Grasping Venus' behavior involves first considering its location in the solar system.

As Venus travels around the Sun inside the Earths orbit, it alternates regularly from evening to morning sky and back, spending about 9- 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 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, Galileo started observing Venus through his crude telescope -- the best available at the time. 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, seeming 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 the movement of Venus is akin to watching an auto race (albeit in slow motion) from the grandstand: All the action takes place in front of you and its necessary to turn only a limited amount either way to see it at all. In contrast, for "superior" planets -- those located in orbits outside Earth's path around 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 to us and looms progressively larger in apparent size; the angle of sunlight striking it, as seen from our Earthly vantagepoint, 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 it 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 is a schedule of how Venus appearance will change in the coming months:

Nov. 29, 2003 Disk 90 percent illuminated: Despite the fact that it appears impressive to naked-eye viewers just after sunset, Venus is not much to look at through a telescope yet. Still on the far side of the Sun at a distance of 137 million miles (220 million kilometers) from Earth, it appears as a small, almost full silvery disk.

Jan. 31, 2004 Disk 75 percent illuminated: Because its still nearly 105 million miles (169 million kilometers) from Earth, Venus continues to appear relatively small in telescopes. Nonetheless, as it continues to come around from behind the Sun, the illumination angle has changed enough to show that it now appears distinctly gibbous (more than half lit) in shape.

March 29, 2004 Greatest Eastern Elongation: Venus arrives at its greatest angular distance east of the Sun (46 degrees). The brilliant planet now swings as far to the east (left) of the Sun as it will get from our 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 kilometers) from both.

In terms of apparent size, Venus now appears more than twice as large as it was at the end of January. In a telescope Venus is now a dazzling silvery-white "half-moon."

A good observing project is to try determining Venuss time of dichotomy -- when the planet appears exactly half lit. 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 gradually becomes a fat crescent while growing ever larger as it swings around in its orbit closer to Earth.

May 2, 2004 -- Greatest brilliancy: Venus is now truly dazzling, shining at magnitude -4.5. Only the Sun and Moon can be brighter on this astronomers scale, in which negative numbers represent brighter objects.

Venus continues to approach the Earth while appearing to curve back in toward the Sun in our sky. In a telescope it is now a big, beautiful crescent that grows larger and thinner with each passing night. The crescent can now be glimpsed even in steadily held binoculars. Venus now stands 42 million miles (68 million kilometers) from Earth, and on May 5 its disk will be 25 percent illuminated. It now appears nearly 40 percent larger in size than it did just one month ago.

May 19, 2004 Disk 12 percent illuminated: The crescent of Venus continues to narrow, but because it also continues to approach our Earth, it appears to greatly lengthen as well. Its now 32 million miles (52 million kilometers) away, nearer than even the recent historic close approach of Mars last August. But Venus is now in a rapid plunge down the sky toward the Sun. Compare the appearance of its two cusps. Can you make out the crescents "cusp extensions" -- threadlike wisps of light extending beyond the crescents points?

May 25, 2004 Disk just 6 percent illuminated: To see Venus now, it is critical to try and locate it 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 stands about 15 degrees above the west-northwest horizon. Your fist on an outstretched arm covers roughly 10 degrees of sky. Venus sets about 100 minutes after the Sun.

Now 29 million miles (47 million kilometers) from Earth, Venus is becoming more and more aligned between us and the Sun and so is turning more and more of its dark side toward us. In another week it will be all but gone from the evening sky.

June 8, 2004 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 first transit of Venus since Dec. 6, 1882. In early 2004, SPACE.com will provide a complete guide to the transit.