The Planets in Opposition
The Solar System >
The operative word this week is opposition. Saturn was at opposition last week. Jupiter will be there Monday night. Republicans and Democrats, it seems, are there continuously.
In astro parlance, opposition means a planet is on the opposite side of
the earth as the Sun. For those of us who enjoy peering at planets through
our telescopes, this is good news. They are then at their closest and
brightest, and rise as the sun goes down meaning many hours of observing
every night.
But opposition means something else, too. You’ll need your thinking
cap here.
This month both Saturn and Jupiter started moving west in the sky. So
what? So this. The outer planets all move around the Sun from the western
part of the skies to the eastern side from Earth’s point of view.
All year we’ve seen these two slowly crawl through Aries eastward
into Taurus.
And then suddenly last month they stopped! But wait, there’s more!
Both Saturn and Jupiter then started moving backwards, toward the west!
And they will continue this motion until February. Then they will stop
again and resume their original eastward motion! What is going on? Let’s
take a trip down History Lane.
Before the Renaissance this motion, called “retrograde,”
was a tough one to explain. Back then - when the Earth was the center
of the universe - all the planets, the Sun, and the stars moved in perfect
circles around us. How could this backward motion be explained in those
days?
Ancient astronomer Ptolemy suggested that each planet
moved on a little circular orbit on top of its big circular orbit around
Earth. Not easy to picture, is it? Then imagine this:
You and a friend at at a park. Your friend has a ball tied to a string
and is swinging it around his head. And your friend is running in a big
circle around you, swinging the ball over his head in this park now filled
with curious onlookers.
Imagine now that you are not embarrassed.
Then in your mind’s eye, make your friend and the string become invisible.
But still picture the ball being swung on its invisible string by your
invisible friend running in a big circle around you. From your point of
view, the ball seems to go forward, then backward, then forward, then
backward against the background of trees and worried onlookers.
This is how the ancients explained retrograde, the unexpected backtracking of the planets. The big problem was: What was the invisible friend and the invisible string which held onto the planet? They couldn’t answer that.
But
then the sun became the center of the universe, and retrograde was easily
explained. We are closer to the Sun than Mars, Jupiter, and Saturn. We
move faster, as well. So as we catch up to them in our race around the
Sun (at opposition), we pass them up. As we pass them, they appear to
go backwards, just as cars appear to go “backwards” as we pass
them on the freeway.
You can observe this ancient ritual in the next couple months using the Pleiades as a reference point. The Pleiades are those six tightly packed stars near Jupiter and Saturn. Watch how the two seem to go westward until the beginning of next year. By the time we get to February, they will start “going east” again.
Need help picturing all this? Images and movies await you at firstlightastro.com. Go to the iColumn page.
Until next time, clear skies!
Mark Ritter is not in opposition to you writing him at ritter@firstlightastro.com.
Posted by Administrator at 2000.11.26 08:59 AM | Comments (0)
