Stellar parallax

parallax shiftparallaxparallax methodparallax measurementsannual parallaxparallax of a starparallax techniqueparallaxessecular parallaxsecular parallaxes
Stellar parallax is the apparent shift of position of any nearby star (or other object) against the background of distant objects.wikipedia
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61 Cygni

61 Cyg61 Cygni A61 Cyg A
It was first observed in 1806 by Giuseppe Calandrelli who reported parallax in α-Lyrae in his work "Osservazione e riflessione sulla parallasse annua dall’alfa della Lira". Then in 1838 Friedrich Bessel made the first successful parallax measurement, for the star 61 Cygni, using a Fraunhofer heliometer at Königsberg Observatory.
In 1838, Friedrich Bessel measured its distance from Earth at about 10.4 light-years, very close to the actual value of about 11.4 light-years; this was the first distance estimate for any star other than the Sun, and first star to have its stellar parallax measured.

Friedrich Bessel

BesselFriedrich Wilhelm BesselBessel, Friedrich
It was first observed in 1806 by Giuseppe Calandrelli who reported parallax in α-Lyrae in his work "Osservazione e riflessione sulla parallasse annua dall’alfa della Lira". Then in 1838 Friedrich Bessel made the first successful parallax measurement, for the star 61 Cygni, using a Fraunhofer heliometer at Königsberg Observatory.
Astronomers had believed for some time that parallax would provide the first accurate measurement of interstellar distances—in fact, in the 1830s there was a fierce competition between astronomers to be the first to measure a stellar parallax accurately.

Astrometry

astrometricastrometricalastrometrist
Even with 21st-century techniques in astrometry, the limits of accurate measurement make distances farther away than about 100 parsecs (or roughly 300 light years) too approximate to be useful when obtained by this technique.
James Bradley first tried to measure stellar parallaxes in 1729.

Aberration of light

aberrationstellar aberrationconstant of aberration
The stellar movement proved too insignificant for his telescope, but he instead discovered the aberration of light and the nutation of Earth's axis, and catalogued 3222 stars.
Aberration should not be confused with parallax.

Heliometer

heliometers
It was first observed in 1806 by Giuseppe Calandrelli who reported parallax in α-Lyrae in his work "Osservazione e riflessione sulla parallasse annua dall’alfa della Lira". Then in 1838 Friedrich Bessel made the first successful parallax measurement, for the star 61 Cygni, using a Fraunhofer heliometer at Königsberg Observatory.
The first successful measurements of stellar parallax (to determine the distance to a star) were made by Friedrich Bessel in 1838 for the star 61 Cygni using a Fraunhofer heliometer.

Koenigsberg Observatory

Königsberg ObservatoryobservatoryKönigsberg
It was first observed in 1806 by Giuseppe Calandrelli who reported parallax in α-Lyrae in his work "Osservazione e riflessione sulla parallasse annua dall’alfa della Lira". Then in 1838 Friedrich Bessel made the first successful parallax measurement, for the star 61 Cygni, using a Fraunhofer heliometer at Königsberg Observatory.
In 1838, the parallax of a star was determined successfully for first time by Bessel using a heliometer of Fraunhofer.

Cosmic distance ladder

standard candlestandard candlesdistance
Stellar parallax remains the standard for calibrating other measurement methods (see Cosmic distance ladder).
This statistical parallax method is useful for measuring the distances of bright stars beyond 50 parsecs and giant variable stars, including Cepheids and the RR Lyrae variables.

Heliocentrism

heliocentricheliocentric modelheliocentric theory
Stellar parallax is so small (as to be unobservable until the 19th century) that its apparent absence was used as a scientific argument against heliocentrism during the early modern age.
Bessel proved that the parallax of a star was greater than zero by measuring the parallax of 0.314 arcseconds of a star named 61 Cygni.

Tycho Brahe

BraheTycho1572 supernova
It is clear from Euclid's geometry that the effect would be undetectable if the stars were far enough away, but for various reasons such gigantic distances involved seemed entirely implausible: it was one of Tycho Brahe's principal objections to Copernican heliocentrism that in order for it to be compatible with the lack of observable stellar parallax, there would have to be an enormous and unlikely void between the orbit of Saturn and the eighth sphere (the fixed stars).
With respect to the stars, Tycho also believed that, if the Earth orbited the Sun annually, there should be an observable stellar parallax over any period of six months, during which the angular orientation of a given star would change thanks to Earth's changing position.

