Wonders of the Universe

THE COSMIC CLOCK

Each day we awake to the rhythm of our planet as it spins at over 1,500 kilometres (932 miles) an hour, relentlessly rolling us in and out of the Sun’s glare. Earth’s ceaseless motion beats out the tempo of our lives with unerring repetition. A day is the twenty-four hours it takes Earth to rotate once on its axis; the 86,400 seconds it takes for anyone standing on the Equator to be whipped around the 40,074-kilometre (24,901-mile) circumference of our planet. This is the most obvious rhythm of the Earth, which comes about because of the spin rate of our rocky, ironed-cored ball that was laid down somewhere in Earth’s formation and 4.5-billion-year history.

Travelling at 108,000 kilometres (67,108 miles) an hour, we move through space in orbit around our star. Racing around the Sun at an average distance of 150 million kilometres (93 million miles), we complete one lap of our 970-million-kilometre (600-million-mile) journey in 365 days, five hours, 48 minutes and 46 seconds, returning regularly to an arbitrarily defined starting point. As we sweep through this place in space relative to the Sun, we mark the beginning and end of what we call a year.

Everywhere we look in the heavens we see celestial clocks marking the passage of time in rhythms. Our moon rotates around Earth every 27 days, seven hours and 43 minutes, and because it is tidally locked to Earth it also takes almost exactly the same amount of time to rotate on its own axis: 27 Earth days. This means that the Moon always presents the same face to Earth. Further out in the Solar System, a Martian day is very similar to our own, lasting one Earth day and an additional 37 minutes. But because Mars is further from the Sun, a Martian year lasts longer, with the red planet taking 687 Earth days to complete an orbit. In the farthest reaches of the Solar System, the length of a year gets progressively greater, with distant Neptune taking over 60,000 Earth days or 165 Earth years to make its way around its parent star. In September 2011, Neptune will have completed its first full orbit of the Sun since it was discovered in 1846.

Here on Earth our calendar is determined by the clockwork rhythm and movement of our planet as it rotates on its axis, working its way through space and along its annual orbit around the Sun.

As we look deep into space, the clockwork of the cosmos continues unabated, but as the distances extend, the cycles become grander, repeating on truly humbling timescales. Just as Earth and other planets mark out the passing of the years as they orbit the Sun, so our entire solar system traces out its own vast orbit. We are just one star system amongst at least 200 billion in our galaxy, and all these star systems are making their own individual journeys around the galactic centre. We are all in orbit around the super-massive black hole that lies at the heart of the Milky Way. It is estimated that it takes us about 225 million years, travelling at 792,000 kilometres (492, 125 miles) per hour to complete one circuit, a period of time known as a galactic year. Since Earth was formed four and a half billion years ago, our planet has made 20 trips around the galaxy, so Earth is 20 galactic years old. Since humans appeared on Earth a quarter of a million years ago, less than one-thousandth of a galactic year has slipped by. In Earth terms, that is the length of a summer’s afternoon.

This is an immense amount of time; difficult to comprehend when we speak of the entire history of our species as the blink of a galactic eye. We live our lives in minutes, days, months and years, and to extend our feel for history across a galactic year is almost impossible. Yet here on Earth there are creatures that have existed for lengths of time that span these grandest of rhythms