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Uncountable stars twinkled brightly as I stepped into the night. Head tilted back, I reveled in a moment of wonder. The fingernail Moon hung among the stars—a tiny sliver of light in the Universe. Venus, named after the Roman goddess of love and beauty, shone brightly just below the moon. As I inhaled deeply, my nostrils froze together, and the cold, dry air in my throat triggered a coughing fit. There went my moment of awe!

Winter stargazing means clear skies and frigid temperatures. The clearest nights are also the coldest. Why? All day, the Earth absorbs energy from the Sun through visible and infrared light. Blacktop roads soak up rays, dark trees warm up, even pale snow captures a little bit of the Sun’s energy. Then, all night long, the Earth radiates heat back up toward the sky.

Clouds are very efficient blankets for our planet. They can trap heat and radiate it back down toward the Earth. If there are no clouds, however, the heat escapes. So, clear nights get colder. In addition, cold air can’t hold as much moisture as warm air, so the number of water molecules standing between our eyes and the stars is reduced even further.

All of this makes winter a great time for contemplating Earth’s place in the Universe. Why are we standing here instead of on Venus or Mars? What makes the Earth special?

First, consider the Moon. While we might think of our Moon as just a lowly satellite revolving around us, it has been integral to the history of our planet. The Moon stabilizes the Earth’s rotation, which keeps our seasons (caused by a slight tilt of the Earth), from going to the extremes. Even its formation was important. Long ago (scientists think), collisions with asteroids flung lighter material away from the Earth, where they coalesced into the Moon. This means that our core is much denser than that of Venus. Today, Venus’s lower density means that its interior is entirely liquid, and oddly calm.

In contrast, the denser Earth has a swirling core that is part liquid and part solid. The movement of our core generates the Earth’s magnetic field. Without our magnetic field, we would be bombarded by harmful radiation from the Sun. Without a magnetic field, solar radiation drove away the water on Venus.

So, instead of fluffy white clouds of water vapor, Venus has opaque clouds of sulfuric acid, and a runaway greenhouse effect. (You could never stargaze on Venus!) The average temperature on Venus is now 864 degrees Fahrenheit. That almost sounds inviting when our mercury dips below zero!

Mars, on the other side, has a different problem. Mars’s atmosphere is about 100 times thinner than Earth’s, so it lacks a thermal blanket. The stars might always be bright on Mars, but the nights are always cold. During the day, the temperatures on Mars can reach a balmy 70 degrees Fahrenheit. However, its average temperature is -80 degrees, and the night-time poles can dip to -225 degrees.

What factors led to our “just right” temperatures and atmosphere? Our mass (not too big and hot like Jupiter, not too small and cold like Mars) is one parameter. The gravity on Earth doesn’t just press us back into our beds in the morning; it also tugs at the atmospheric blanket and prevents it from slipping away. Mars has a smaller mass than the Earth, and therefore lacks sufficient gravity to hold onto its atmosphere.

Our distance from the Sun is also ideal, since it partly controls how much radiation we get from our closest star. Not too far, not too close, we got it just right.
“…dear star, that just happens to be where you are in the universe to keep us from ever-darkness, to ease us with warm touching, to hold us in the great hands of light…” –Mary Oliver, from “Why I Wake Early.”
In the end, almost all the factors that make the Earth special relate to the presence of liquid water, and our ability to retain it. Liquid water, and the seasonal presence of solid-state water (hooray for fishable ice and skiable snow!), are certainly important in my life!

On that starry night, the scarcity of liquid water above me was both a blessing and a curse. The chance to gaze out at the sparkling Universe emphasized that there is no place like home…even if home is where your nostrils freeze together.

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI, at 13470 County Highway M. The current exhibit, “Deer Camp: A Natural and Cultural History of White-tailed Deer,” opened in May 2013 and will remain open until April 2014.

Find us on the web at www.cablemuseum.org to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot, http://cablemuseumnaturalconnections.blogspot.com/.