This is an exciting week for skywatchers, because it will bring an old friend back into our evening sky: the planet Venus.

From our terrestrial vantage point, Venus appeared to go behind the Sun on Jan. 14. This alignment, called superior conjunction, rendered the planet invisible, mired deep in the brilliant glare of the Sun.

Now, with each passing day, Venus has been moving on a slow course toward the east and pulling slowly away from the Suns general vicinity. Finally, during this week, it should emerge as an evening "star" very low in the western twilight.

The challenge

But by March 1 the bright Venus is in the same location 55 minutes after sunset, giving less experienced skywatchers a fighting chance to get their first glimpse.

Continuing to swing east of the Sun during March, Venus will soon become plainly visible in the western evening sky even to the most casual observers. Appearing as a brilliant white star-like object of magnitude 3.9, our sister planet will set at least an hour after the Sun by the end of the first week of March.

Magnitude is the standard by which astronomers measure the apparent brightness of objects that appear in the sky. The lower the number, the brighter the object. The brightest stars in the sky are categorized as zero or first magnitude. Negative magnitudes are reserved for the most brilliant objects. The brightest star is Sirius (-1.4); the full Moon is -12.7; the Sun is -26.7.

Venus will get slowly higher each evening to adorn the western evening sky all during spring and summer. By early June, it will set as late as 2 hours after the Sun.

Interestingly, the planets greatest altitude at sunset will also occur in June, when the apparent path of the Sun, Moon and planets throughout the year, called the ecliptic, becomes nearly vertical with respect to the western horizon for observers in central and northern latitudes.

Venus' elongation, or distance from the Sun as seen from Earth, will be greatest on Aug. 22, when the planet is 46 to the east of the Sun. It is brightest in early autumn as it heads back down toward the Sun, reaching its greatest brilliancy for this apparition on the evening of Sept. 26 at magnitude 4.6.

No star or planet ever gets brighter.

Venus then quickly fades, vanishing from view in mid-October, and passes inferior conjunction on October 31st. Within a week it reemerges as a morning "star" in the southeast.

If you have a small telescope, you can see the phases of Venus -- just like on the Moon.

Phases are caused by the constantly changing illumination angle between the Sun, Earth and the object that is being illuminated.

You can readily demonstrate this for yourself by taking a tennis ball outside into bright sunlight. Imagine the ball represents the Moon, and your eye is the Earth. Hold the ball at arm's length. As you change its position relative to your eye and the Sun, you can see the ball going through phases, just like the Moon.

In fact, the next time you see the Moon in the daytime sky, get out the tennis ball and hold it at arm's length, carefully positioning it right next to the Moon. You'll immediately note that both the Moon and the ball will be displaying the same phase, since the illumination angle for both will be the same!

Between now and October, repeated observation will show the complete range of phases and disk sizes. Venus is currently full, being 99 percent sunlit on Feb. 22, and it will be a tiny, dazzling gibbous disk.

The term 'gibbous' is used to describe a phase between half and fully illuminated. It is a Latin word which directly translated means 'humped shaped,' which pretty much describes what the Moon looks like when it's midway between half and full.

Venus will become noticeably less gibbous by late spring. In late August, it reaches dichotomy, displaying a "half moon" shape. Then, for the rest of the year it will look like a large crescent as it swings near the Earth.

Indeed, those using telescopes will note that while the Earth-Venus distance is lessening, the apparent size of Venus disk will grow, doubling from its present size by Aug. 3.

When it has doubled again in size on Sept. 26, its large crescent shape should be easily discernable even in steadily held 7-power binoculars, a typical instrument that many people have stuffed in their closets.

More about Venus

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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.