Astrometry is the term for measuring the positions of objects in the night sky. Long before telescopes were around, it was the main tool used by astronomers. 

The method uses lots of maths to work out where things are in space. There are special coordinate systems to define positions in the sky. These can use the position of stars in the night sky compared to things on Earth, like the North Pole and equator. In modern astronomy, they include the distance from Earth, making them 3D. All the systems used need us to be able to measure very accurate angles. These angles are very small. In space, we use units called arcminutes and arcseconds to measure angles, rather than degrees. There are 60 arcminutes in 1 degree. And there are 60 arcseconds in 1 arcminute. 

Thousands of years ago people first mapped the planets and bright stars they could see in the night sky. As technology got better many more millions of stars have been found inside our own galaxy. In the 1900s we found other galaxies all around us in the Universe. So today maps of the night sky contain many, many more objects. 

Astrometry is used to track objects in the sky and to understand their motions. We can use it to see trails of stars from small galaxies which have been pulled into larger systems. It can also help us work out the distances to far away objects in space, by comparing them to things that we know are close to us.

It has even helped us find planets going around other stars by seeing small wobbles in their position. This happens because the planet's gravity pulls on the host star.
It might seem simple to map positions, but in space, it is very hard to make accurate measurements. We have even launched satellites, like Gaia, to help. Gaia has mapped one billion stars inside the Milky Way, better than ever before. But this is still only 1 percent of the stars in our galaxy. It can see much fainter stars than telescopes on Earth, more than 4000 times fainter than we can see with the human eye. So far the mission has found stars in double, triple, and even quadruple systems. These are complicated with 4 stars all in orbit around each other. Gaia is predicted to also find thousands of new exoplanets and even new supernova explosions.