Astronomy is the study of the greater environment, that part of the Universe that appears to us as the celestial sphere—the sky. By day, it is dominated by the Sun, our very own star, but by night it becomes a wonderland as the earth’s shadow dims the sunlight and allows us to see the Milky Way. With the naked eye we can see, depending on conditions, the Moon (the Earth’s only substantial natural satellite), 6 planets, part of the Milky Way galaxy, and our sister spiral galaxy, Andromeda M31. Of course, there are also transient phenomena that come and go from view relatively quickly, like artificial satellites, comets, and meteors. And Jumbo jets!
Without the assistance of optical instruments, everything we can see is part of our home galaxy—except Andromeda, naturally—and with the exception of comets, meteors, the Moon, and the Sun, all appear as points of light on the sky. For untold millennia, earthbound observers studied the heavens and wondered what these things really are, how far they are, how they interact with each other and with us, and what may lie beyond the horizon of our vision. To tell you the truth, we are still pondering those very same questions, because the more we know, the more there is to know.
In the 16th century, Polish cleric Nicolaus Copernicus established in general terms the dynamics of the Solar System, placing the Sun at the focus and the planets in orbit around it. A distinction was drawn between planets and stars on this basis, and a new understanding of the cosmos dawned. Naked-eye astronomy had revealed a tremendous body of knowledge, culminating in the meticulous observational data of Danish astronomer Tycho Brahe, and the mathematical analysis of those data by Johann Kepler, the basis upon which Isaac Newton could build his vision of existence.
In 1609, four hundred years ago, a telescope was applied to the night sky, and modern astronomy was born. Galileo was able to use the new invention, a simple refractor with a magnification of not more than 30, to reveal some marvels of the world around us. He was able to resolve the six known planets as disks, rather than points of light, and thus conclude that they were similar to Earth and the Moon. He discerned craters on the face of the Moon, and from the shadows gained the first 3-dimensional view of a celestial object. He proved that the Moon was not a disk, but a sphere. He sketched the positions of sunspots, and showed that the Sun appeared to be rotating. He saw the phases of Venus, exactly like the phases of the Moon, and thereby provided the first observational verification of Copernicus’ theory. He witnessed four moons of Jupiter.
He realised that Jupiter emulated the Earth, and he even saw what was later to be identified as the rings of Saturn. Slowly, the family we know as the Solar System was revealed, and celestial mapping gained a new dimension.
Seventy years later, Isaac Newton published his monumental Philosophiae Naturalis Principia Mathematica, and gave to the world of science a set of laws that are as vividly accurate today as they were centuries ago. Astronomers now had the formulae (and, thanks to Newton, the calculus) to quantify orbital motion, and even more importantly, to predict unseen events. In this way, the existence of the planet Neptune with great positional accuracy a year before it was seen for the first time. Although Isaac Newton is best remembered for his mechanical laws of motion and gravitation, he also invented the reflecting telescope—still today the standard optical design—and in addition, his definitive 1709 classic Opticks formally introduced us to the study of light as a branch of physics.
At the dawn of the 20th century, although by the end of the first decade we had already light spectroscopy, astro-photography, Planck’s Quantum Hypothesis, and Einstein’s Special Relativity, it is astonishing to contemplate that we still had no physical conception of a Universe beyond our own galaxy. In the early 1920s, Edwin Hubble made the first foray into extragalactic astronomy when he was able to resolve individual stars in the so-called “spiral nebulae” catalogued by Messier, and thus refute the idea that they were just weirdly shaped, diffuse clouds of gas and dust. They were, in the language of the time, examples of other “island universes”, just like the Milky Way appeared to be.
Suddenly, the enormous scale of the Universe became apparent, and the advent of super-telescopes, with mirror diameters exceeding 5 metres, gathered light at an unprecedented rate, giving us images of distant objects with stunning resolution. Our observational knowledge of the cosmos grew enormously during this era of visual exploration, but sadly, the 1930s marked also the beginning of complete domination by mathematical theorists, and the steady decline of visual imagery as our first-rank clues to the details of cosmology. During this time, Big Bang Theory emerged from the field equations of General Relativity, and meta-mathematics became gospel. The prime position of the observational astronomer at the front line of astrophysical discovery was usurped by mental gymnasts, constructing poorly-understood equations with their eyes closed, and everything in cosmology was henceforward calculus and differentials and poly-dimensional geometry. In a subtle coup, observers were sidelined and silenced unless their pictures matched the model. The tail was happily wagging the dog.
I suppose, in real terms, the story of pure astronomy closes with that chapter, but there are further stories to tell. We have explored the Solar System, put telescopes into space, and acquired the most beautiful photographs imaginable. We have walked on the Moon and driven around on Mars. We have dared to go near the Sun, and sent ships to the Kuiper Belt. We have seen that things cling together on ever-increasing scales—stars into galaxies, galaxies into groups, groups into clusters, clusters into superclusters, and now we see what looks like great big walls and huge empty voids at the limits of measurement. If only our large telescopes had eyepieces…