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Will Earth Pass through the Galactic Plane on December 21, 2012?

The Short Answer Is No

No, the Earth and solar system will not pass through the galactic plane on December 21, 2012! Astronomers believe the Earth and solar system reside north of the plane of our Milky Way galaxy. It is not known with precision how far north we are of the galactic plane, but estimates place us at a few to several dozen light-years.
What's more, the solar system is going northward of the galactic plane as we speak at the rate of 1.5 astronomical units per year. In about 15 million years, we're expected to be farthest north of the galactic plane, perhaps to reach a peak of 230 light-years. After that, we'll start to plunge southward toward the galactic plane. In short, we won't be crossing the galactic plane in 2012 or anytime soon!
As the solar system revolves around the center of the Milky Way galaxy in a period of roughly 230 million years, it does bob up and down through the galactic plane in periods of roughly 35 million years. But none of this has any bearing on what is to happen on December 21, 2012.

Galactic Equator Passes Near Solstice Points

As seen from Earth, the Sun appears to be south of the galactic equator (or galactic plane) for half the year, and north of the galactic equator for the other half of the year. Twice a year - on or near the solstices - the Sun crosses the galactic equator. Around the June solstice, the Sun (appears) to cross the galactic equator going from south to north. Six months later - on the December solstice - the Sun (appears) to cross the galactic equator going from north to south.
Incidentally, the galactic equator is farthest from the Sun around the equinoxes. As seen from Earth, the Sun appears to swing a maximum number of degrees south of the galactic equator near the March equinox; and six months later - around the September equinox - the Sun appears to swing a maximum number of degrees north of the galactic equator. Click here for constellation maps showing the galactic equator and the ecliptic upon the celestial sphere. The ecliptic marks the annual pathway of the Sun in front of the backdrop stars.
If we wish to accept the galactic coordinates as defined by the International Astronomers Union (IAU) in 1959, then the solstice points exactly coincided with the galactic equator in 1998. That means the December solstice point will be a slightly west of the galactic equator on December 21, 2012. But even if the Sun were to cross the galactic equator at the exact instant of the December solstice in 2012, the Earth and solar system still won't physically pass through the galactic plane.

We Don't Physically Pass Through Galactic Plane

Yes, even though the Sun appears to cross the galactic equator (plane) twice a year, we don't physically pass through the galactic plane. I admit the concept seems difficult to grasp, but let's see if we can bring it all down to Earth.
Step outside with a Hula-Hoop, and if possible, find an unobstructed horizon. Stick your head in the middle of this Hula-Hoop, and incline the Hula-Hoop plane about 60 degrees to the plane of your horizon. (No need to fuss. Guessing 60 degrees works just fine.) Note that half of the Hula-Hoop is backdropped by the sky, and the other half is backdropped by the terrain. Note that two points on the Hula-Hoop align with the horizon.
Obviously, the points at which the Hula-Hoop intersect the horizon are not points at which the Hula-hoop physically passes through the horizon. In this thought-experiment, your eye represents the Sun, the Hula-Hoop depicts the Earth's orbital path, and the terrain out to the horizon represents the galactic plane. The two points on the Hula-Hoop that intersect the horizon are analogous to the two points on the Earth's orbital path where the Earth and Sun appear to align with the galactic equator (plane).
The Earth's orbit no more goes south of the galactic plane than the Hula-Hoop plunges beneath the horizon.

Visualizing Our Place in the Galaxy

Astronomers tell us that the diameter of the galactic disk equals about 100,000 light-years, with Sun and solar system residing about 27,000 light-years from the galactic center. The ecliptic - the plane of Earth's orbit around the Sun - is inclined about 60 degrees to the galactic plane. For a diagram that helps you to see our place in the galaxy, click here, then click on Dust and earth's motion and scroll down to the third illustration. As seen from the north side of the ecliptic, The Earth and planets revolve counter-clockwise around the Sun, but as seen from the north side of the galactic plane, the Sun and solar system revolve clockwise around the center of the Milky Way galaxy.
This illustration is not drawn to scale. According to this NASA Kid's Page, if the solar system all the way to the orbit of Pluto were shrunk to the size of a quarter, the Milky Way would be the size of North America.

copyright 2009 by Bruce McClure

March 2009 Feature * May 2009 Feature