As the stars come out on clear evenings this month, face where the sun went down and look very high. Shining up there is one star brighter than the others: Arcturus, an orange giant 37 light-years away, tinted like a drop of ginger ale sparkling in the July sun.
That is your landmark for identifying more celestial objects down below. A little left of straight down, you will see Saturn standing above Spica. They are separated by about three finger-widths at arm’s length. They have been that way for months. Spica is the brightest star of Virgo, the big constellation that has housed Saturn for the last 2½ years and will for another year to come.
Well off to the right of Saturn and Spica, and a bit lower, you will see Mars. It glows a deeper orange than pale Arcturus.
Having found this scene once, you will recognize it at a glance in the coming days and weeks. Keep watch on it, because things are happening.
Planets got their name from the Greek word for wanderer. They move around against the backdrop of the “fixed stars.” And they do not move at the same speeds. Distant Saturn appears to move hardly at all with respect to Spica in the coming weeks. But Mars, being much closer to the sun and Earth, moves leftward by a noticeable amount almost day by day.
‘Watch the long triangle that Mars makes with Saturn and Spica. Mars is heading straight toward the gap between the other two, and it will fly between them in mid-August.’
Watch the long triangle that Mars makes with Saturn and Spica. The triangle is shortening. Mars is heading straight toward the gap between the other two, and it will fly between them in mid-August.
Picture Saturn and Spica as the front of a slingshot and Mars as the pebble. The slingshot is shooting Mars leftward in slow motion. By the time Mars goes flying free in late August, however, the whole arrangement will be sinking low and away into the glow of sunset.
How the night sky made modern life
The motions of the planets, the five visible to the unaided eye, certainly add interest to the night sky. So does our unusually large moon. When you think about it, we are lucky to have eyes good enough to see stars and planets at all; seeing things that far away serves no survival purpose. Moreover, we are lucky to live on a world that has a clear atmosphere allowing us a view of them.
Without such an interesting view of the cosmos, would humanity have gotten as far as it has?
It is a real question. Predicting the motions of the heavens was central to the invention of science itself: the idea of making careful, numerical investigations of the natural world, puzzling out this data mathematically to discover deep principles of why things happen, and using these principles to correctly predict new and unexpected things.
This was a radical and unnatural innovation in human life. Ancient people had always assumed that the world worked by the unpredictable whims of gods or spirits, conscious beings who had choice and agency, like us. This assumption about why things happen seems to be wired right into human nature. Every baby is born thinking this way.
The crucial step away from this kind of thinking — toward a universe that runs by impersonal, predictable laws rather than the whims of gods — happened pretty much once in human history: in the Ionian islands of eastern Greece in the sixth century BC. And there, trying to figure out the heavens was a key driver.
Of course, people in all cultures had used observation and reason for thousands of years. But rarely if ever did they carry these things to the level of finding productive fundamental theories of nature, as opposed to mere rules of thumb. The idea that you could do this, if you pursued things with mathematical rigor, had never before taken root in some 8,000 years of civilization among divergent cultures around the globe.
Ionian Greece became the point from which modern science grew and remade the world. Without it, the next 2,500 years would probably have been pretty much like the previous 2,500. The world of 2012 would support only a tiny fraction of its present population, and if you were lucky enough to be one of them, you would probably be a hard-labor farmer or slave sweating out a life brutish, painful, and short.
If your present life seems better, maybe you can thank the stars.
Whole-sky maps
Easy-to-use maps of stars and constellations across the entire evening sky are available at SkyandTelescope.com/
gettingstarted.