Astronomy & Celestial Mechanics
the sky, computed
Concept

Horizon coordinates — where it appears

Altitude and azimuth, measured from the observer's own horizon and meridian. They are what a telescope on an altazimuth mount is pointed in, and they are valid for one place and one instant, since the sky turns through them.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

The sky from latitude 52°. The celestial sphere seen from latitude 52 degrees. The pole stands 52 degrees above the horizon, the celestial equator meets the horizon due east and west, and a star at declination 20 degrees traces the drawn circle once a day. Everything below the horizon is drawn faint.

The sphere that is not there, and why it is still the right model

The stars are at wildly different distances and the celestial sphere is a fiction. It is also the most useful fiction in observational astronomy, because for pointing at things, distance is exactly the information to throw away.

sky · Celestial sphere One star, two coordinate systems, at latitude 52°. The equatorial grid and the horizon grid drawn on the same sphere for an observer at latitude 52°. The star marked has declination 20° and hour angle -40° in the first, and altitude 45.5° and azimuth 239.4° in the second. The two frames differ by a single rotation through the co-latitude 38°, which is why the celestial pole stands 52° above the northern horizon.

Where a star is depends on who is asking

The sky needs two coordinate systems because two different things stay still in it — the observer's horizon and the stars themselves. One rotation converts between them, and the angle of that rotation is the time.

sky · Celestial sphere

Named alongside it

The objects these essays reach for when they reach for this one.

Celestial sphereCircumpolarDeclinationEpochEquinoxSidereal timeAstrometryNutationParsecReference framesSolstice

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