Turn back time to 230 million years ago, when dinosaurs began roaming the Earth. Some relied on a plant-based diet, while others were meat eaters. For a long period, they ruled the lands beneath their feet, never knowing of a different world displayed above their heads that we call the cosmos.
Imagine how dark the starry sky must have looked back then, with perhaps the only light pollution being an active volcano. It was during the Mesozoic era, about 66 million years ago, when the dinosaur’s last perfect day came to an end.
One fateful day, there was a blinding light lit the sky as a 10-kilometre wide asteroid or comet, travelling about 20 kilometres per second, collided with the Earth off the coast of what we now call Mexico, near the present-day village of Chicxulub on the Yucatan Peninsula. This event is a popular theory on what killed the dinosaurs and 75 percent to 80 percent of all life on Earth.
Museums around the world proudly exhibit the fossilized remains of these once-mighty dinosaurs. You might think this is the only way to travel back to their time period, but you would be wrong.
According to Einstein, the speed of light is the fastest thing there is. But even with a mind-boggling speed of 300,000 kilometres per second—the equivalent of travelling seven and a half times around the planet in one second—it still takes time for light photons to reach our eyes from great distances.
Our closest star other than the sun is Proxima Centauri, which is about 4.2 light-years from Earth. That means it takes light from that star 4.2 years to reach our eyes. Therefore, when we look at Proxima Centauri through a telescope, we are seeing that red-dwarf star exactly as it was more than four years ago. So when you look at any celestial body—either through a telescope, binoculars, or with the naked eye—you are, in effect, looking back through time.
The bright star Sirius, located to the lower left of Orion, is 8.6 light-years away. Keep in mind that a light-year is about 10 trillion kilometres. In contrast, our moon is an average distance of 386,000 kilometres from Earth (1.3 light-seconds), and the sun is 150 million kilometres (8.3 light-minutes).
Even the light from Saturn, about 1.6 billion kilometres away, takes 90 minutes to travel to us from its farthest point away in our orbits around the sun.
These distances pale in comparison to those of remote galaxies (which rare just like our Milky Way) that hold 200 billion to 400 billion stars or more apiece. Today’s amateur telescopes help reveal these distant islands of stars residing tens to hundreds of millions of light-years away. During my observing career, I have seen many of these faint objects whose light started its journey to my eyes about the time dinosaurs were running around on Earth.
It is difficult to put into words how it feels to view such vast celestial bodies and know how long their light has taken to reach us. The farthest galaxy I have seen with my telescope is identified by catalogue number IC4617. It is located in the constellation Hercules and resides an astonishing 500 million light-years from us. Its light left this galaxy after the so-called Cambrian explosion occurred on Earth, which saw a burst of life in the oceans some 550 million years ago.
The Pleiades star cluster (the heart of Taurus the Bull) is thought to have formed during the Earth's Cretaceous period, a mere 100 million years ago. And some distant stars may have already exploded (gone supernova) long ago, which means we are now looking at what could be called their ghosts.
So the next time you gaze up at the night skies, imagine what events were occurring here on Earth during that period your eyes are actually seeing.
Till next time, clear skies.