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Two Stars Flag the Sun's Path through the Milky Way


Two exceptionally bright stars - Sirius and Vega - stand across from one another on these January evenings, flaging the Sun's orbital path through our Milky Way galaxy. You need a fairly unobstructed horizon to see both stars at the same time, with Vega lording over your northwestern horizon and Sirius predominating over the southeastern sky. Be sure to catch Vega early, because it sets by mid evening. At northern temperate latitudes, these stars are in the sky together around 7:30 p.m. local time in early January, 6:30 p.m. at mid month and 5:30 p.m. by the month's end. Click here for the precise rising and setting times of these stars in your sky.
Vega, the brightest star in the constellation Lyra the Harp, marks the general direction of the solar apex - the direction in which the Sun is traveling through space. Sirius, the Dog Star, the brightest in the nighttime sky, shows you the general direction of the solar antapex - the direction that the Sun is traveling from. Vega lies about 25 light-years distant from Earth, and Sirius looms about 9 light-years away. These two stars are only about 34 light-years from one another.
On a galactic scale, Vega and Sirius are our next-door neighbors. Astronomers estimate that our solar system resides some 27,000 light-years from the center of the Milky Way galaxy, and that the circumference of the Sun's orbit is somewhere around 150,000 light-years. Even though our Sun travels at the breakneck speed of 225 kilometers (140 miles) per second, our solar system takes some 225 million years to complete one revolution around the galaxy!

The Sun Travels along the Galactic Equator

For a more precise positioning of the solar apex and antapex, look roughly 10 degrees southwest of Vega and Sirius. The plane of Sun's orbital path around the center of the Milky Way galaxy is profoundly different than the plane of the ecliptic - the plane of the Earth's orbit around the Sun. This sky chart, courtesy of Professor Jim Kaler, shows you galactic equator passing to the east (or left) of Sirius and the ecliptic passing to the north of the constellation Orion. The galactic equator is inclined about 60 degrees to the plane of the ecliptic.
If you could look down upon our solar system from the the North Ecliptic Pole in the constellation Draco the Dragon, you'd see that the Earth and planets go counter-clockwise around the Sun. In direct contrast, if you looked down at the Milky Way galaxy from the North Galactic Pole in the constellation Coma Berenices, you'd see that the Sun goes clockwise around the center of the Milky Way galaxy. Relative to the Sun's revolution through the galaxy, the Sun rotates and the planets revolve around the Sun in retrograde - or opposite the direction of the Sun's revolution.
To view a diagram that helps to clarify the contents of the previous paragraph, click here. Proceed to click on "Dust and earth's motion" and scroll downward to the 3rd illustration.

Orion for Reference

For orientation, refer to the constellation Orion the Hunter. Orion shines close to both the ecliptic and the galactic equator, though it's to the south of both. As seen from the South Ecliptical Pole (almost in alignment with the Large Magellanic Cloud) in the constellation Dorado, the Earth and planets would appear to revolve clockwise around the Sun. As seen from the South Galactic Pole in the constellation Sculptor, the Sun orbits the galaxy in a counter-clockwise direction.

The Sun Bobs Up and Down through the Galactic Plane

According to the astronomer Ken Croswell and the American Scientist magazine, the Sun (and other stars) bob up and down through the galactic plane (equator) like horses on a merry-go-round. Presently, the Sun resides some 88 light-years north of the galactic plane and is flying northward at 7 kilometers per second. In 15 million or so years, the Sun will crest at its northernmost point, hovering roughly 230 light-years above the galactic plane. Then the Sun will go southward, eventually to cross the galactic plane, and to plunge 230 light-years south of the plane of our Milky Way galaxy. The oscillating Sun crosses the galactic plane every 33 million years.
Yo-yoing some 230 light-years north and south of the galactic plane might sound like a long way, but on a galactic scale it really isn't. It's like someone forever going around the world at the Earth's equator, and straying - at most - 55 miles north and south of the Earth's equator.

copyright 2007 by Bruce McClure

December 2007 Feature * February 2008 Feature


Zoom In & Out of the Milky Way Galaxy