The nine planets whirl merrily around the sun in an endless hoedown. Their orbits are far apart and each planet travels at its own speed. Their positions in relation to each other change every minute. What's more, we only can figure their paths from the earth which is always whirling around its own orbit.

The orbits of the planets are more or less on a level with the sun's equator. However, the orbits are slightly tipped, some more so than others, so that the whirling Solar System is not quite as flat as a carousel. They are also slightly oval, some more so than others. They are separated by millions of miles except where Pluto swings nearer the sun and comes inside the orbit of Neptune. What's more, the inner planets travel much faster than those farther from the sun.

The earth occupies the third orbit. Mercury and Venus, the two inner planets, are closer to the sun than we are, while all the outer planets are farther away. Mercury travels around at about 30 miles a second, the earth at about 18 miles a second and little Pluto pedals around the farthest orbit of the Solar System at only three miles a second.

The year of a planet is the time it takes to make one orbit and Mercury, on the inside track, makes about 1,000 orbits while Pluto, on the outside track, is making only one. The inside planets, then, are forever overtaking those on the outside tracks and passing them. For a moment, the two planets are in line with each other, but the inside planet then pulls ahead and orbits around to the opposite side of the sun.

The slower outer planets take longer to pass each other and while two of them are more or less in line, the inner planets will zoom past them several times.

The earth may pass Jupiter while Jupiter is passing Saturn and the three planets might, just might, make a straight line. However, remember that the orbits are slightly tipped and one of the three could pass a little above or below the other two.

T o get in line, the nine planets would have to be on the same side of the sun, which is very rare. Eight of them would have to be passing Pluto at the same time and it is not likely that all would pass on the same level. If this miracle did happen, Mercury and Venus would be directly between us and the sun. All the outer planets would be close together in the night sky   arid the amazing event would be over in a moment.

We see the planets against a background of fixed stars and they move in a belt of sky called the zodiac. During a year, we see the entire zodiac, which is a great circle of 12 signs or constellations, and the planets seem to wander from one sign to another. We can track them, even from the moving earth, from their movements against this background of fixed stars.