Snow melt has swollen the ditches, creeks and rivers. So much water is coursing through them, the roar of small creeks is audible from a half-mile away. I was enjoying the frothy foam floating down Amity creek yesterday when I noticed my reflection in the bubbles’ hemispherical domes. Often times we think of the sky as a dome over us, an impression reinforced if you’ve ever gone to a planetarium. Astronomical diagrams frequently depict the sky as a dome. The bubbles were reversed domes of sorts, with the sky from zenith to horizon wrapped around the bubble’s outer surface.
The strong curvature of the bubbles turned them into short-lived fisheye lenses. Notice also that the size of my reflection varies according to bubble size. The smaller the bubble, the smaller the reflection had to be to fit on the reduced surface area available. Big bubbles have more room to spare.
So is the dome of sky shaped like a hemispherical bubble?
Well, no, not really. In a diagram it might look that way, but go out and have a look for yourself. It seems like we’re looking farther away when we direct our gaze toward the horizon. The overhead point actually feels closer, as if the sky there were flattened or depressed. So our impression of the sky is more of a stretched-out, flattened bubble.
Now imagine an airplane crossing the sky from one side to another. When the plane is near the horizon, it’s very tiny because it’s much farther away. When it crosses overhead, we see its outline and other details much more clearly because it’s closer to us. Bingo! Our perception of the flattened sky is correct. Things in our atmosphere or otherwise bound to Earth really are closer to us overhead than off toward the horizon. The sky isn’t a snow globe after all.
How does this play out in other ways? Let’s take the International Space Station (ISS). Have you ever noticed that the ISS is fainter when it first becomes visible in the west and again when it drops away in the east? Its altitude remains fixed at ~215 miles, but the station is closer to us overhead than near the horizon just like the plane. The lower we direct our gaze, the greater the horizontal distance between us and what’s in the sky depending on the object’s altitude. For a bird near the horizon, the horizontal distance might be a mile, but for the space station it’s 1,200 miles!
By the time you get to something as far away as the moon, the difference in distance at the horizon versus overhead is equal to one Earth radius (half our planet’s diameter) or about 4000 miles. That sounds like a lot of miles until you realize the moon is 240,000 miles away. Put those numbers in your calculator and you’ll discover that the moon is 2% smaller at the horizon compared to overhead, a real difference but one we can’t perceive with the naked eye.
Hey, wait a minute! Isn’t the moon bigger at the horizon? Yes, so it appears, but that’s nothing more than an illusion created by how we perceive the world where the ground interfaces with the sky. As for the stars, they’re so far away that it doesn’t matter what shape we imagine the sky. From horizon to zenith, the view goes on virtually to infinity.
As long as we’ve touched on the space station, here are time for viewing the ISS this week, as it transitions from morning to evening sky. For times for your location, please login to Heavens Above or key in your zip code at Spaceweather’s satellite flybys site.
Times are for the Duluth region:
* Tuesday morning April 12 beginning at 4:28 a.m. across the northern sky. A second bright pass occurs at 6:01 a.m. across the southern sky.
* Wednesday April 13 at 4:53 a.m. A brilliant overhead pass!
* Thursday April 14 at 5:18 a.m. across the south
* Friday April 15 at 5:43 a.m. very low in the southwest
* Sunday April 17 in the evening at 9:26 p.m. Brief pass in the southwestern sky
* Monday April 18 at 9:50 p.m. in the south-southwest. Enters Earth’s shadow and disappears around 9:53 near the star Regulus in Leo.