There is a release of new astronomical data by the European Space Agency satellite called “Gaia.” The main purpose of GAIA is to report accurate distances to stars
in the Milky Way. GAIA does this by measuring parallaxes.
This method relies on measuring what we might colloquially call “perspective.” The idea is that a high precision snapshot of a nearby star will make a pattern on the sky with respect to its fixed stellar neighbors. If you then wait for six months and take another snapshot image of that same nearby star, you will see a slightly different pattern.
The Difference between the two images is called parallax.
Physically, what happens is that view of a nearby star against the background of fixed stars change when the Earth is one one side of its orbit compared to when the Earth is on the other side of its orbit. Similar an artist will paint a completely different picture of the same room depending on which corner the artist sits down.
The ancient Greeks attempted to measure parallaxes to nearby stars 2500 years ago, but failed. As a result, these scientists concluded that the Earth must be at the center of the Solar System. It was simply not realized at that time that even the nearest stars are so distant that our eyes cannot discern this difference in parallax (or in perspective).
The first parallax measurements would have to wait until the 19th century. But by this time we had already adopted the Sun-centered Solar System and figured out that the apparent lack of parallax was an indication of vast distances between us and the stars.
Moving forward to the 21st centure, what GAIA can bring us is parallax measurements not just of the nearest stars, but of 1.7 billion stars. Put another way, GAIA measured the distances to about one percent of the stars in the Milky Way.
Because many measurements were made over a five year period, this gives us the ability also to measure the motions of stars and the orbits of clumps of densely-packed stars called globular clusters. This is leading to a better understanding of our place in the Galaxy, and the data are offered for free to the public.