Observing our Solar System: A beginner’s guide

Introduction

Every star may be a sun to someone.

Carl Sagan

The Solar System comprises the Sun and a vast number of orbiting worlds ensnared by its gravitational influence – most of which are practically just pieces of debris by comparison to their host star. A small number of the most significant objects, including our planet Earth, can be considered environments of their own, but most are lifeless fragments – leftovers from a turbulent birth some billions of years ago. This system operates on a scale that vastly exceeds our everyday experience, yet viewed against the expanse of the Galaxy beyond, it seems remarkably local. So, cosmically speaking, the Solar System is the region of space most appropriately termed our ‘neighbourhood’. As with our earthly neighbourhoods, it is as familiar to us as it would be foreign to a visitor from another star system, and just as we take an interest in the buildings that share our streets, astronomers scrutinise the behaviour of the planets, their many moons, and countless other companions of the Sun.

Shutterstock / Pavel Dyachenko

Centuries of insatiable curiosity and pioneering spirit have delivered an extraordinarily rich picture of the Solar System. We live at a time when spacecraft have completed the reconnaissance of the planets, robotic emissaries have landed on numerous other worlds, and enormous surveys continue to chart the most remote reaches of the outer Solar System to astonishing distances. There remain yet more questions than answers, but the wealth of knowledge gained so far greatly enriches our observing experience, lending context to the small images in our telescopes, and allowing us to predict spectacular events with surgical precision. In concert with these scientific advances, affordable telescopes and eyepieces of excellent quality have become ubiquitous. We can all follow in the footsteps of early trailblazers like Galileo, whilst enjoying a much clearer view than they could have imagined possible, and with a surprisingly low cost of entry.

This book’s purpose is to serve as your personal guide to Solar System observation, and support your ambitions whatever they may be. It could be that you intend to become a skilled visual observer with a subject focus on the planets or the Moon; perhaps you’d like to chase down comets and follow their progress to possible greatness; maybe you hope to make your own high-resolution images revealing details and colour beyond the reach of the eye – there are many avenues to exploring our neighbouring worlds. My advice is based upon many years of experience as a keen visual observer and astrophotographer, who has been fortunate enough to witness almost the complete checklist of ‘must see’ Solar System highlights, including events that occur only once or twice in several generations. I hope that it will steer you well on your journey to mastering your cosmic neighbourhood.

To maximise the practical value of this guide, I have curated the text to emphasise information fundamental to observation. Naturally, some scientific insight into the objects we’re looking at will enhance the experience and better prepare us to make the most of our equipment, but this will only be included insofar as it is useful. As such, we will not explore Solar System objects in exhaustive scientific detail – that would entail a much thicker and heavier book! After a short summary of the observational history and discovery of the Solar System, we’ll briefly explore its formation, structure, size and motion. Then we’ll look at resources that enable accurate forecasting of positions of Solar System objects and timings of special events. From here, we can begin to dissect the various categories of these objects and events for a granular look at how best to observe them, but not before a discussion about the limits of the naked eye and the importance of optical aid. Many wonderful sights can be enjoyed by eye alone or with modest binoculars, but the in-depth portions of this guide will be better suited for telescopes. In some cases, telescopes are essential for observations to be possible. Following a guide to telescope designs and accessories, their practical limits and the atmosphere, we’ll look in detail at the planets and Moon, the Sun, minor planets including asteroids and comets, and finally special events such as eclipses, occultations, transits and conjunctions. The closing chapters cover Solar System imaging, with a broad overview of theory and equipment, followed by example workflows that illustrate different approaches to various targets.

I wish you enduring clear and steady skies for many exciting nights at the eyepiece of your telescope. Let it carry you upwards and connect you to other worlds.

1: HISTORY OF SOLAR SYSTEM OBSERVATION

Prehistory

It is not difficult to close your eyes and bring to mind the image of another planet like Mars or Saturn. We grow up with a clear impression of how these worlds appear from space-probe imagery and informed artists’ impressions. But the familiarity we enjoy has existed for just a few short centuries – a vanishingly small fraction of the human timespan. For most of human history, our ancestors regarded the Solar System with a mixture of bewilderment and superstition. That is not to say that there were no important milestones of understanding before the advent of the telescope, but the pace of discovery since the early 17th Century virtually eclipses literally tens of thousands of years of speculation that preceded it.

We can only speculate about what early humans thought about the objects and events in the Solar System.

© Tom Kerss

Strictly speaking, there is no prehistoric model of the Solar System, because the concept did not exist in human thought about the natural world. Early humans regarded the ground and the sky as two entirely different domains – the latter being forever out of reach – and this likely brought them comfort when strange, inexplicable things occurred up there. In that vast gulf of unrecorded time, people witnessed eclipses of the Sun and Moon, the arrivals and departures of comets, and dazzling meteor storms, but in their rarity, many of these events will have become mythology within just a few generations. Surviving observations from this period are sparse and subject to considerable speculation.

