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Jeff Root · #5 (**permalink**) 13-November-2008, 01:18 PM

Grant,

Could you restate that to make clear that the plane referred to is not
the actual plane of the Galaxy's disk, but merely a line projected onto
Earth's sky of that plane? The alignment involves this projected line,
the position of the Earth, and the position of the Sun, but not the
position of the Galaxy.

I'll make a stab at it by way of analogy.

In place of the Galaxy, imagine a huge office building, wider than it is
tall, but still fairly tall. You are in a position where you can see it from
the side. Maybe you are at ground level, or on a slight hill, or a few
stories up in another building, but you aren't very far up or very far
down. Just like Earth in relation to the Milky Way Galaxy. You draw
an imaginary horizontal line through the center of the building. That
is, the centerline from your point of view. Someone higher up or lower
down than you would see the centerline in a slightly different place.

A window washer works on the outside of the building every day.
He starts at the top and works across from east to west. Each day
he moves his platform a little to get at the windows that are farther
down. When he finishes the windows on the bottom he starts working
his way slowly back up again. It takes him six months to wash all the
windows from top to bottom, and then another six months to wash
them all again, from bottom to top. Twice a year you see him cross
the imaginary centerline of the building. Those correspond to the two
times of the year when the Sun crosses the galactic plane. What is so
special about those times? Nothing. Nothing special happens to you
when the window washer crosses the centerline of the building, and
nothing special happens to Earth when the Sun crosses the centerline
of the Galaxy.

-- Jeff, in Minneapolis