People are out looking around a lot on these warm summer evenings. "What is that bright star in the sky?" many have asked me lately. Before I answer, I must ask a question of my own. "What time of night are you referring to?" Usually the answer is, "Evening, just after dark." Then I need to ask, "Do you mean the star setting in the west, or the one rising in the east?" I have been getting questions about both of them, for we currently have two gleaming objects gracing our early evenings, but they are on opposite sides of the sky. Both of them are planets.
"If you are asking about the one low in the west as it gets dark," I reply, "it is the planet Venus, and it will get brighter over the next several months.""Even brighter than it is now?" they usually ask.
"Yes, much brighter. I hope you will watch it each clear evening and see how it steadily changes its place as it brightens, and you can watch it pull up on Mars to see the two planets side-by-side about the middle of October." Because Venus is coming into our sky from the far side of its orbit, it changes brightness slowly for several months as it hangs for a while following the sun down.
Then I always want those asking about such things to know the other bright planet in our sky right now. "If you are asking about the planet rising toward the east, that is Jupiter, named after the king of the Olympian gods." This is the very best time in 1997 to enjoy this brilliant planet, for it reached opposition Saturday, Aug. 9.
Opposition is when a planet with an orbit beyond Earth's is opposite the direction of the sun, rising as the sun sets and setting at sunrise. Another way to think about this is to remember that Earth, being much closer to the sun, is now moving between the sun and Jupiter. Thus, Jupiter is closest to us now. Although it begins to dim as Earth pulls away from Jupiter, the planet will remain nice and bright and easy to recognize throughout coming months. It will rise earlier each day and and progress toward the west in our night-time sky.
Jupiter has always been one of the dominant objects of Earth's sky for human eyes. Although the ancients had no way of knowing that this was the largest of the planets, they named it well. In their minds, the hurler of thunderbolts looked down from lofty Mount Olympus, so it seemed appropriate to associate him with the great luminary looking down from heaven, nearly always visible, and always brilliant. The planet only disappears for a couple of months when it is lost in the glare of the sun, being on the far side of its orbit as we view it.
There are many things the old Greeks did not know about this great light of heaven, but the more we study it the more appropriate it seems that they named it as they did. Jupiter has a diameter about 12 times that of Earth, yet it rotates on its axis more than twice while Earth rotates once. Jupiter has nearly 318 times more mass than Earth - indeed nearly three times more mass than all the other planets combined. Except for the sun itself, Jupiter is the giant of the solar system. It is nearly large enough to have become a star. Think about that.
If Jupiter had been a bit more massive we would be living in a binary star system. Or would we? If Jupiter had become a star, would life have been possible on Earth? Probably so, for Jupiter would have been a dim star, orbiting five times farther out from the sun than Earth is. For us, its distance constantly changes between four and six times the solar distance. As Earth sweeps between Jupiter and the sun it is four times farther from us than the sun, and when Jupiter is beyond the sun, on the far side of its orbit it is six times farther from us than the sun.
Thus, if Jupiter had been a star, orbiting just as it currently does, it would be an extremely bright but constantly changing light. How different things would be. We would not have had nearly as much darkness, for whenever Jupiter would be in the sky, it would cast its much greater brilliance into our otherwise dark night - far more light than the full moon we know. The famous science fiction author Arthur C. Clarke included the idea of Jupiter be-coming a star in his series of books, "2001," "2061" and "3001."
Although Jupiter does not truly rule the solar system, for that is the role of the sun, it does have a family of its own. At last count we know of 16 natural satellites orbiting Jupiter. None of these were known to the Greeks, for telescopes were required to see them, but as soon as Galileo looked at Jupiter through his lenses in 1610, he discovered four tiny dots accompanying the planet. That was all for the better part of 300 years, but as telescopes became better and bigger, another Jovian satellite was found in 1892. Additional ones were discovered in 1904, 1905, 1908 and 1914.
A pair of them were discovered in 1938, then another in 1951. Space science technologies discovered one in 1974 and 1979. Now we have seen these members of the Jovian family in some detail through the eyes of space probes, and they are among the most astonishing worlds we have ever seen. Each and every one of them is unique, crying out for further exploration. In addition to these larger bodies, we have learned through space programs that Jupiter sports a ring composed of tiny orbiting particles.
Out of space for this article, I can only hope that you will go out and locate Jupiter in our evening sky now. "You really can not miss it," as they say. Look somewhat south of east as it gets dark. You will have to wait longer if you have high mountains off in that direction. You will know it when you see this eye-dazzler, and once you have found it you can follow it for the rest of your life. As you enjoy it, think about its role in ancient history. Consider how much we have to learn about it; then contemplate the many secrets it still holds for future discovery through exploration.