How the Sky Turns · 8 of 8

Celestial Coordinates

How do you give a star an address a telescope can find?

Right Ascension and Declination: the two numbers that turn 'over there somewhere' into an address a telescope can drive to.

You’ll be able to read a star’s Right Ascension and Declination and find where on the sky that address is.

11 steps · about 8 min

Before this: Why the Sky Moves (8 min)

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This grid is Earth's latitude and longitude, inflated until it reaches the stars. Its pole is where Earth's axis points, and Polaris sits just short of it. Its on the readout is your latitude: the two grids are one grid.

Polaris's height is your latitude, from Finding the North Star

Both grids at once: latitude and longitude on the globe, and the same mesh hanging on the sky beyond it, sharing one axis. The sky's equator stands over Earth's equator, and its poles over Earth's poles.

Declination is latitude on the sky: degrees north or south of the , up to +90° at the pole. With north at the top, as on a map, read three off their tags — Capella well north, Mintaka on the line, Sirius south.

The celestial equator is Earth's equator projected outward, and Mintaka sits on it. Declination decides how long a star is up: at 0 it rises due east, sets due west and is up half the day, from every latitude. Run two hours: Mintaka comes up at the E.

Watch the grid, not the stars. Over four hours the whole mesh turns with the sky as one rigid thing, while your horizon and compass points stay put. Addresses belong to the stars; "high in the south" is only where one is right now.

The same turning, from space: the sky's grid holds still and the Earth turns inside it. Here the analogy breaks: longitude is bolted to the ground, while the sky's hours are bolted to the stars. So the sky needs a zero line of its own, like Greenwich.

The sky's zero line runs through the ring, where the Sun's path, the , crosses the equator going north each March: Right Ascension 0h. Still called the First Point of Aries, though it has slid into Pisces (The Wandering Equinox).

The equinox slides west, from The Wandering Equinox

Right Ascension counts east in hours, because the sky turns 15° in one. So RA is a clock: Procyon crosses the meridian, the line drawn from due south straight overhead, at its highest, three hours after Aldebaran.

A catalogue address: RA 6h 45m, Dec −16.7°. Start from Betelgeuse's tag and drive there — a little east, well south — and find what any telescope on Earth would, given those two numbers.

One catch: the equinox drifts, so catalogues state an epoch — here J2000.0 — and by 2026 a star can sit 22 from that address, the ring. Dec is latitude; RA is hours east of the March point; both ride with the stars.

Every star's address, read off the grid. Any chart or app that quotes RA and Dec now tells you where to look.

Dec is latitude on the sky
RA is hours east of the March point
Both ride with the stars

Celestial Coordinates

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