Dr. Carl Sagan once remarked that astronomy is a “humbling and character-building experience.” Anyone who’s looked up in a dark night sky, away from streetlights or cities could not help but be awed at the distant planets, stars, galaxies, and nebula. Peering through even a relatively cheap set of binoculars reveals as many as 100 times as many distant objects. But because the light that we see left those objects long ago, what we are seeing is not how they look today, but how they seemed thousands or even hundreds of millions of years ago. When we look at distant astronomical objects, we are looking back in time.
I first developed a passion for astronomy in 1994, when a close friend gave me a small telescope for my birthday that year. It was small, only about 2 inches, but it let me look at the moon, and a few larger, closer star clusters like The Pleiades, shown here.
That we are looking backward through time seems counterintuitive. The idea is relatively recent and is a result of Einstein’s theory of relativity, which asserts that light travels at a particular speed (300,000 km/second or 186,000 MPH). This is so fast that on a daily basis we never experienced any consequence of this speed limit. But when we look at far distant objects through a telescope or binoculars, we see things as they were long ago. While gazing at very near objects, for example, the Moon, we are seeing the light that reflected from the sun around 1.25 seconds ago, hardly worth mentioning. Light from our Sun takes about eight and a half to minutes to reach us, and if you were visiting nearby planets, sunlight would require six minutes to get to Venus, and over twelve minutes to get to Mars. Jupiter, the largest of the planets has to wait 43 minutes for light from the Sun, while Neptune must wait over four hours. Lowly Pluto, no longer a planet, since alone in the dark waiting over 5 ½ hours for sunlight. What this means is that if you were standing on Pluto (in a space suit of course otherwise you would freeze solid) and the Sun exploded, you would not know it for over five hours!
As incredible as the seems, our solar system is extremely compact. Astronomically speaking, all the planets are practically the same place, as is our sun. The nearest star outside of our solar system, Proxima Centauri is over 4.23 light years from Earth. A popular target for amateur astronomers is our closest Galactic neighbor, Andromeda, also known as M-31, 2.5 million light-years away. To look at it another way, when the light we see tonight left the Andromeda galaxy, Homo Erectus had yet to appear. Our own Milky Way galaxy and our neighbor Andromeda galaxy make up part of the local group, a cluster of about 30 galaxies, spread across over 10 million light years. Objects like the galaxy MACS1149-JD are so distant that when light first left on its voyage through space, over 13 billion years ago, our sun and planets would not form for another 5 billion years. In a sense, astronomy is history, because we see things not as they are, but as they were long ago.
So the next time you’re unable to sleep, or find yourself curious, grab a pair of binoculars and go outside and look up at the night sky. And travel back in time.