Why astronomers measure sky positions in hours rather than degrees
A star's right ascension is written in hours, minutes and seconds, not degrees. The habit comes from how positions were first measured: by timing when each object crossed the meridian as Earth turned. One hour of right ascension equals 15 degrees, and 24 hours take you all the way around the sky.
The equatorial coordinate system is the standard way to pin down where objects sit in the sky. Picture Earth's equator projected outward onto an imaginary celestial sphere: that is the celestial equator, the system's base plane. The starting direction points to the March equinox, one of two spots where the Sun's yearly path crosses that equator. The frame is centred on Earth's core, as if the planet were see-through, and although it lines up with our equator and poles, it does not spin with us. It stays nearly fixed against the stars.
Two numbers do most of the work. Declination, written with the Greek delta, is the celestial version of latitude, running positive north of the celestial equator and negative south, up to plus 90 degrees at the north celestial pole. Right ascension, with the Greek alpha, counts eastward from the March equinox. Distant objects then share the same coordinates for every observer on Earth, unlike horizon-based positions that change with where you stand and keep shifting as the planet rotates. Equatorial telescope mounts with setting circles use these numbers, along with a star chart, to swing straight to a target.
The frame is not truly still. Earth's axis precesses, dragging the whole grid slowly westward around the poles of the ecliptic, one full turn in roughly 26,000 years, with a smaller nodding called nutation layered on top. So any position must name an epoch. The commonest is J2000.0, a fixed reference that lets catalogues from different dates be compared directly; others use the mean or true equinox of the observation date, the latter including every wobble.
Some observers prefer hour angle, which runs westward from their own meridian and grows steadily as Earth turns, effectively a clock of time since the object last stood highest in the sky.
Source: Equatorial coordinate system
