![]() ![]() The first successful stellar parallax measurements were done by Thomas Henderson in Cape Town South Africa in 1832-1833, where he measured parallax of one of the closest stars ― alpha Centauri. In the second quarter of the 19 th century, technological progress reached to the level which provided sufficient accuracy and precision for stellar parallax measurements. Measurement of annual parallax was the first reliable way to determine the distances to the closest stars. The split lens of the Bamberg Heliometer (late 19th century) History of measurement Early theory and attempts For meaningful results in stellar astronomy, Dutch astronomer Floor van Leeuwen recommends that the parallax error be no more than 10% of the total parallax when computing this error estimate. The approximation is far more accurate for parallax errors that are small relative to the parallax than for relatively large errors. Where d is the distance and p is the parallax. The approximate distance is simply the reciprocal of the parallax: d (pc) ≈ 1 / p (arcsec). Because stellar parallaxes and distances all involve such skinny right triangles, a convenient trigonometric approximation can be used to convert parallaxes (in arcseconds) to distance (in parsecs). 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. Stellar parallax measures are given in the tiny units of arcseconds, or even in thousandths of arcseconds (milliarcseconds). The more distant an object is, the smaller its parallax. Thomas Henderson, Friedrich Georg Wilhelm von Struve, and Friedrich Bessel made first successful parallax measurements in 1832-1838, for the stars alpha Centauri, Vega, and 61 Cygni. Stellar parallax is so difficult to detect that its existence was the subject of much debate in astronomy for hundreds of years. 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). 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 opposite sides of the Sun in its orbit, giving a baseline distance of about two astronomical units between observations. By extension, it is a method for determining the distance to the star through trigonometry, the stellar parallax method. Stellar parallax is the apparent shift of position ( parallax) of any nearby star (or other object) against the background of distant stars. (1 AU and 1 parsec are not to scale, 1 parsec = ~206265 AU) Stellar parallax is the basis for the parsec, which is the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond. ![]()
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