Astrometry

Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry to explain their origin and their overall evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, meteoroids, asteroids, and comets. Relevant phenomena include supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, astronomy studies everything that originates beyond Earth's atmosphere. Cosmology is the branch of astronomy that studies the universe as a whole.

Astronomy is one of the oldest natural sciences. The early civilizations in recorded history made methodical observations of the night sky. These include the Egyptians, Babylonians, Greeks, Indians, Chinese, Maya, and many ancient indigenous peoples of the Americas. In the past, astronomy included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars.

In astronomy, the fixed stars (Latin: stellae fixae) are the luminary points, mainly stars, that appear not to move relative to one another against the darkness of the night sky in the background. This is in contrast to those lights visible to the naked eye, namely the planets and comets, which appear to move slowly among those "fixed" stars. The fixed stars include all the stars visible to the naked eye other than the Sun, as well as the faint band of the Milky Way. Due to their star-like appearance when viewed with the naked eye, the few visible individual nebulae and other deep-sky objects are also counted among the fixed stars. Approximately 6,000 stars are visible to the naked eye under optimal conditions.

The term fixed stars is a misnomer because those celestial objects are not actually fixed with respect to one another or to Earth. Due to their immense distance from Earth, these objects appear to move so slowly in the sky that the change in their relative positions is nearly imperceptible on human timescales, except under careful examination with modern instruments, such as telescopes, that can reveal their proper motions. Hence, they can be considered to be "fixed" for many purposes, such as navigation, charting of stars, astrometry, and timekeeping.

Proper motion is the angular speed of a celestial object, such as a star, as it moves across the sky. It is an astrometric measure, giving an object's change in angular position over time relative to the center of mass of the Solar System. This parameter is measured relative to the distant stars or a stable reference such as the International Celestial Reference Frame (ICRF). Patterns in proper motion reveal larger structures like stellar streams, the general rotation of the Milky Way disk, and the random motions of stars in the Galactic halo.

The components for proper motion in the equatorial coordinate system (of a given epoch, often J2000.0) are given in the direction of right ascension (μ_α) and of declination (μ_δ). Their combined value is computed as the total proper motion (μ). It has dimensions of angle per time, typically arcseconds per year or milliarcseconds per year.

Spherical astronomy, or positional astronomy, is a branch of observational astronomy used to locate astronomical objects on the celestial sphere, as seen at a particular date, time, and location on Earth. It relies on the mathematical methods of spherical trigonometry and the measurements of astrometry.

This is the oldest branch of astronomy and dates back to antiquity. Observations of celestial objects have been, and continue to be, important for religious and astrological purposes, as well as for timekeeping and navigation. The science of actually measuring positions of celestial objects in the sky is known as astrometry.

In geodesy and astrometry, earth orientation parameters (EOP) describe irregularities in the rotation of planet Earth.EOP provide the rotational transform from the International Terrestrial Reference System (ITRS) to the International Celestial Reference System (ICRS), or vice versa, as a function of time.

Earth's rotational velocity is not constant over time. Any motion of mass in or on Earth causes a slowdown or speedup of the rotation speed, or a change of rotation axis. Small motions produce changes too small to be measured, but movements of very large mass, like sea currents, tides, or those resulting from earthquakes, can produce discernible changes in the rotation and can change very precise astronomical observations. Global simulations of atmosphere, ocean, and land dynamics are used to create effective angular momentum (EAM) functions that can be used to predict changes in EOP.

A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars using a telescope, in which case they are called visual binaries. Many visual binaries have long orbital periods of several centuries or millennia and therefore have orbits which are uncertain or poorly known. They may also be detected by indirect techniques, such as spectroscopy (spectroscopic binaries) or astrometry (astrometric binaries). If a binary star happens to orbit in a plane along our line of sight, its components will eclipse and transit each other; these pairs are called eclipsing binaries, or, together with other binaries that change brightness as they orbit, photometric binaries.

If components in binary star systems are close enough, they can gravitationally distort each other's outer stellar atmospheres. In some cases, these close binary systems can exchange mass, which may bring their evolution to stages that single stars cannot attain. Examples of binaries are Sirius and Cygnus X-1 (Cygnus X-1 being a well-known black hole). Binary stars are also common as the nuclei of many planetary nebulae, and are the progenitors of both novae and type Ia supernovae.

