The 16th century Danish astronomer Tycho Brahe — arguably the most exacting observer of the pre-telescopic age — proved, contrary to the prevailing idea that comets were atmospheric phenomena, that they actually lay beyond the moon.
Tycho (pronounced “TEE-koh”) used a surveyor’s technique: an object’s distance can be determined by observing its direction from each end of an established baseline, since two angles and the length of one side is sufficient to fully describe a triangle. The shift of the distant object’s position is called parallax.
To extend his reach, Tycho used the entire earth as his baseline. By observing a celestial body at the start of the night, then again at the end, earth’s rotation carried him across its diameter, creating a baseline thousands of miles long.
Despite his most diligent efforts, however, Tycho was unable to detect the parallax of stars, implying they lay many earth-diameters away.
Indeed, not until 1838 (237 years after Tycho) did telescopes attain sufficient precision to measure stellar parallax, not using Earth as a baseline, but its 186 million mile wide orbit (and then only the nearest stars, with great difficulty).
Ground-based telescopes are still limited to measuring the nearest stars’ parallaxes, due to atmospheric distortion. A huge leap forward came in the 1990s with the HIPPARCOS satellite, which measured accurate parallaxes out to about 1000 light years. But that’s still only about 1 percent of the galaxy’s diameter.
HIPPARCOS’s successor, the Gaia satellite, is currently reaching even further out. Its most recent data release (the second of five planned) includes the distances of 1.3 billion stars. By the time its primary mission is completed in 2022, Gaia will have reached as far as the Galactic Center, giving us the most accurate map ever of the Milky Way.
Next column: The scariest thing in the universe.