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Started on GPT-6-Sol, then moved to GPT-6-Sol

Astronomy studies celestial objects and phenomena, including stars, planets and galaxies. Astrometry measures their positions and motion; astrophysics uses physics to explain their properties and behaviour.

Much of astronomy begins with incoming light. An image tells us which direction the light arrived from, rather than directly providing the source's three-dimensional position. Two stars close together on the sky may be very far apart in space. The celestial sphere organizes these viewing directions; locating an object on it is a different task from measuring how far away that object is. Repeated observations turn this limitation into an opportunity: the changing direction of light contains evidence about both Euclidean distance and motion.

The particularly satisfying combination is stellar parallax with proper motion. The first supplies a geometric Euclidean distance, while the second measures how rapidly the viewing direction changes after accounting for the observer's annual motion. Together they give a transverse speed, connecting tiny angles on an image to kilometres travelled each second. This bridge from a flat sky map to moving stars is one reason astrometry matters to the study of the Milky Way.

A representation used to predict or explain the apparent arrangements and motion of celestial objects. Agreement with a limited observation set need not identify a unique physical arrangement.

A model in which the Earth is stationary at the centre of the relevant celestial system. Distinguish this broad class from a particular ancient planetary arrangement.

A geoheliocentric arrangement with the Sun circling a stationary Earth and other planets circling the Sun. It can reproduce the phases of Venus without an orbiting Earth.

An astronomical model in which Earth and the other planets orbit the Sun. Its acceptance and detailed mathematical form have distinct histories; heliocentrism alone does not imply elliptic orbits.

The angle between two lines of sight. For a small transverse physical separation $s$ at distance $D$, $\theta \simeq s/D$ in radians. At cosmological distances $D$ is the appropriate angular diameter distance.

A planispheric astrolabe uses a stereographic projection of the celestial sphere as a physical calculating device. A rotating star map is compared with a latitude-specific plate; a sighting rule measures an object's angular altitude. Aligning that measurement with the engraved geometry permits time and celestial-configuration calculations.^[ref]

A sighting rule used to measure a direction or angle. On a conventional astrolabe, it rotates over the back's angular scale; it is distinct from the front rule used to read the projected sky geometry.

The map from a three-dimensional astronomical distribution to its observed sky position or integrated surface signal. For a scalar volume emissivity $j(\mathbf{x})$ in simple transparent geometry, surface brightness is proportional to $\int jdl$ along the ray. Reconstructing the spatial distribution requires additional geometry or symmetry assumptions; a projected surface distribution is not a unique three-dimensional density.

Scientific collaboration responsible for analysis of the Planck mission data.

A solar eclipse occurs when the Moon obscures part or all of the Sun as seen from a specified location. Totality allows observations of nearby background stars that ordinary daylight hides.

An astronomer whose observations of aberration of light provided evidence for the motion of Earth and a finite speed of light.

An astronomer whose work includes the primordial Earthshine hypothesis for the lunar crustal dichotomy.

Astronomer (1473–1543) whose 1543 book placed Earth among planets orbiting the Sun. The heliocentric model altered the geometric organization of planetary motion.

Astronomer and first author of a 1984 cold-dark-matter galaxy-formation model.

Astronomer and author of research on X-ray plasma spectroscopy.

Radio astronomer and first author of a Very Long Baseline Array solar-deflection measurement published in 2009.

An astrophysicist studying stellar black holes and X-ray binaries.

An astronomer studying star-forming galaxies, active galactic nuclei and galactic outflows.

An astrophysicist studying galaxy formation and cosmological gas accretion.