The following nearest bright stars are found within 15.0 parsecs (48.9 ly) of the closest star, the Sun, and have an absolute magnitude of +8.5 or brighter, which is approximately comparable to a listing of stars more luminous than a red dwarf. Right ascension and declination coordinates are for the epoch J2000. The distance measurements are based on the Hipparcos Catalogue and other astrometric data. In the event of a spectroscopic binary, the combined spectral type and absolute magnitude are listed in italics.

Gaia was a space observatory of the European Space Agency (ESA) that was launched in 2013 and operated until March 2025. The spacecraft was designed for astrometry: measuring the positions, distances and motions of stars with unprecedented precision, and the positions of exoplanets by measuring attributes about the stars they orbit such as their apparent magnitude and color. As of May 2025, the mission data processing continues, aiming to construct the largest and most precise 3D space catalog ever made, totalling approximately 1 billion astronomical objects, mainly stars, but also planets, comets, asteroids and quasars, among others.

To study the precise position and motion of its target objects, the spacecraft monitored each of them about 70 times over the five years of the nominal mission (2014–2019), and about as many during its extension. Due to its detectors degrading more slowly than initially expected, the mission was given an extension, lasting until March 27, 2025, when scientists at the ESA switched off Gaia after more than a decade of service. Gaia targeted objects brighter than magnitude 20 in a broad photometric band that covered the extended visual range between near-UV and near infrared; such objects represent approximately 1% of the Milky Way population. Additionally, Gaia was expected to detect thousands to tens of thousands of Jupiter-sized exoplanets beyond the Solar System by using the astrometry method, 500,000 quasars outside this galaxy and tens of thousands of known and new asteroids and comets within the Solar System.

Hipparcos was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions and distances of celestial objects on the sky. This was the first practical attempt at all-sky absolute parallax measurement, something not possible with groundside observatories, and thus represented a fundamental breakthrough in astronomy. The resulting high-precision measurements of the absolute positions, proper motions, and parallaxes of stars enabled better calculations of their distance and tangential velocity; when combined with radial velocity measurements from spectroscopy, astrophysicists were able to finally measure all six quantities needed to determine the motion of stars. The resulting Hipparcos Catalogue, a high-precision catalogue of more than 118,200 stars, was published in 1997. The lower-precision Tycho Catalogue of more than a million stars was published at the same time, while the enhanced Tycho-2 Catalogue of 2.5 million stars was published in 2000. Hipparcos's follow-up mission, Gaia, was launched in 2013.

The word "Hipparcos" is an acronym for High Precision Parallax Collecting Satellite and also a reference to the ancient Greek astronomer Hipparchus of Nicaea, who is noted for applications of trigonometry to astronomy and his discovery of the precession of the equinoxes.

35P/Herschel–Rigollet is a Halley-type comet with an orbital period of 155 years and an orbital inclination of 64 degrees. It was first discovered by Caroline Herschel on 21 December 1788. Given that the comet has a 155-year orbit involving asymmetric outgassing, and astrometric observations in 1939 were not as precise as modern observations, predictions for the next perihelion passage in 2092 vary by about a month.

A critical-list minor planet (critical list numbered object or critical object) is a numbered minor planet for which existing measurements of the orbit and position are especially in need of improvement.

The IAU's Minor Planet Center (MPC) regularly publishes a list of these critical objects in their Minor Planet Electronic Circular. The list typically contains asteroids that have been observed at a small number of apparitions, especially on opposition, or that have not been adequately observed for more than 10 years, while other observatories create their own, customized lists. The MPC also lists currently observable critical objects on their website, providing differently formatted lists of orbital elements to the worldwide astrometric community.

Barnard's Star is a small red dwarf star in the constellation of Ophiuchus. At a distance of 5.96 light-years (1.83 pc) from Earth, it is the fourth-nearest-known individual star to the Sun after the three components of the Alpha Centauri system, and is the closest star in the northern celestial hemisphere. Its stellar mass is about 16% of the Sun's, and it has 19% of the Sun's diameter. Despite its proximity, the star has a dim apparent visual magnitude of +9.5 and is invisible to the unaided eye; it is much brighter in the infrared than in visible light.

Barnard's Star is among the most studied red dwarfs because of its proximity and favorable location for observation near the celestial equator. Historically, research on Barnard's Star has focused on measuring its stellar characteristics, its astrometry, and also refining the limits of possible extrasolar planets. Although Barnard's Star is ancient, it still experiences stellar flare events, one being observed in 1998.