.

During its apparition Venus exhibits phases, much as the Moon does. At the beginning of the evening apparition, Venus appears as a full disk on the far side of the Sun from Earth. As the evening apparition progresses, Venus moves around its orbit and begins to get closer to Earth. At the same time, the relative angle (at Venus) between Earth and Sun changes, too. Venus grows a shadow along its back edge and becomes a gibbous phase.

At greatest elongation, Venus appears as far from the Sun as possible. At that time, its appearance in a telescope is much like a featureless half-moon. The phase is 50 percent illuminated. Later, a crescent phase begins where less than half of its shining Sun-illuminated clouds are visible.

All this time, Venus is getting closer to Earth, increasing the angle taken up by its apparent disk. The combination of overall size plus the phase determines the apparent lit area, the illuminated extent, and hence its brightness. The maximum point is the greatest illuminated extent.

Venus just happens to be at its greatest illuminated extent tonight. From here until inferior conjunction, it will continue to increase in apparent diameter, but the phase is now dwindling at a faster rate, thereby reducing the overall illuminated extent.

By the end of the first week of January 2014, Venus will become lost from view as it speeds toward inferior conjunction on Jan. 11, 2014. After the inferior conjunction, Venus will return as the so-called the Morning Star. This begins its morning apparition.

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