Bad Astronomy: Misconceptions and Misuses Revealed, from ...

THE DISASTER THAT WASN’T 127
All the planets in the solar system, including the Earth, orbit
the Sun. They move at different speeds, depending on how far they
are from the Sun. Tiny Mercury, only 58 million kilometers from
the Sun, screams around it in just 88 days. The Earth, almost three
times as far, takes one full year—which is, after all, how we define
the year. Jupiter takes 12 years, Saturn 29, and distant, frigid Pluto
250 years.
All the major planets in the solar system formed from a rotating
disk of gas and dust centered on the Sun. Now, nearly 5 billion
years later, we still see all those planets orbiting the Sun in the
same plane. Since we are also in that plane, we see it edge-on.
From our vantage point, it looks like all the planets travel through
the sky nearly in a line, since a plane seen edge-on looks like a line.
Since all the planets move across the sky at different rates, they
are constantly playing a kind of NASCAR racing game. Like the
hands of a clock only meeting every hour, the swifter planets can
appear to “catch up” to and eventually pass the slower-moving
ones. The Earth is the third planet out from the Sun, so we move
in our orbit faster than Mars, Jupiter, and the rest of the outer
planets. You might think, then, that they would appear to pass
each other in the sky all the time.
However, the planets’ orbits don’t all exist perfectly in the
same plane. They’re all tilted a little, so that planets don’t all fall
exactly along a line in the sky. Sometimes a planet is a little above
the plane, and sometimes a little below. It’s extraordinarily rare for
them to actually pass directly in front of each other. Usually they
approach the same area of the sky, getting perhaps to within the
width of the full Moon, then separate again. Often they never even
get that close to each other, passing many degrees apart. For this
reason, surprisingly, it’s actually rather rare for more than two
planets to be near each other in the sky at the same time.
Every so often, though, it does happen that the cosmic clock
aligns a bit better than usual, and some of the major planets will
appear to be in the same section of the sky. In 1962, for example,
the Sun, the Moon, and all the planets except Uranus, Neptune,
and Pluto appeared to be within 16 degrees of each other, which is
roughly the amount of sky you can cover with your outstretched