Outer space has no latitude or longitude, no north or south, no vertical or horizontal. And yet direction―or sometimes the lack of it―is crucial to understanding our place in the universe. Take the Milky Way, the starry band of light that encircles the sky. Observers in the 19th century discovered an odd pattern: No spiral nebulas could be found within its boundaries, even though they were common in other parts of the heavens. So prominent was this absence that scientists called the region around the Milky Way the Zone of Avoidance. During the 1910s, American astronomer Heber Curtis and others deduced that dusty gas in our galaxy blots out the light of more distant objects when we look lengthwise through our flattened galaxy. In directions roughly perpendicular to the plane of the Milky Way, such as toward the Big Dipper, we look through much less obscuring material and get an open window to the larger universe beyond. Curtis concluded that spiral nebulas must lie well outside the bounds of our galaxy. We now know that those whirlpools are other galaxies, many as majestic as our own.
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