Ever since it returned to the evening sky just over a month ago, our "sister planet," as Venus is often called, has been the centerpiece of the current evening sky.
As Venus travels around the sun inside the Earth's orbit, it alternates regularly from evening to morning sky and back, spending about 9 1/2 months as an "evening star," and about the same length of time as a "morning star."
Some ancient astronomers actually thought they were seeing two different celestial bodies. They named the morning star after Phosphorus, the harbinger of light, and the evening star for Hesperus, the son of Atlas. It was the Greek philosopher and mathematician Pythagoras who first realized that Phosphorus and Hesperus were one and the same object.
To the ancients, such behavior was puzzling and was not really understood until the time of Galileo. After moving to Pisa in the autumn of 1610, he started observing Venus through his crude telescope.
One evening, he noticed that a small slice seemed to be missing from Venus' disk. After several more months, Venus appeared in the shape of a crescent — in other words, it seemed to display the same phases as the moon. This was a major discovery, which ultimately helped to deliver a deathblow to the long-held concept of an Earth-centered universe.
Venus wanders only a limited distance east or west of the sun, since, like Mercury, it is an "inferior" planet (orbiting the sun more closely than Earth does). Watching its movement is akin to watching an auto race from the grandstand: all the action takes place in front of you and it’s necessary to turn only a limited amount either way to see it at all. [Amazing Photos of Venus]
In contrast, for "superior" planets (those located in orbits beyond the Earth from the sun), viewers on Earth are like the pit crews inside the racetrack who must turn in all directions to follow the cars.
When Venus is on the opposite side of the sun from us, it appears full (or nearly so) and rather small because it is far from us. But because Venus moves with a greater velocity around the sun than Earth, it gradually gets closer and looms progressively larger in apparent size; the angle of sunlight striking it as seen from our Earthly vantage point also appears to change as well.
Ultimately, as Venus prepares to pass between the Earth and the sun, it appears as a thinning crescent. And since, at this point in its orbit, it is nearly six times closer to us compared to when it was on the opposite side of the sun, Venus appears much larger to us as well.
Here then, is a schedule of how Venus' appearance will change during the coming months:
2011 November 26 – Disk 90% illuminated: Despite the fact that it appears impressive to the naked eye, Venus is not much to look at through a telescope. Still on the far side of the sun, at a distance of 136 million miles (219 million kilometers) from Earth, it appears a small, almost full silvery disk.
2012 January 28 – Disk 75% illuminated: Because it’s still nearly 105 million miles (169 million km) from Earth, Venus continues to appear relatively small in telescopes. Nonetheless, as it continues to come around from behind the sun, the illumination angle will change enough to show that it now appears distinctly gibbous in shape.
2012 March 27 – Greatest Eastern Elongation: Venus arrives at its greatest angular distance east of the sun (46 degrees) on this date. The brilliant planet will now swing as far to the east (left) of the sun as it will get from our Earthly viewpoint, setting four hours after sundown and continuing to wane in phase as well as slowly enlarging in size.
In our solar system geometry, Venus now makes a right angle with both the sun and Earth, and is equally distant (66 million miles or 106 million km) from both. In terms of apparent size, Venus will now appear more than twice as large as it was at the end of January. In a telescope, Venus will now appear as a dazzling silvery-white "half-moon." A good observing project is to try to determine Venus’ time of dichotomy: when the planet appears exactly half lit. [The Greatest Mysteries of Venus]
The dichotomy of Venus usually occurs when Venus should still appear slightly gibbous, roughly a week before greatest eastern elongation. In the nights that follow, it will gradually become a fat crescent while growing ever larger as it swings around in its orbit closer to Earth.
2012 April 30 -- Greatest Illuminated Extent: Venus will now be at the pinnacle of its great brilliance. It is so bright now that it can be seen easily with the naked eye in a deep blue, haze-free afternoon sky. It will continue to approach Earth while appearing to curve back in toward the sun in our sky.
In a telescope it will now appear as a big, beautiful crescent that grows larger and thinner with each passing night. The crescent will now be visible even in steadily held binoculars. When Venus displays a full (or nearly full) disk, it appears relatively small. Conversely, when it appears very large, it’s a very narrow crescent. At this point though, we’re at the midpoint between these two extremes; hence the term "Greatest Illuminated Extent."
Venus will now stand 42 million miles (66 million km) from the Earth, and on May 2, its disk will be 25 percent illuminated. The planet will now appear nearly 40 percent larger in size than it did just one month ago.
2012 May 16 – Disk 12% illuminated: The crescent of Venus will continue to narrow, but because it will also continue to approach Earth, it will appear to greatly lengthen as well. Venus will now be 32 million miles (51 million km) away. But Venus will now be in a rapid plunge down the sky toward the sun. Compare the appearance of its two cusps. Can you make out the crescent's "cusp extensions" — threadlike wisps of light extending beyond the crescent’s points?
2012 May 23 – Disk just 6% illuminated: It will now be critical to try to locate Venus as early as possible when it is still high in the sky in a steady atmosphere. Well before sunset is best. At sunset, as seen from mid-northern latitudes, Venus will stand about 15 degrees above the west-northwest horizon and will set about 100 minutes later. Now 29 million miles (46.6 million km) from the Earth, Venus is becoming more and more aligned between us and the sun, and as such, will turn more and more of its dark side toward us. In another week it will be all but gone from the evening sky.
2012 June 5 – Inferior Conjunction: Venus will finally transition from an evening to morning star and will appear to pass directly between the Earth and the sun on this day. The result will be a very rare “transit”: Venus will be seen in silhouette as a black dot, moving across the disk of the sun. This will be the last transit of Venus until Dec. 11, 2117 ... and the last visible from North America until Dec. 8, 2125.
SPACE.com will have more to say as we draw closer to this very special celestial event. Stay tuned!
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.