It's a good question when looking up at the night sky because the brightest "stars" may not be stars at all, but planets in our own solar system. You can begin a lifetime of planet watching on the next clear evening.

Since prehistoric times skywatchers have distinguished the five planets visible to the unaided eye from myriad stars overhead. They watched and named them, and recorded their appearance and movement. Today, we call them Mercury, Venus, Mars, Jupiter and Saturn. These planets appear as bright "stars" and they are often the brightest objects in the sky. (Excluding, of course, the Moon and Sun, which do not look like the nighttime stars.) In fact, the planets are sometimes referred to as "morning stars" or "evening stars" because they shine brightly in the dawn and dusk skies when the genuine stars have been washed from view by the light of the Sun.

More importantly, as the planets move along their orbits about the Sun, they appear to wander slowly across the background of the stars. In fact, the word "planet" derives from planetes, Greek for "wanderer." Ancient people in many cultures recognized and tracked these "wandering stars." They also noted that the Sun and Moon wander regularly across the background of the stars. Observing the objects over a few evenings (or mornings) will reveal this movement.

Today, we recognize the importance ancient people gave to these seven wanderers (the five visible planets, the Sun and the Moon) when we consider the origin of the seven-day week. In many different languages, the day names derive from the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. Our ancestors did not know that Earth was just another planet, so no day is named for Terra -- Earth. Nor are there days named for Uranus, Neptune and Pluto, more recently discovered planets -- but the search for these dimly lit worlds is a story for a future column.

Finally, the visible planets do not "twinkle" quite as much as stars. The difference is subtle, but the light from a planet is steadier to the eye. Stars appear as pinpoints of light, and the atmosphere distorts the light so that they twinkle. Look for a steady "star" when seeking a planet.

So, how can you find a planet? Begin by simply going outside at sunset and looking for the brightest objects. If the Moon is visible, draw a line eastward across the sky from where the Sun went down. The planets that are visible will be near this arc. For people living in the Northern Hemisphere, the arc is across the southern sky. For points south of the equator, like Australia and New Zealand, the Sun, Moon and planets will be found along an arc across the northern sky.

For early risers, look for planets just before dawn, starting at the position of sunrise and tracing the arc to the Moon. If the Moon is not visible, simply look for bright, steadily shining objects. The real stars twinkle and twinkle; the planets twinkle less, if at all. You will find zero to five visible planets at any given time. They may all be in the evening sky or all in the morning sky or split between evening and morning or aligned too closely to the Sun to see.

[uplink]

To be better prepared, you can use your computer to predict what's up and where to look.

Computer planetarium software such as Starry Night [a product of SPACE.com Canada], is a good place to start. It can be set for your location, the date and time, and allows you to create a map to identify any visible planets.

Or you can consult an online sky calendar like the Abrams Planetarium Skywatcher's Diary by David Batch, Department of Physics and Astronomy at Michigan State University. This diary gives day-by-day instructions for what's up and where to look. An illustrated version with a sky map, the Abrams Sky Calendar, is available by subscription. Another good source is Star Trak by Hal Kibbey of Indiana University, which offers a narrative description for skywatching each month.

Finally, you can find star maps with information on the current location of the planets in popular astronomy magazines: Sky & Telescope, Astronomy and Mercury.

In future columns, I'll write about the search for "invisible planets" in our solar system and beyond. But for now, go outside, look up and see another world. It's one of the first steps toward finding life beyond our home planet, Earth.