Part IV · The fix · Page 3 of 5
The running fix
In daytime there is often only the Sun. One Sun line in the morning and another a few hours later, when its bearing has changed, can still make a fix: move the first line forward by the boat's run between the sights.
- Run between the sights
- 6 kn × 3 h = 18.0 nm
- First line
- moved along the track, keeping its direction
Every point of the first line moves with the boat, so the whole line slides along the course for the distance run. Where the moved line crosses the second is the running fix. Its accuracy depends on the course and speed you used: use the course and speed made good, not just the heading.
- Plot the first line as usual.
- Work out the run: speed × time (2 h 30 min is 2.5 hours).
- From any point on the first line, step off the run along the course, and draw the line again through that point, parallel to the first.
- Plot the second line. Where it crosses the advanced line is the running fix, for the time of the second sight.
The second line is often the noon latitude. A latitude is a line of position too, running east–west. To plot it like any other line, use the DR as the AP: Zn is 180° if the Sun bore south (000° if north), and the intercept is the difference between the DR latitude and your noon latitude, toward the Sun if your latitude is nearer it.