Science is about discovering where we came from and where we are metaphorically going. And, sometimes, literally.

The sun orbits the Milky Way’s center for the same reason the planets orbit the sun: gravity. The planets’ orbits are essentially elliptical, but the sun’s galactic orbit is far more complex.

Because the sun is tiny compared to the planets’ orbits, its gravitational field behaves as though it’s issuing from a central point. But the galactic disk, home to its spiral arms, hosts a significant fraction of the Milky Way’s mass, and thus its gravity. So, not only does the sun move under the influence of the billions of stars concentrated in the galaxy’s central bulge, but also the disk in which it is embedded.

The sun orbits the galactic center in just under a quarter-billion years (a “galactic year”), moving at about half a million miles per hour. It also periodically swings above and below the disk, with each bobbing cycle spanning roughly 70 million years. Finally, the sun’s speed varies as it passes through the local galactic spiral arms, just as cars speed up and slow down in a traffic jam.

The galactic center, around which the solar system revolves, lies about 27,000 light years away in Sagittarius, currently low in the southwest around 9 p.m. The broad, diffuse glow of the galactic bulge is visible in dark skies. About a fist-width at arm’s length to the lower left of Vega (the brightest star currently visible, very high in the west) is the “Solar Apex,” the direction in which the sun is going (and we, along with it). The solar apex is a little off the band of the Milky Way because the sun crossed the galactic plane around 3 million years ago, and is still heading away.

Next column: The many kinds of months.

Sky calendar through October 25th:


One hour after sunset:

Saturn: SW, low

One hour before sunrise:

Venus: E, very low

Mars: ESE, low

Moon: Last quarter 6:25 a.m. 10/12. Close to Mars 10/17. New moon

1:12 p.m. 10/19. Close to Saturn 10/23-24.

Other data: Annual Orionid meteor shower peaks 6:00 a.m. 10/21. Weak shower (max. 20/hr.).


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