The Celestial Sphere — drag to rotate · drag star (✦) to reposition
Observer Properties
sidereal time:h
latitude:°
1.0×
Star Properties
right ascension:h
declination:°
you can also change the star's position by dragging it
hour angle: −2.00 h
Hour Angle = Sidereal Time − Right Ascension
altitude: +45.0° azimuth: 220.0°
Display
Sidereal Time and Hour Angle Demonstrator
This simulator shows how sidereal time and right ascension determine the hour angle of a star as seen from an observer on Earth.
Sidereal Time (ST) is the right ascension of the local meridian — the great circle passing through the zenith and the celestial poles.
Hour Angle (HA) = ST − RA. Negative HA means the star is east of the meridian (rising); positive means it is west (setting). HA = 0 at upper transit.
The green disk is the local horizon. The blue arc is the local meridian. The gold circle is the celestial equator. The white arc shows right ascension from the vernal equinox; the red arc shows declination. The dashed white circle is the star’s diurnal path. The faint dashed line ending in a Z dot marks the zenith — the point directly overhead; it moves as latitude changes. When “show hour angle arc” is checked, the orange arc along the diurnal path shows the angular distance the star has traveled from the meridian (its hour angle).
Drag the sphere to rotate the view. Drag the star (✦) to reposition it.