Sky Watch: Planetary trio on display

The next couple of weeks offer early morning stargazers a great chance to check out our solar system's two innermost planets as they shine near each other in the dawn sky, along with the farthest planet visible to the unaided eye.

You'll have little trouble spotting the most brilliant of these: Venus. It glows with a luminous white light in the southeastern sky before sunrise and, except for the sun and moon, is the most radiant of all celestial objects.

Below and to its left, you should be able to spot Mercury in the southeastern horizon, appearing as a much fainter, yellowish "star." And midway up in the south-southwestern sky - just above and to the upper right of a similarly appearing star named Spica - lies the ringed planet Saturn.

Aim a small telescope toward these distant worlds, and you'll discover that you can see much more than is visible to the unaided eye. Venus and Mercury, for example, will show a quarter phase, but they don't always appear this way.

Over the next two months, Venus will appear to shrink in size and display more of a "full" phase as it rounds its orbit toward the far side of the sun. Mercury will do the same but, since it's the closest planet to the sun and whirls much more rapidly around its orbit, will appear to change its size and phase by the end of January.

It's always fun to ask stargazers why they believe that if Mercury is the nearest planet to the sun, it's not also the hottest of our solar system. That honor belongs to Venus, nearly twice as far from the searing heat of our star. The reason for this is rather simple. Venus is shrouded by a thick, cloudy atmosphere of mostly carbon dioxide that behaves like a blanket and holds in heat. Mercury has no atmosphere at all, so while it's still quite hot there, much of its daytime heat can escape back into space at night.

Before you pack up your telescope to head indoors, don't forget to aim it toward Saturn. We all know Saturn as the ringed planet, and its glorious ring system is quite visible to even the smallest of backyard telescopes. You might even get a glimpse of its largest moon, Titan, lurking nearby.