The oldest science
Astronomy is the study of everything beyond Earth's atmosphere: planets, stars, galaxies and the universe as a whole. Ancient cultures tracked the Sun, Moon and planets to build calendars and navigate. Modern astronomy combines observation with physics to explain how these objects form and change.
Light is the messenger
Almost everything we know about the cosmos arrives as electromagnetic radiation. Visible light is only a narrow band; radio, infrared, ultraviolet, X-rays and gamma rays each reveal different processes. Cool dust glows in infrared, hot gas around black holes shines in X-rays.
Spreading light into a spectrum reveals dark or bright lines at specific wavelengths. Each element leaves its own fingerprint, so a spectrum tells us what a star is made of, how hot it is and how fast it moves towards or away from us (the Doppler shift).
Measuring the sky
Positions are measured as angles. The full Moon is about half a degree across. Brightness is measured in magnitudes, where smaller numbers are brighter and a difference of 5 magnitudes is a factor of 100 in brightness.
Distances are the hardest measurement. For nearby stars astronomers use parallax — the small apparent shift as Earth orbits the Sun. ESA's Gaia mission measured parallaxes for almost two billion stars.