Antiquity

The Nebra Sky Disc depicts the Moon alongisde stars, possibly including the Pleiades cluster, as well as the Sun.

© State Office for Heritage Management and Archaeology Saxony-Anhalt, Juraj Lipták.

Artefacts from the Bronze Age, such as the Nebra Sky Disc, speak to the growing importance with which ancient people considered the sky at this time, and their fascination with it would have a profound impact on the emerging philosophies of the natural world. The ancient Greeks devoted a great deal of attention to understanding the workings of the heavens, with a special focus on the asteres planetai (‘starry wanderers’). It is from this phrase that we get the modern word planet, meaning wanderer, and during the antiquity, the five visible to the unaided eye (Mercury, Venus, Mars, Jupiter and Saturn) were grouped with the Sun and the Moon to form the group of seven classical planets. This scheme is immortalised in the names of the days of the week, which originate from this period.

Days of the week with their corresponding Latin names and classical planets.

Understanding cosmology was a central pursuit for many generations of prolific Greek philosphers in the ancient world.

Shutterstock / Dimitrios P

Of course, today we understand planets to be mechanically different from the Sun and Moon, but to ancient people they were associated by their common ability to move through the sky among the ‘fixed’ stars. Greek astronomers such as Plato committed much thought to the cause of this special behaviour, whilst the chiefly superstitious generally regarded them as deities. Centuries after Plato’s time, the Greek polymath Ptolemy published his seminal work – the Almagest. It contained an early attempt at arranging the Solar System in a series of orbits, building upon the ideas of Aristotle.

Ptolemy’s Almagest (‘Great Work’) included a geocentric model of the Universe.

Joshua Brown /Research Gate

Ptolemy’s model was sophisticated, and prevailed for over 14 centuries, but he was fundamentally mistaken in placing the Earth at the centre of the Solar System. As such, he had to correct for deviations in the positions of the planets by placing them on smaller paths within their orbits. Remarkably, a model with the Sun at the centre had been proposed centuries earlier by Aristarchus of Samos. In retrospect, we can see how far ahead of his time he was, but his ideas of a heliocentric cosmology failed to gain support in the ancient world. For most, it was simply easier to accept that the Earth held a special importance, and did not move.

The Copernican Revolution

By the 16th Century, astronomers were sitting on a wealth of data about the Solar System – tables that had been made by skilled observers. Considering the fact that the telescope had not yet been invented, the accuracy of these observations is a marvel. In 1543, the Polish astronomer Nicolaus Copernicus shocked the scientific world with his publication of a robust, heliocentric theory of the Solar System, placing the Sun at the centre and resolving many issues with the Ptolemaic model. It was supported by the best observational evidence available, and triggered what has become known as the Copernican Revolution – a dramatic shift in the application of mathematics in astronomical investigation.

Portrait of Nicolaus Copernicus, who proposed a revolutionary heliocentric model.

Wikimedia Commons / Wellcome Collection

Copernicus was precisely correct in his solution, but the adoption of his model came reluctantly for those who could not abandon the notion of the Earth being central to the Cosmos. The prolific Danish astronomer Tycho Brahe, one of the last great observers of the pre-telescope era, developed his own system, in which the planets were allowed to orbit the Sun, and the Moon allowed to orbit the Earth. But in his view, the Sun also orbited the Earth. The Tychonian system was excessively complicated, but fortunately one of his own students – a freethinking young German called Johannes Kepler – would be drawn back to the elegance and simplicity of Copernicus in the years after Brahe’s death.

Tycho Brahe sought to unify the Copernican and Ptolemaic systems, resulting in a complicated model.

Wikimedia Commons

The Telescope

Johannes Kepler was Brahe’s student, and enjoyed unrivaled access to his world-class observation tables.

Wikimedia Commons

At the turn of the 17th Century, Tycho Brahe passed away, leaving Kepler to continue his work with his superb trove of observational data. He had for years been struggling to reconcile the motion of Mars with the Tychonian system, and had long suspected that Copernicus offered the better solution. Holding a prestigious position, whilst moonlighting as an astrologer, Kepler turned his brilliant and creative mind to a complete understanding of celestial mechanics. In 1608, something happened which no living astronomer could have expected. A Dutch lens-maker called Hans Lipperhey submitted a patent for a refracting telescope, which could magnify the appearance of distant objects. It is unclear whether he invented the device himself, but word of the patent spread rapidly through the academic community.

Hans Lipperhey’s telescope, which used lenses to form and magnify an image, was utterly transformative to astronomical science.

Early in the following year, the English scientist Thomas Harriott purchased one, and on 26 July that same year,