The Parallax Limit
2. Pushing the Boundaries of Measurement
So, if parallax is so great, why don't we use it to measure the distance to every star? Well, there's a catch. The farther away a star is, the smaller its parallax angle becomes. Eventually, the angle gets so small that it becomes incredibly difficult, if not impossible, to measure accurately. It's like trying to see that tiny shift of your thumb against a background that's light-years away!
The limitation comes down to the precision of our instruments. Ground-based telescopes have to contend with the blurring effects of Earth's atmosphere. This makes measuring those tiny parallax angles a real challenge. For a long time, the practical limit for ground-based parallax measurements was around 100 parsecs (about 326 light-years).
However, things changed dramatically when we started launching telescopes into space. Free from the distorting effects of the atmosphere, space-based observatories can achieve much higher precision. The Hipparcos satellite, launched in 1989, revolutionized parallax measurements, allowing astronomers to determine distances out to several hundred parsecs with good accuracy.
But even Hipparcos had its limits. The current champion is the Gaia spacecraft, launched by the European Space Agency in 2013. Gaia is designed specifically to measure the positions and motions of billions of stars with unprecedented accuracy. Its basically creating a 3D map of our galaxy. With Gaia, we can now measure parallax out to thousands of parsecs!
