Judy Armstrong, age 10, of Montgomery, Alabama, for her question:
How can we be sure which are the planets?
The experts tell us that we can tell a planet from a star just by staring at it. A star twinkles and a planet does not. But many ordinary star gazers, including Andy, do not trust this simple test to work all the time. So we use a few extra tests to be sure.
The planets, as we know orbit around the sun in widely separated traffic lanes. They travel at different speeds and the faster inner planets are forever catching up and passing the slower planets in the longer, outer traffic lanes. This is why the planets change their positions in our skies from night to night and month to month. When seen at their best, almost anybody can tell them from the stars. At their brightest, Venus and Mars, Jupiter and Saturn are bigger and brighter than the stars. And instead of twinkling, they shine with steady light. This clue is a helpful guide to spotting the planets among the stars. But sometimes it is hard to use. As the planets move farther away, they often grow too small and pale for human eyes to observe them in detail. Then they tend to lose themselves among the dim stars.
However, there are ways for a young star gazer to identify each planet that happens to be appearing in the night sky. First you get your bearings in the Solar System. Chances are you already know the distances, orbits and yearly periods of the major planets. This is your basic picture. The next step is to check where the orbiting planets happen to be right now in relation to the earth. For this you may use an almanac that has charts showing their positions on certain dates. Better still, you can get more details on this from one of those fascinating monthly magazines written for amateurs and young star gazers.
The charts show that Venus now shines brightly after sunset in the western sky. Saturn also appears this evening but look for it in the east. Through the night its bright steady eye climbs up and over the southern sky. You can see Mars and Jupiter before sun¬rise, rising close together in the east. But the dazzling rays of the rising sun soon outshine them and hide them from sight. When you know where to look, check your discovery with the nearby twinkling stars. If it shines with a steady light, it is a planet. Chances are you may want to go on checking the planet positions month by month. As you do so, you catch on to the changing celestial traffic. You discover where the planets are on their orbits and where they are moving in relation to the earth. Mars, for example, is now about 200 million miles from us on the far side of the sun. This is why it looks much dimmer than Jupiter in the morning sky. Venus is coming closer, getting bigger and brighter every night.
If you make a hobby of planet watching, you will need your charts plus a small three inch telescope. There is always some wonderous sight to be spied and your charts will tell you ahead of time where and what to look for. With a telescope, the main at¬traction is now Saturn. Train your eye piece upon it and you will behold the most beauteous planetary spectacle of the entire Solar System. The giant planet is now positioned and tilted to show its sparkling rings almost at their very best.