All in a Row

Shortly after sunset, all of the visible planets can be seen spread out in aa line in the western sky: Mercury, Venus, Mars and Saturn (in that order) will be very close to each other. By next week they can be covered by your fingernail. Jupiter will be blazing nearby. A close alignment of the planets, like this one, is a beautiful sight. In fact, though, ther plasnets are always spread out in a line in the sky. This line is called the ecliptic.

Why are all the planets lined up this way? Well, about five billion years ago, a giant cloud of gas and dust in space was contracting under the force of gravity. As it contracted, it formed a spinning disk, like a phonograph record. The orbiting planets condensed out of this disk, with the Sun at the center. The Earth was one of these planets.

When we look at the Sun, or the planets, we're looking along the plane of this disk. It's as if the sky were the skin covering an apple with us at the center. Slice the apple through the center; the red line left by the skin is the ecliptic. This lineup of planets is a reminder of the original cloud of dust and gas that formed of our solar system.

We can see most of the planets with our unaided eyes. Mercury, Venus, Mars, Jupiter, and Saturn are visible without optical aid, though we need binoculars to see Uranus and Neptune and a large telescope to see Pluto. The blazing sun reflects our own position. The moon acts as if it too were a planet. But the real seeing is in our minds, not our eyes. The sky is a kind of mirror, and what we see when we look at it with understanding is our own history.

(4/24/02)