Without Twilight You'd Be Dead

Discoveries about the Divine fine-tuning of planet Earth for human life continue to pour in and continue to amaze. Take something so mundane as twilight; everyone experiences it twice a day, at each sunrise and sunset. True they are beautiful sights, often spectacularly beautiful, sometimes literally painting the sky in a blaze of golds, reds, pinks, and other vibrant colors. Interestingly, some of the least scenic places on Earth, such as the steps of Central Russia, are home to some of the most spectacular displays of this atmospheric artistry, thereby giving it the name “the Land of the Firebird.”

True this overture to each day and night adds beauty to human life and has served as a source of inspiration to numerous poets, painters, composers, and romantics of every stripe, but is it really important? If twilight vanished tomorrow, would it really change anything? After all we could live without it, couldn’t we? If you think so, think again.

The October issue of Astronomy, in the “Strange Universe” section carried a story entitled “The Real Twilight Zone,” which inspired this Blog. The author points out that the Earth is the only planet in the Solar System that has twilight. Mars almost has a little, but its thin atmosphere and lack of water vapor fail to produce a palate of colors or a long duration of low light, which gradually trickles off. The other planets and planet size satellites either have no atmosphere, going from bright sun light to pitch dark immediately, or they have dense gaseous atmospheres, absorbing and diffusing the sun’s light before it reaches any great depth. The same appears to be true of the 200-plus planets that have been discovered outside of this solar system. Not one of them is a good candidate for producing twilight.

“Okay,” you say. “Not many planets, perhaps no others, have any twilight, so what?” Stop to consider this: we could not exist on this planet without twilight.

Evening Twilight starts when the sun touches the horizon and lasts until it has fallen 18 degrees below the horizon. The length of time this takes depends on one’s location on the globe and the time of year. It is shortest at the equator and longest at the poles. As the sun falls lower on the horizon, its light progressively passes through more and more of the atmosphere. The gases, water vapor, and dust in the atmosphere change the spectrum of the light as the sun moves lower. This produces a gradient effect on the light that reaches the Earth’s surface. This gradient is critical because it triggers certain biochemical cascades in different organisms. Some biochemical processes are only triggered in certain organisms by light of a particular wavelength, and that wavelength is only in sufficient intensity when the sunlight is coming into the earth’s atmosphere at a certain angle. This light plays a role in many types of animal behavior, from the singing of some birds to the feeding habits of many fish. However, this is another story, which is far too complex to go into here.

There are two other major phenomena, which are currently known and there are probably more. First, the thermal equilibrium of the atmosphere is strongly influenced by twilight. It helps moderate the shift in temperature between day and night. On Mars temperatures can easily swing 200 degrees Fahrenheit from day to night, but on Earth it is rarely more than 40 degrees. This narrow temperature range reduces the intensity of storms, is essential in maintenance of the water cycle, and damps down wind velocities. On Jupiter it is not unusual to see sustained wind speeds of 1,000 miles per hour. Imagine something like that on Earth!

The second major phenomenon is the maintenance of the Oxygen (O2)/Carbon Dioxide (CO2) balance of the atmosphere. While plants take in CO2 and produce O2 during the day, the process is reversed at night, when O2 is taken in and CO2 is given off. In order to maintain the balance needed for higher life forms over geologic time, the additional input of twilight to the photosynthetic process is essential. Without it, O2 levels would slowly decline.

So the next time you see a beautiful sunrise or sunset, take in a deep breath, and remember that it was put there for you.