Illuminated by the Galaxy

When I began running in the mornings, I used to run later during the winter since the sky was dark. However, I was always reluctant to run later because I was ready to run just as early in the winter when it's dark as in the summer when it's light. So, I would delay starting my run later as long as the light would hold. I discovered over the years that I didn't have to delay my run at all. On mornings when the moon would shine, there was plenty of light to see. On the mornings when no moon was visible, to my surprise I found that I had just enough light to see the outlines of the road as I ran. I wondered where the light originated because few houses had outdoor lights shining early in the morning and where I live there are no street lights. I figured it must be light pollution from Santa Fe or the shopping center that opened two miles from my house. It isn't a lot of light but it had to come from somewhere.

I read an article in the newspaper this week about ambient light from the galaxy. It is particularly bright at this time of year in the early morning hours. It originates along the path that the sun, moon and planets trace through the sky, the so-called ecliptic path, which is also the location of the Zodiac constellations. The article I read didn't explain why the light from the stars along that particular path gives off more light than the stars in other parts of the sky, but I have some knowledge that completes the explanation. The ecliptic path also happens to coincide with the edge-on view of the Milky Way Galaxy in which our solar system resides. Since we're in the Galaxy, when we look in certain directions, we're looking into the heart of our galaxy and more stars are visible in that direction.

Since the galaxy is a flat spiral, being in the galaxy is like living in a pancake. Looking in the direction of the center of the pancake, I would see more pancake than looking outward, away from the center. More importantly, if I look up or down rather than toward or away from the center, I look straight out of the pancake into the emptiness that surrounds it. Likewise, in the Milky Way, looking up or down, in a direction perpendicular to the direction of the center of the galaxy, I won't see very many nearby stars. Most of them are in the direction of the center of the galaxy.

The part of the ecliptic path visible in the morning sky in the autumn must display the center of the galaxy for there to be more light shining during this time of year. The galaxies and quasars visible in other directions are just too far away to transmit much light to earth. The galaxy center reveals the greatest cluster of stars nearer to earth than any other direction, so the billions of stars in that direction transmit sufficient light to illuminate the earth more brightly before sunrise on autumn mornings.

The path I followed as I ran this morning was illuminated by the light of the Milky Way Galaxy. No wonder I felt so light as I ran.