stjernetid

Stjernetid is the Danish term for sidereal time, a form of timekeeping that measures rotation of the Earth relative to distant stars rather than the Sun. It is used in astronomy to track the apparent positions of stars and other celestial objects and to navigate and point telescopes. Because the stars appear to move across the sky as the Earth rotates, stjernetid advances at a different rate than civil or solar time.

The word is formed from stjerne, meaning star, and tid, meaning time. In common usage, stjernetid often refers to local sidereal time (LST), which corresponds to the right ascension currently crossing the local meridian. LST increases by about four minutes per solar day, so a full 24-hour cycle of sidereal time is completed in roughly 23 hours 56 minutes of solar time.

In practice, stjernetid is used to predict when a given star or celestial object will transit the local meridian, which is when it reaches its highest point in the sky and is best placed for observation. Observers convert a celestial object's right ascension into a time that aligns with the local sidereal time to plan telescope tracking and exposure windows. Greenwich sidereal time (GMST) and local longitude are typically involved in translating between universal time and stjernetid for a specific observing location.

Historically, stjernetid played a central role in celestial navigation and astronomical observations. Today it remains a standard concept in astronomy education and software, helping both amateur and professional observers align instruments with the stars.