The plane of the Earth’s orbit around the sun is called the ecliptic. The plane of the ecliptic projected onto the stellar sphere marks the sun’s annual path in front of the background stars. Although the sun appears to move eastward through the stars at about one degree per day, this apparent motion is really a reflection of the Earth orbiting the sun.
The ecliptic is an important reference and is often highlighted on sky charts. Because the planets of the solar system circle the sun on nearly the same plane that the Earth circles the sun, the planets are always found on or close to the ecliptic. The plane of the moon’s orbit around Earth is only somewhat askew to the plane of the ecliptic, so the moon is always found on or near the ecliptic, too. When the new moon aligns with the ecliptic, we have a total eclipse of the sun. When the full moon aligns with the ecliptic, we have a total lunar eclipse.
Where is the ecliptic in relation to the Milky Way?
The equator (or plane) of the Milky Way galaxy is tilted by about 60o to the plane of the ecliptic. Quite by coincidence, the ecliptic intersects the galactic equator on or near the June and December solstices. But does that mean the Earth literally crosses the plane of the Milky Way’s disk at these times? The answer is no!
The ecliptic as marked on sky charts represents an outward projection of the Earth’s orbital plane onto the distant stellar sphere. By galactic standards, the Earth’s orbit around the sun is so puny that it shrinks to the size of a point. Earth’s orbit is about 16.5 light-minutes in diameter, whereas the diameter of the Milky Way disk is a whopping 100,000 light-years wide.
The vastness of the Milky Way dwarfs our solar system to almost nothingness. If the solar system, from the sun out to Pluto, were shrunk to the size of a quarter, the Milky Way galaxy would be the size of North America.
The three major great circles on the so-called celestial sphere – the ecliptic, celestial equator and galactic equator – make up an elaborate but convenient fiction of a bounded sphere above our heads and beneath our feet. The equally-large yet imaginary great circles enable astronomers to create a viable coordinate system for locating the positions of solar system bodies, stars and deep-sky objects.
By galactic standards, the Earth and solar system reside right next to the galactic plane. But by solar sytem standards, we’re far from the plane of the galactic disk. Astronomers estimate that we’re several dozen light-years north of the plane of our Milky Way galaxy.