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 great circle of the ecliptic intersects the great circle of the celestial equator at the equinox points, and intersects the great circle of the galactic equator near the solstice points . Galactic equator not shown on illustration.
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.
There’s no such thing as a coincidence.
Last I heard, the sun (and every other star in the Milky Way Galaxy) orbits the center of the galaxy. If it\’s true that \”we’re several dozen light-years north of the galactic plane\”, it\’s also true that the sun/solar system will be several dozen light-years SOUTH of the galactic plane after half an orbital period. Perhaps you\’d like to explain how we can do that without passing THROUGH the galactic plane…
Given that we\’ve crossed the plane twice in each galactic orbit for as long as the sun\’s been in existence, I guess it\’s safe to assume that nothing catastrophic happens at the stellar scale. Hell, galaxies are so sparse that \”interesting\” events are rare even during galactic COLLISIONS!
On the other hand, what about galactic debris? If there is roughly the same amount of \”stuff\” above the galactic plane as below, and everything changes sides in half an orbit- wouldn\’t it seem reasonable that collision probability is maximum at the equator?
Something the size of a small asteroid hitting the sun might be but a blip on the stellar scale, but on the planetary scale it could ruin your whole day!
Hi Chip.
Yes, the sun and the stars of our galaxy revolve around the center of the Milky Way. The astronomer Ken Croswell (in his article The Brightest Red Dwarf in the July 2002 issue of Sky & Telescope magazine) reports, “As they orbit the Milky Way, stars bob up and down through the galactic plane like horses on a merry-go-round.” Our sun stands as no exception. According to the astronomer Karen Masters, the sun crosses the galactic plane in periods of about 35 million years. One revolution of the sun around the galactic center takes an estimated 230 million years.
Presently the sun lies north of the galactic plane. Moreover, the sun is continuing to travel northward from the galactic plane at some 7 kilometers per second. The sun last crossed the galactic plane several millions of years ago, and isn’t expected to cross the galactic plane again for many millions of years to come.
Bruce
That being said, our sun is 4.5 billion years old, we complete a revolution every 230 million years, and cross a plane every 35 million years, that makes no sense unless you realize the center of the center of the galaxy rotates faster the we do some 27K miles light years away, like how the milk in the center of your coffee spins faster that the milk our near the edge of the rim…
We have crossed the plane 128.57 times and are due to actually cross again in about 15 million years, then will visually cross on a map about 27 thousand years later.
I don’t think these calculations are even close to being correct since I believe light slows without relation to time as it exits the gravitational center of our galaxy therefore our visual representations of the universe based on photos are completely incorrect without proper correction for time/speed based compensations of the visual light we can see and plot on paper with regard for the actually true current locations based on time and assumed directions of travel.
Computer based extrapolations around the world have all calculated the actual true physical crossing a plane to occur in December of 2012.
Full of win.
Siempre,
Given that the solar system resides north of the galactic plane and is traveling north of the galactic plane at the rate of 7 kilometers per second, that means we’ll be farther out from the galactic plane in 2012 than we are at present.
Bruce
Far best explanation I have read on forum where they say:
“the sun will pass through the galactic center
(from earth’s vantage point)” …”that the three points of the earth sun and galactic center will
form a straight line with the sun at the center.”
http://www.physicsforums.com/showthread.php?t=208935
1. If that can be proved it shows clearly that it is “Galactic alignement” – alignement – in the right meaning of the word itself … 3 points lying on the line. Not that Solar system is crossing Galactic plane. I did not find when the “crossing” might be, anyone knows?
2. Solar system should (or never) cross notional Galactic plane from “above” to “below”. As I thing, It can be only twice during Galactic period. (app. 230 mil years takes to orbit Sun all around our Galaxy)
3. Rotation of Spiral patterns is faster. Wikipedia says Spiral pattern rotation period is 50 million years, Bar pattern rotation period is 15 to 18 million years, Sun’s galactic rotation period is 250 million years(negative rotation).
Question: What is the time when Solar system cross the Galaxy Spiral arms on its orbit arround Galaxy center?
There is theory (of Michael Gillman and Hilary Erenler) which I found could be interesting and covers those crossings and its relation to mass extinctions of species – (found link only)
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=1851772
Tomas,
Most sources with which I am familiar claim the solar system last passed through the galactic plane around 3 million years ago. Although – as you say – the sun’s revolutionary period around the center of the Milky Way galaxy is somewhere around 250 million years, the sun does bob up and down through the galactic disk like a merry-go-round horse, crossing the plane every 35 million years or so. I give you a link that gives a brief summary of the solar system and the galactic plane at http://www.2012hoax.org/galactic-plane.
Bruce
I think I see it now. We need to stop thinking of the Galaxy as just a bigger version of our Solar System, with just 2 nodes for each planet. It’s much more dynamic, more like the rings of Saturn, with ripples and waves constantly interacting between the spiral arms, the central bar structure, and the galactic center. So I can see why the oscillations could be on the order of 35 million years or less.
What is the relationship of the terrestial axis of rotation to the galactic plane?
Given that the TAR during its total 26K year precessional cycle changes its angle about 45 degrees
Sonja,
At present, the inclination of the Earth’s equator to the galactic equator is about 63 degrees, and the tilt of the Earth’s rotational axis to the galactic equator is about 27 degrees. So if you live at or near 27 degrees north latitude, the galactic equator aligns with the horizon on May evenings. Conversely, if you live at or near 27 degrees south latitude, the galactic equator aligns with the horizon on November evenings, or some six months later. Read our upcoming May 7 program: Milky Way encircles the horizon on May evenings at http://earthsky.org/tonight/milky-way-encircles-the-horizon-on-may-evenings.
Bruce
[...] Where is the ecliptic in relation to the Milky Way? [...]
What does the galactic center’s postition have in relation to the alignment between the ecliptic and the galactic plane?
Sandra,
The direction of the galactic center resides a bit south of where the galactic equator and the ecliptic intersect in the constellation Sagittarius. See the sky chart at http://earthsky.org/astronomy-essentials/will-earth-pass-through-galactic-plane-in-2012
Bruce
Hi Bruce McClure
How can you tell if we are at the north or south of the galactic plane and what direction we are heading? i saw on a website showing that the solar system including earth travels like a sine wave along the plane and currently heading towards the plane.
check ” Nasa Confirmation 2012 Pole shif t”
Rodney,
Astronomers claim the solar system is north of the galactic plane and is traveling northward – away from the plane of the Milky Way galaxy – at some 7 kilometers per second. Yes, the sun and solar system bob up and down through the galactic plane like a merry-go-round horse, crossing the plane of the galactic disk in periods of about 35 million years. It takes some 250 million years for the sun to go around the galactic center full circle.
I can’t claim to know how astronomers figured all this stuff out. However, you may be able to access scientific abstracts by doing a search, using the the key words “sun’s distance,galactic plane,abstract.”
Good luck!
Bruce