Expert: Tom Whiting - 9/5/2006

Question
I don't understand the concept of the ecliptic plane. Is it an imaginary outline of the earth's orbit around the sun?

Answer
Hi Audrey,
 Yes, one can think of it that way...but the real definition,.....
it's the plane (like a table top - a 2 dimensional "surface")
formed by two points...the center of the sun and the center of the Earth, and that plane is extended out to infinity.
And yes, the Earth, or rather the Earth's orbit around the sun,  is always on the plane of the ecliptic, by definition....and so is the sun, by definition.

So yes, if we assume a stationary sun, it would be the PLANE
(not the outline of the orbit) that the Earth's elliptical orbit is always orbiting on.  
NOTE: not the Earth's equator, but only the "Earth's orbit"...
around the sun.  IF the earth's axis were not tilted but rather
straight up and down vertically, then the ecliptic plane and
the celestial equator would be Identical in our sky, but they are not because......
Since our planet has a tilted axis, 23 1/2 degrees from the
vertical, the ecliptic plane is not an extension of the Earth's
equator,.... there is a 23.5 degree difference between the
Earth's equatorial plane, and the ecliptic plane. By convention in the night sky, it's easier for us in astronomy
to keep the Celestial Equator....the Earth's equator extended
out into the night sky....at a  "constant" elevation, right over the equator, and allow the ecliptic plane to vary from
23.5 degrees above (north of), and 23.5 degrees south, of the celestial equator.  And where the two planes intersect,
that's where the sun is located in our sky on the first day
of spring on, or about March 21 (also called the Vernal Equinox and "First Point of Aries")...... and the first day of autumn, on or about September 23.

Since the entire Solar System is basically flat, and nearly,
on that same ecliptic plane, that is the "line" across our
night sky where all the planets and moon (and sun, by
definition) are located.  There is a small amount of deviation
(just a few degrees for most of the major planets, and 5
degrees for the moon's orbit),
so we in astronomy define a band 8 degrees (about 1/2
a full hand span at arm's length)  on each side
of the ecliptic plane, called the "zodiac band"....and that
is where you/we find all the other members of the Solar system, in our night sky with a few minor exceptions....some main belt asteroids and Pluto with it's many Kuiper asteroid Belt members can stray out of the zodiac band.

Of course, all these planes and lines are just for our easy reference to the night sky....for identification purposes only.
That is, they help form a "longitude and latitude" coordinate
system in our overhead sky with which we can easily locate all the other objects.  {The term longitude is replaced by
Right Ascension, and latitude with....Declination,  the Vernal
equinox acting as the "ZERO" line/point... in the sky...the "prime meridian"...if you wish, but that's another story.}

In reality, the sun is moving at high speed, like 200 miles
per second, around the center of the Milky Way galaxy,
thus all the planets' orbits are actually "cork-screws"
in space as we orbit a fast moving sun.  Plus the galaxy
itself is moving at high speed as we, the galaxy, orbit the
Virgo Supercluster of Galaxies out there some 65 million
lightyears....so we, the Earth and Solar System,  are never back to.....where we were....a year ago....
we are always 'cutting new ground' as we
move around the sun, and around the galaxy.

Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA