Sky watch: Orbital travels of the moon

If you've been paying attention to the evening sky recently, you most likely have spotted the two bright "stars" in the western sky; perhaps you even have noticed that they appear to be approaching each other as time goes on.

Well, if they were truly stars, I'd be worried. But they're not; they're planets -- the two brightest in the heavens. The lower of the two is Jupiter, the giant planet of our solar system, and the upper (and significantly brighter) is VenusJupiter and Venus appear to be changing positions in the evening sky because they, along with our Earth, orbit the sun, and our constantly changing viewpoint makes them appear to drift against the more distant and "fixed" stars.

Over the next week, another solar system body will enter the scene -- the moon. Because our celestial neighbor orbits the Earth only once a month, we can see its movement much more clearly.

The moon must make its 360-degree journey around our planet in one month. To do so, it must move about 12 degrees per day, and anyone who goes outside on consecutive evenings can see this change in position.

What's more, to make this journey, the moon must move one-half degree per day; that's equivalent to its apparent diameter in the sky. So if you're really perceptive, you might even notice this as the moon drifts by other, more distant, celestial bodies.

The moon enters the scene March 25, when it appears as a sliver of a crescent just below Jupiter. You'll need to get outside not long after sunset to spot it low in the southwestern sky at dusk.

By the next night, the moon will have moved eastward in its orbit and will appear as a slightly thicker crescent between the two planets. On March 27, it will lie just to the left of brilliant Venus -- and so on. Of course, this is an illusion caused by our inability to see true depth in the cosmos, but it results in a truly pretty picture.And speaking of pretty pictures, why not try your hand at photographing the spectacle? You don't need fancy or expensive equipment -- just a camera and tripod will do fine -- though a zoom lens will help you to produce a larger image.

This week, the trio appears low enough in the sky at dusk for you to photograph: frame the scene with a foreground subject -- a dramatic tree, building, sculpture or person, for example. If you shoot about 30 to 40 minutes after sunset, you probably can trust your camera's automatic settings. If not, try setting your camera to ISO 100 or 200 and shooting one-third of a second or so at f/8.

You'll need to adjust settings as you go, however, because the light changes rapidly at dusk, so don't be afraid to experiment. Remember that with digital photography, it costs you nothing; in fact, my very best friend on the camera is the "delete" button.

Enjoy the sky show, and please email me if you get some nice shots. I'd love to share in your success.