The sky is deep, friends

Which is the closer of Saturn's two moons - Dione at top or Enceladus? This photo was taken by the Cassini spacecraft on December 1, 2010. Credit: NASA/JPL/Space Science Institute

I hope you don’t mind exploring one more astronomical illusion before moving on to other topics. This one has to do with distance. Look at the recently released photo of Saturn’s moons Dione (die-OH-nee) (top) and Enceladus (en-SELL-uh-duss). It’s tempting to think that Dione is the closer, but it only appears that way because it’s more than twice as large as Enceladus – 698 miles vs. 313 miles. Enceladus, with its bright, reflective, icy surface was closer to the spacecraft when the picture was taken.

If you weren’t familiar with the surface features of these two moons and just happened to drop by for a look, you’d be hard pressed to tell which is closer. We rely on our stereoscopic vision and other visual cues to see the world in 3-D. At best, stereo vision is good out to about a mile and a half. Beyond that, it becomes very difficult to estimate distance without other cues.

An astronaut works with the lunar rover on the moon. In an airless and alien environment, it was tough for the astronauts to get a feel for how close or faraway points of interest were. Credit: NASA

The space environment, which has no atmosphere or haze to dim or soften the outline of a more distant object, is unfamiliar territory to Earthlings. The Apollo astronauts had a difficult time telling distance on the airless moon for the same reason. With neither atmosphere nor familiar objects to mark distance and size, far away craters and boulders looked closer than they really were.  Another factor at play may be small mineral crystals and proteins that shift about on hairs in our inner ear. Gravity and other forces affect these tiny granules allowing us to sense gravitational pull and acceleration. It’s possible that in the moon’s lesser gravity, we lose our internal gyroscope, contributing to our inability accurately gauge distance and size.

At left is the outline of Orion, but if you could spin the constellation through 90 degrees and view it from the side, you'd see how each of its stars lies at a different distance from us in space. Our eyes can't sense depth in the sky because the stars are all too far away.

The sky’s greatest illusion is its two-dimensional appearance. The stars are all so far away that we simply cannot sense which are closer than others. As with Dione and Enceladus, we might be tempted to think that the brighter ones are closer. In many cases we’d be correct. One of the main reasons Vega in the Summer Triangle is so bright is because it’s a mere 25 light years away. Compare that to the fainter set of stars that comprise the outline of the Big Dipper. Most of those are at least three times as far. On the other hand, Barnard’s Star, only visible in a good pair of binoculars, is 2 1/2 light years closer to us than Sirius, the brightest star in the sky! Some stars are intrinsically brilliant but dulled by distance. Others, like Sirius, are fairly ordinary but appear brilliant because they’re nearby. A candle on your nightstand casts a bright light, but would be hard to recognize a mile away.

As humans, we hate being fenced in. To combat our ignorance of distance, we use the length of Earth’s orbit and trigonometry – called the trigonometric parallax method – to get hard data on stellar distances out to around 300 light years. Beyond that, the astronomer’s tool bag is jam packed with yardsticks like variable stars, supernovas and the expansion of the universe itself which are put to good use to measure the incomprehensible vastness of space. I encourage you to visit the ABC’s of Distances website to appreciate the variety of methods devised to open up our perspective on both size and distance.

Cross your eyes while looking at this stereogram of the constellation Orion and you'll see it in three dimensions. A "third" stereo image will appear between the two. Credit: Brian May

The stereo map of Orion was created using well known distances to the constellation’s brightest stars. That information was then converted into a stereo view of the constellation to simulate what it might look like if we could see it in 3-D. Sit back about a foot from your computer screen, relax and cross your eyes. The black dots at top will help. When you merge them to form a single dot in the middle of the panes, Orion should appear in stereo. Give yourself some time. The longer you look, the better your brain will interpret the stereo effect. Although the map has a few inaccuracies, it gives you a great feel for the “depth” of the sky.

The galaxy NGC 1300, located 70 million light years away, in Eridanus fills the field of view in this photo taken by the Hubble Space Telescope. If you look closely, much more remote galaxies on the order of 10 times farther away are sprinkled about the field. Nine are circled but there are even more. Credit: NASA/ESA

Our final look into the abyss of space includes this photo of the magnificent barred spiral galaxy NGC 1300 located some 70 million light years away in the winter constellation of Eridanus (err-RID-uh-nuss) the River. Scattered about the image are what appear to be at least a dozen “mini galaxies”. By now, you’ve probably guessed they’re anything but. Each is its own galaxy just as spectacular as NGC 1300 but diminished by distance to bit players around the mighty NGC 1300. These “little ones” are probably hundreds of millions of light years away.

The step-by-step, century-by-century solving of the cosmic distance riddle has bequeathed us a universe that is vast beyond imagination. The sky is deep, friends. Do drop in sometime.

2 Responses

  1. Sebastien

    “At best, stereo vision is good out to about a mile and a half.”
    I was pretty sure the human stereo vision stops completly after circa 200 yards only, because of our 1/60° angular definition and our natural parallax of 2,5″.

    1. Profile photo of astrobob

      Hi Sebastien, you are correct for real life situations when we’re moving about. The upper limit was achieved under controlled conditions in experiments. I chose to go with the upper limit to show that even a mile and a half is nothing compared to anything remotely cosmic. Thanks for your comment.

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