Wonders of the Universe

THE BLUE MARBLE

Even the most dogmatic flat-Earther would have a problem explaining away ‘The Blue Marble’. This photo, taken by the astronauts on board Apollo 17 during its journey to the Moon on 7 December 1972, has caused some to speculate that this beautiful picture of our fragile world is perhaps the most distributed image in human history. But why is Earth a sphere? Actually, why are all planets and all stars spherical?

As we’ve discussed, we know that planets and stars are formed by the gravitational collapse of clouds of dust. You could say that the force of gravity pulls everything together, which is one way of looking at it, but another way of saying the same thing is that all the little particles in the primordial cloud of dust had gravitational potential energy, because they were all floating around in each other’s tiny gravitational fields. Just like the water droplets that fell as rain high up in the mountains above the Fish River Canyon, these particles would all try to fall ‘downhill’ to minimise their gravitational potential energy. This leads us to a very general and very deep principle in physics, and you can pretty much explain everything that happens in the Universe by applying it: things will minimise their potential energy if they can find a way of doing so. So, you could answer the question ‘why does a ball roll down a hill?’ by saying that the ball would have lower gravitational potential energy at the bottom of the hill than the top, so it rolls down. You could also, of course, say that there is a force pulling the ball down the hill. Physicists often work with energies rather than forces, and the two languages are interchangeable.

‘The Blue Marble’…photo has caused some to speculate that this beautiful picture of our fragile world is perhaps the most distributed image in human history.

With a collapsing cloud of dust, the shape that ultimately forms will therefore be the shape that minimises the gravitational potential energy. The shape must be the one that allows everything within the cloud to get as close to the centre of it as it possibly can, because anything that is located further away from the centre will have more gravitational potential energy! So, the shape that ensures that everything is as close to the centre as possible is, naturally, a sphere, which is why stars and planets are spherical

‘The Blue Marble’ is perhaps one of the most famous photographs ever taken of Earth, and has inspired numerous images since. The photograph, taken by the Apollo 17 crew on their 1972 journey to the Moon, made history as the first true-colour image of our planet which showed Earth in unprecedented detail.

NASA

VERY LARGE ARRAY

A very large array indeed – the 27 dishes on the Plains of San Augustin are an impressive sight, stretching into the horizon. Through these, the radio astronomy observatory can take some even more impressive images.

In the US state of New Mexico, on the Plains of San Augustin between the towns of Magdalena and Datil, lies one of the most spectacular and iconic observatories on the planet. The Very Large Array (VLA) is a radio astronomy observatory consisting of 27 identical dishes, each 25 metres (82 feet) in diameter, arranged in a gigantic Y shape across the landscape. Although each dish works independently, they can be combined together to create a single antenna with an effective diameter of over 36 kilometres (22 miles). This allows this vast virtual telescope to achieve very high-resolution images of the sky at radio wavelengths.

Radio astronomy has a history dating back to the 1930s, when the astronomer Karl Jansky discovered that the Universe could be explored not just through the visible part of the electromagnetic spectrum, but also through the detection of radio waves. Over a period of several months, Jansky used an antenna that looked more like a Meccano set than the VLA to record the radio waves from the sky. He initially identified two types of signal: radio waves generated by nearby thunderstorms, and radio waves generated by distant thunderstorms. He also found a third type, a form of what he thought was static. The interesting thing about the static was that it seemed to rise and fall once a day, which suggested to Jansky that it consisted of radio waves being generated from the Sun, but then over a period of weeks the rise and fall of the static deviated from a 24-hour cycle. Jansky could rotate his antennae on a set of Ford Model T tyres to follow the mysterious signal, and he soon realised the brightest point was not coming from the direction of the Sun, but from the centre of the Milky Way Galaxy in the direction of the constellation of Sagittarius.

Coinciding with the economic impact of the Great Depression, Jansky’s pioneering work did not immediately lead to an expansion in the new science of radio astronomy, but ultimately exploring the radio sky has become one of the most powerful techniques used in understanding the Universe beyond our solar system