English: The figure illustrates the definition of the hour angle system coordinates hour angle and declination. It shows a section of the sky measuring 200° × 70° with the horizon (green straight line) near the bottom. It is designed to be valid at the geographic latitude φ = 55°. Three of the four compass directions are shown at the horizon. In the right part of the figure, a yellow star is shown, whose horizon coordinates, azimuth Az and altitude h are marked in red. The blue curve shows the celestial equator, which can be thought of as an extension of the Earth's equator onto the celestial sphere. It intersects the horizon in the east (E) and in the west (W). In the hour angle system, one angular coordinate, the hour angle τ (tau), is measured from the intersection of the equator with the meridian to the star’s foot point on the equator; the hour angle is marked with magenta color. The second angular coordinate is the declination δ (delta), which is measured from the foot point up to the star. The star's coordinates in the horizon system here are (Az, h) = (40°, 50°) and in the hour angle system (τ, δ) = (26° 07′, +20° 11′) = (1h 44.5m, +20° 11′). The west point has the coordinates (τ, δ) = (90°, 0°) = (6h, 0°), as the hour angle is traditionally also given in time measure. Since 24h = 360°, 1h = 15°. The east point E has the coordinates (τ, δ) = (18h, 0°). The left part of the figure shows meshes in the hour angle system with an extent of 10° × 10°. By the projection onto the horizon system, the meshes have become deformed.