Trelleborg has been used as a measuring instrument with following functions:
1. Determining daymarks and aetter (sharing the horizon like a compassrose)
2. determining length of the day - telling time without a clock
3. determining days of the year - calendarfunctions
4. measuring the height of the sun (altitude) for navigating purposes:
- for a special amplitude (azimuth) on a selected latitude
- for special meridians
Daymarks and aetter has been used constructing all bearing-dials and sunstones shown on this website. The orientations of the marks dividing the horizon on the left and above are S - N and N - S. The Vikings used front and backsite of the bearingdials for sailing to and from homeport SE - NE and SW - NW and S - N or N - S knowing and controlling the amplitude (sunrise and sunset) and the altitude of the noon sun.