Hour circle

[1] Together with declination and distance (from the planet's centre of mass), it determines the location of any celestial object.

As such, it is a higher concept than the meridian as defined in astronomy, which takes account of the terrain and depth to the centre of Earth at a ground observer's location.

The location of stars, planets, and other similarly distant objects is usually expressed in the following parameters, one for each of the three spatial dimensions: their declination, right ascension (epoch-fixed hour angle), and distance.

[2] A meridian on the celestial sphere matches an hour circle at any time.

The hour circles make for easy prediction of the angle (and time due to Earth's fairly regular rotation, approximately equal to the time) between the observation of two objects at the same, or similar declination.