= Astronomy
{wiki}
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>.
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