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July 2003: Apex of the Sun's Way


The Apex of the Sun's Way, or the solar apex, refers to the direction that the Sun travels through space. This is not to be confused with the Sun's apparent motion through the constellations of the zodiac, which is illusionary -- this supposed motion being caused by the Earth revolving around the Sun. Astronomers determine solar motion from observing the apparent motions of the stars.

When you drive down the road, the trees ahead of you appear to widen apart as you approach them, whereas the trees in the back of you appear to converge. In like manner, stars in the front of the Sun's direction of motion are expected to spread apart, while those receding behind should come closer together. To actually observe this seems like a labor of Hercules to me, but the astronomer William Herschel is said to have found the solar apex with this method as early as 1783.

Luckily, the bright star Vega marks the solar apex for us -- at least within the ballpark. At early evening, Vega, the brightest star of the Summer Triangle, stands relatively high in the east. It's hard to miss, and the solar apex lies to the southwest of this star, in the constellation Hercules. In winter, the brightest star of the night sky, Sirius, marks the approximate position of the solar antapex -- the direction opposite of the solar apex and Vega.

The Sun and its entourage of planets travel in the general direction of Vega (away from Sirius), orbiting the center of the Milky Way at some 140 to 150 miles per second. Even at this breakneck speed, it takes something like 225 million years for the Sun to complete one revolution around the Galaxy. The Sun, as I understand it, travels 12 miles per second faster than Vega, thereby closing the gap between it and Vega by this amount every second.

At nightfall, look low to your southeast to find where the center of the Milky Way Galaxy resides. It lies some 26 light years distant from us (though unseen), between the tail of Scorpius and the "teapot" of Sagittarius. (Here's another map showing the direction of the galactic center.)

by Bruce McClure

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