Two decades after the end of the Hipparchos mission, another breakthrough arrived. The spacecraft orbited Earth for about four years, allowing astronomers to probe the neighbourhood of the sun up to the distance of 300 light-years with the accuracy of 0.001 arcseconds. This mission, launched by ESA in 1989, measured the positions and parallaxes as well as proper motions (the motion of a star on the sky observed over the years that is not caused by the parallax but reflects the actual movement of the star in space), for nearly 120,000 stars. (Image credit: ESA) Breakthrough in parallax measurements and galaxy mappingĪ real breakthrough in parallax measurement and therefore in determining distances of stars in our galaxy, the Milky Way, came with a mission called Hipparchos, after the ancient Greek astronomer that first used the method to estimate the distance of the moon. Because stars also move in space on their own trajectories, these circles actually turn into a spiral.
The parallax effect causes the stars to seemingly perform tiny circles in the sky every year. "Today, with advanced technologies such as adaptive optics and interferometry, we can reach accuracies of a few dozen micro-arcsecond on large ground-based telescopes," Jos de Bruijne, an astronomer at the European Space Agency (ESA) said in a statement. But the flickering effect caused by Earth's atmosphere and the distortion of the telescope observations caused by Earth's gravity prevented astronomers from reaching a precision better than about 0.01 arcseconds (one arcsecond is an angular measurement equal to 1/3600 of a degree).
His catalogue was extended to about 6,000 stars by Louise Freeland Jenkins in 1952, and to over 8,000 stars by William van Altena in 1995. In 1924, American astronomer Frank Schlesinger published a catalogue with the parallaxes of almost 2,000 stars, probing stellar distances out to a few dozen light-years from Earth. Over the following decades, astronomers, aided by the improvements in telescope technology gradually grew the catalogs of stellar distances using the parallax method. By the early 20th century, the list of stars with measured parallaxes grew to a few hundred, mostly thanks to the work of Dutch astronomer Jacobus Kapteyn. In the late 1830s, Bessel’s contemporaries and rivals Wilhelm Struve and Thomas Henderson provided one parallax measurement each, bringing the total number to three. This was the beginning of the long and tedious process of building a three-dimensional map of the universe. Based on his observations, Bessel calculated that the star 61 Cygni, one of the stars in the Cygnus constellation, must be about 10 light-years away from Earth. The first person to succeed at measuring the distance to a star using the parallax method was German astronomer Friedrich Bessel in 1838. Cassini subsequently used those measurements to compute the parallax determining Mars' distance from Earth. In 1672, Italian astronomer Giovanni Cassini and his colleague Jean Richer made simultaneous observations of Mars, with Cassini in Paris and Richer in French Guiana. The ancient Greek astronomer Hipparchus reportedly used observations of a solar eclipse from two different locations to calculate the distance of Earth's celestial companion. The first known astronomical measurement using parallax didn't involve a star but the moon.
The history of parallax measurements in astronomy
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