Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way. Astrometry also helps us to discover new planets, asteroids, comets, and other objects in the sky by detecting their effects on the motions of nearby stars.

Astrometry has a long history that dates back to ancient times, when people observed the sky with naked eyes and recorded the positions of stars and planets. With the invention of telescopes, astrometry became more accurate and detailed. Today, astrometry uses sophisticated instruments and methods, such as optical and radio telescopes, space probes, interferometry, and digital image processing. Astrometry also relies on mathematical models and standards to define reference frames and coordinate systems for the sky.

One of the most popular and useful applications of astrometry is Astrometry.net , a web service that allows anyone to upload an image of the sky and get back astrometric calibration meta-data, plus lists of known objects falling inside the field of view . Astrometry.net can handle images from any source, such as amateur telescopes, professional observatories, or even mobile phones . Astrometry.net aims to create correct, standards-compliant astrometric meta-data for every useful astronomical image ever taken, past and future, in any state of archival disarray . Astrometry.net hopes to help organize, annotate and make searchable all the world’s astronomical information .

Astrometry is a fascinating and important science that helps us understand our place in the universe. By measuring the sky with precision and accuracy, astrometry reveals the secrets of the stars and their motions.

One of the main challenges of astrometry is to measure very small angles and distances in the sky. For example, the nearest star to the Sun, Proxima Centauri, is about 4.2 light-years away, which means that its angular diameter is only 0.0002 arcseconds. To measure such a tiny angle, one needs a very high-resolution telescope and a very stable reference frame. Another challenge is to account for various sources of error and uncertainty that can affect the measurements, such as atmospheric turbulence, instrumental noise, and systematic biases. Astrometry requires careful calibration and correction of the data to achieve reliable and accurate results.

One of the main achievements of astrometry is the creation of star catalogs and maps that provide the positions, distances, and motions of millions of stars in the sky. Some of the most famous and comprehensive catalogs are the Hipparcos catalog, which contains data for about 120,000 stars, and the Gaia catalog, which contains data for about 1.3 billion stars. These catalogs are essential for many fields of astronomy, such as stellar evolution, galactic structure, and cosmology. They also help us to identify and study rare and exotic objects, such as binary stars, pulsars, quasars, and black holes.

Astrometry is not only a science of measurement, but also a science of discovery. By analyzing the subtle changes in the positions and motions of stars over time, astrometry can reveal the presence and properties of unseen companions or planets orbiting around them. This is how astrometry has contributed to the field of exoplanet research, which aims to find and characterize planets outside our Solar System. Astrometry can also detect the gravitational lensing effect, which occurs when a massive object bends the light from a distant source, creating multiple or distorted images. This effect can be used to measure the mass and distance of the lensing object, as well as to magnify the source object.