Copernican heliocentrism

Copernican systemCopernican heliocentricheliocentric theory
It is clear from Euclid's geometry that the effect would be undetectable if the stars were far enough away, but for various reasons such gigantic distances involved seemed entirely implausible: it was one of Tycho Brahe's principal objections to Copernican heliocentrism that in order for it to be compatible with the lack of observable stellar parallax, there would have to be an enormous and unlikely void between the orbit of Saturn and the eighth sphere (the fixed stars).
Yet it ascribes to the Earth, that hulking, lazy body, unfit for motion, a motion as quick as that of the aethereal torches, and a triple motion at that.” Likewise, Tycho took issue with the vast distances to the stars that Copernicus had assumed in order to explain why the Earth's motion produced no visible changes in the appearance of the fixed stars (known as annual stellar parallax).

TAU (spacecraft)

TAU
TAU (spacecraft) (an abandoned space mission project that would have used parallax)
One scientific purpose would be to measure the distance to other stars via stellar parallax.

Gaia (spacecraft)

GaiaGaia spacecraftGaia'' spacecraft
The European Space Agency's Gaia mission, launched 19 December 2013, is expected to measure parallax angles to an accuracy of 10 microarcseconds for all moderately bright stars, thus mapping nearby stars (and potentially planets) up to a distance of tens of thousands of light-years from Earth.
To determine the intrinsic luminosity of a star requires knowledge of its distance. One of the few ways to achieve this without physical assumptions is through the star's parallax. Ground-based observations would not measure such parallaxes with sufficient precision due to the effects of the atmosphere and instrumental biases. For instance, Cepheid variables are used as standard candles to measure distances to galaxies, but the accuracy in their own distance measurement is poor. Thus, quantities depending on them, such as the speed of expansion of the universe, remain inaccurate. Measuring their distances accurately has a great impact on the understanding of the other galaxies and thus the whole cosmos (see cosmic distance ladder).

Star catalogue

star catalogNLTTLTT
Automated plate-measuring machines and more sophisticated computer technology of the 1960s allowed more efficient compilation of star catalogues.
It is particularly notable for its parallax measurements, which are considerably more accurate than those produced by ground-based observations.

Photometric parallax method

photometric parallax
Other uses of the term parallax in astronomy, with different meanings are the photometric parallax method, spectroscopic parallax, and dynamical parallax.
Unlike the stellar parallax method, photometric parallax can be used to estimate the distances of stars over 10 kpc away, at the expense of much more limited accuracy for individual measurements.

Moving-cluster method

moving cluster method
Moving cluster method
Stellar parallax

Earth's orbit

its orbitorbitEarth orbits
Created by the different orbital positions of Earth, the extremely small observed shift is largest at time intervals of about six months, when Earth arrives at exactly opposite sides of the Sun in its orbit, giving a baseline distance of about two astronomical units between observations.

Astronomical unit

AUastronomical unitsAUs
Created by the different orbital positions of Earth, the extremely small observed shift is largest at time intervals of about six months, when Earth arrives at exactly opposite sides of the Sun in its orbit, giving a baseline distance of about two astronomical units between observations. The parallax itself is considered to be half of this maximum, about equivalent to the observational shift that would occur due to the different positions of Earth and the Sun, a baseline of one astronomical unit (AU).

Parallax

trigonometric parallaxsolar parallaxmotion parallax
The parallax itself is considered to be half of this maximum, about equivalent to the observational shift that would occur due to the different positions of Earth and the Sun, a baseline of one astronomical unit (AU).

Trigonometry

trigonometrictrigonometricaltrigonometrically
Once a star's parallax is known, its distance from Earth can be computed trigonometrically.

Parsec

Mpcpckpc
Even with 21st-century techniques in astrometry, the limits of accurate measurement make distances farther away than about 100 parsecs (or roughly 300 light years) too approximate to be useful when obtained by this technique.

Electromagnetic spectrum

spectrumspectrawhite light
Other techniques, such as spectral red-shift, are required to measure the distance of more remote objects.

Redshift

red shiftzred-shift
Other techniques, such as spectral red-shift, are required to measure the distance of more remote objects.

Minute and second of arc

masarcsecondarc second
The European Space Agency's Gaia mission, launched 19 December 2013, is expected to measure parallax angles to an accuracy of 10 microarcseconds for all moderately bright stars, thus mapping nearby stars (and potentially planets) up to a distance of tens of thousands of light-years from Earth. Stellar parallax measures are given in the tiny units of arcseconds, or even in thousandths of arcseconds (milliarcseconds).

Cathetus

cathetileglegs
The distance unit parsec is defined as the length of the leg of a right triangle adjacent to the angle of one arcsecond at one vertex, where the other leg is 1 AU long.

Right triangle

right-angled triangleright angled triangleright angle triangle
The distance unit parsec is defined as the length of the leg of a right triangle adjacent to the angle of one arcsecond at one vertex, where the other leg is 1 AU long.