Measuring the Universe

posted: 3146 days ago, on Thursday, 2009 Mar 12 at 23:53
tags: astronomy, Milky Way.

After a particularly hectic editing marathon, one of the large muscles in my leg went into a spasm. Not long afterward I found myself face-down on the physiotherapists table, looking down through a hole, thinking about the size of our Universe.

Measuring distances in the cosmos works on a scaffold principle. Larger distances are built on knowledge of closer distances. The more certain you are of the shorter distances, the more certain you can be of the really big ones.

Astronomers have figured out a "distance ladder", in which the distance to some objects are known in terms of distances to other objects. Once you can actually measure one of these distances, then all the others follow. This game of cosmic ladders has kept astronomers busy for centuries, and is still a major topic of research.

Quasars, for example, are some of the most distant objects we know of today. There's one quasar that lies 100 000, 000, 000, 000, 000, 000, 000 km away from us. All those zeroes show that kilometres aren't a very convenient unit of measurement when you're plumbing the cosmos. Astronomers prefer to use the "light year" the distance that light can travel in one year.

Light sallies forth at about 300 000 kilometres per second, so in one year, it will travel about 9.5 trillion kilometres. That distant quasar thus lies 10 000 000 000 (10 billion) light years away. Still a lot of zeroes, eh?

A quasar is a monster galaxy with a massive black hole in its centre. As matter is annihilated by the black hole's enormous gravitational pull, it releases tremendous amounts of energy, making the quasar shine very brightly. In fact, a quasar can shine with the light of 100 000 000 000 (100 million) stars. Lots of zeroes and lots of light.

The nearest quasar to Earth (labelled "3C273") is about two billion light years away, and can be seen in a cheap (few zeroes) little telescope.

An ordinary galaxy, like our own Milky Way, hosts about 400 billion stars. The nearest ordinary galaxy to us the Andromeda Galaxy (Messier 31) is about two million light years away, and it can be seen in binoculars. In fact, from a dark site, it can readily be seen with the naked eye.

Just like the Andromeda Galaxy, our Milky Way also has spiral arms, and just recently, a black hole was discovered lurking in the centre of our galaxy. No need to get worried, though, its some 300 000 000 000 000 000 kilometres away. Suddenly all those zeroes are comforting.

The Sun and planets lie in the Milky Way, tucked into the Orion Arm, in a region pretty rich in young, bright, stars. The nearest bright star to us is alpha Centauri, some four light years away. We see it as one of the two Pointers showing the way to the Southern Cross.

The Sun, of course, is also a star, and its just one-sixty-thousandths of a light year from Earth. The Moon, our nearest celestial neighbour, is just one-twenty-five-millionth of a light year away. The Earth's diameter is about a billionth of a light year.

So how did modern astronomers come up with all these numbers? In order to determine the distance to the quasars and galaxies, and then to the centre of our Milky Way, we first need to measure the distance to individual stars. But to do that, we first need to measure the distance to the Sun. One way of doing that is to first measure the distance to the Moon. But for that to work, you first need to know how big the Earth is. And to figure that out, a hole would be handy.

Some 2,200 years ago there lived a clever Greek called Eratosthenes. He noticed as had thinkers before him that the length of the shadow cast by an upright stick changed during the course of day. He then asked himself, what would the shadow of an identical stick look like, at the same time of day, if this stick was somewhere far away.

On one particular sunny summer day, at 12:00, he noticed there were no shadows at all. In fact, he went over to a nearby well and looked down the hole: the Sun was shining directly down it. Just how Eratosthenes reacted is not known (he didn't, for example, go running naked through the streets yelling 'Eureka!') but what he realized was that the Earth was spherical and he also knew how big it was, too! And he knew that, because he happened to know that 800 kilometres away, in the city of Alexandria, the Sun was not directly overhead, but cast short shadows. His measurements showed the Sun still had to move 7 in the sky before reaching its highest point over Alexandrian holes.

If 7 corresponds to 800 kilometres, then 360 (a full circle) corresponds to 41 143 kilometres which has to be the circumference of the Earth! This simple measurement gets a passing grade of 97% pretty amazing, huh?

With the size of the Earth fixed, the first step was taken towards measuring distances in the rest of the Universe. And all because someone looked down a hole.

nothing more to see. please move along.