It was a clear, sunny day on Mars, with just a few streaks of cirrus cloud. A range of low hills defined the horizon. This was one of the images sent back to us by one of the robots we have exploring the Red Planet.

Then we have pictures transmitted back from Titan, Saturn’s largest moon, showing the rocky bed of a dried up stream. We have rock samples from the Moon, from an asteroid, and close-up images of many bodies in the Solar System. We even have images from the surface of Venus, sent back before the spacecraft melted.

For most of its history, astronomy consisted mainly of observing dots in the sky. Our ancestors assessed their brightnesses and recorded their positions. They noticed that some dots were in groupings that were unchanging.

These were stars, and those groupings became the constellations we know today. There were a few other dots that moved to and fro along a defined path in the sky; these were the planets. One thing that our ancestor astronomers noticed is that the heavens were orderly, stable and highly predictable.

This contrasted strongly with the unpredictability of life down here on the ground. The relative stability of the heavens is why people panicked when something new turned up in the sky, such as a comet.

Later, when telescopes were invented, we could see that planets are different from stars, and we could even see surface features on some of them. The Moon changed from a smooth heavenly body to a rock ball, pocked with craters and old lava flows.

In those days, sciences like geology, meteorology and biology, were aimed at better understanding our own planet. Applying them to other worlds was left largely to science fiction writers. Now things have completely changed. Thanks to improved telescopes, spacecraft observing bodies in the Solar System from up close, or even landing on them, we can now study other worlds almost as freely as we can study our own. We can now buy textbooks on the geology of Mars, or the Moon. We can even study the weather on other worlds. They have become places we can relate with ours.

Not long ago we thought there are planets orbiting other stars, but we did not know. Since then we have discovered thousands of them. Almost every star has planets, some of which could be like our own. We even know whether some of these planets have atmospheres, and scientists are actively searching those atmospheres for chemical markers of life. We now know we live on one world among many, orbiting one star among billions, in one galaxy among billions. It is hard to imagine that our planet could be the only inhabited one.

Thanks to our rapidly expanding knowledge of other worlds we can apply what we used to think of as the Earth sciences, such as geology, to other worlds. This is making it possible to compare the forces moulding other planets with those shaping ours. These comparisons are raising important questions, such as if Venus, Earth, and Mars were similar when they formed, why are they so different now? We thought we knew why our Solar System is structured as it is, and assumed other systems would be similar. However, now we know the Solar System is by no means typical. Few extrasolar planetary systems are anything like ours.

On 14 February, 1990, NASA’s Voyager 1 spacecraft, then around 6 billion kilometres away, and leaving the Solar System, looked back and took a picture of the Earth. It showed our planet as a tiny, faint blue dot against the blackness of space; it was just another dot in the sky. Astronomy has changed from a science aimed at improving our understanding of what is going on “out there,” to a science that is also improving our understanding of what is going on down here.

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Saturn lies in the south after sunset, with Jupiter in the east. Venus rises in the early hours. The Moon will be Full on Nov. 27.

Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory, Penticton.