Alberto is a professional and amateur astronomer. He is an adept piano player and also a skilled computer programmer, both abilities he began learning as a child. In 2001 he got together with some like-minded friends and founded what is now one of the largest amateur astronomy institutions in Brazil. He currently works in the Data Processing and Analysis Consortium of the ESA-GAIA satellite.

This article explains the basics of measuring the Universe. It's a story that begins with Hipparchus of Nicaea, who lived around 140 BC, and ends with the prediction of astrometrics labs aboard futuristic spacecraft. In between we'll learn about the motions of celestial objects, how to observe moving stars and the trick of using distant and exotic quasars as astronomical measuring sticks.

Astrometry

What is this astrometry all about? What does an astrometrist do? Simply put, astrometry is the measure of positions, distances and movements of bodies in the Universe.

A bit of history

Classically, it is done by measuring very tiny angles in the sky, and it has been like this for millennia. Hipparchus of Nicaea, who lived around 140 BC, could perhaps be called one of the first astrometrists. He is considered to be the greatest astronomical observer from antiquity as he created the first catalogue of positions and brightness of the stars. He was also one of the fathers of trigonometry - yes, now you know who to blame!

So astrometry is about positions and movement of bodies in space. Why do we want to know those things? Apart from the obvious question "where are we?", if we want to answer questions like "what is the Universe?", "how does a star work?", "is there any dark matter there?", "how old is the Galaxy? and the Universe?", we really need to learn how things move and where things are... and the more precise you know this, the better you will be able to answer those questions.

Measuring distances

Let's take the most obvious example: distances. We cannot know anything physically if we do not know its distance, and measuring distances is one of our specialties.

Measuring distances using ideas from school-level geometry is possible. For example, to answer "are all the stars at the same distance?" you could assume that the solution is "yes, they are all at the same distance, in a crystal sphere around the Solar System". And in fact, that was a perfectly valid answer during a long time in human history.

Nonetheless, this answer is simply not acceptable any more, since now we can observe that stars appear to move periodically in the sky. This apparent motion is due to the rotation of the Earth around the Sun and the effect is called annual parallax. This is a motion that is similar to what you can observe closing one eye or the other alternately.

Objects near you will move more than those far away. Try to do it! So, if you measure the angle the object moves and you know the distance between your eyes, you can compute the distance between you and the object by solving a simple triangle.

You see, generally we don't like to make anything unnecessarily complicated, since complicated things are more likely to contain errors and/or amplify unavoidable uncertainty. But of course, in some cases we can get as complicated as is possible. This is the case with the European Space Agency's GAIA satellite, which will measure the distance, movements and colours for more than one billion stars spread all over our Milky Way Galaxy!

So, parallaxes are measured as a periodic motion of a star in the sky. But how do we observe this movement? As we observe any other motion: by comparing the position of a body with the position of other bodies, considered as fixed. Wow, that is strong: we are saying that there are fixed bodies in the Universe. In fact, we don't believe that there is such thing, but the farther a body is from us, the smaller its motion due to the parallax. Perhaps you've got the point: we need to use very far away bodies to create a sort of "quasi-fixed" reference. In astronomy we do exactly this, using very distant but bright objects called Quasars to create what we call the International Celestial Reference Frame.

Taking the measure of the Universe

Now you know how to measure distances to other bodies in the Universe: you can use parallax. But perhaps you have noticed that if the object you want to measure is far, far away, this motion would be very, very small and almost impossible to measure accurately. That is the reason why parallaxes are not used to measure distances to other galaxies. Nonetheless, the parallax is the first step in what we call the "cosmic distance ladder".

But what is this ladder? Let's answer with an example: if you want to be able to say that the Universe is expanding or contracting, you will need to use different astronomical methods for measuring distances up to a great number of galaxies. All those methods depend one on each other, and the basis of all of them is the parallax. Just like building a big skyscraper you need to use a very good and reliable basis, in this case you need to use good parallaxes, if you want to arrive at reliable conclusions. Why? Because any errors in your parallax figures will propagate in all those other methods.

So, imagine the size of the error at the end of the ladder if you were not careful with your parallaxes. Would you dare to climb a high ladder that is not stable? Better to have good parallaxes!

That is not all!

The measurement of distances is just a part of astrometry although it is a very important one, since otherwise we wouldn't be able to know the nature of things in the Universe. But apart from all the contributions of this millenary part of astronomy to human knowledge (such as taking the Earth from the centre of the Universe), astrometry is always of utmost importance to all kinds of navigation.

If someday the human race really wants to get outside the Earth, just like the first caravel navigators and explorers left Europe (who also used astrometry), and explore the stars and galaxies alike in spaceships, astrometry will be needed. Perhaps those spaceships will even have "astrometric labs" to indicate the path to "boldly go where no-one has gone before"...

This feature article was written as part of the Cosmic Diary Cornerstone project for the International Year of Astronomy 2009. To find out more, check out www.cosmicdiary.org and www.astronomy2009.org.