Course / The intercept method

Part III · The line of position · Page 1 of 8

Hc: the altitude at the AP

The line of position module took Hc as given. Here is where it comes from. The computed altitude, Hc, is the altitude the body would have at the assumed position. Its circle of equal altitude passes through the AP; your own circle (from Ho) passes through the boat. Both are centered on the same ground position.

APboattoward the Sun, 156°circle through the AP: Hccircle through the boat: Ho
About 40 miles across. Both circles are centered on the Sun's ground position, far off to the east.
Hc, the altitude at the AP
51°45.6'
Ho, measured at the boat
51°51.6'
Ho − Hc
+6.0′ = 6.0 nm toward

Slide the boat along its circle: Ho does not change, so neither does the intercept. The sight says how far nearer the Sun you are than the AP, not where along the line.

Intercept = Ho − Hc, in minutes = miles; toward the body if Ho is greater

So the intercept is measured from the AP, along the bearing Zn, and the LOP is drawn square to Zn through that point. Sight reduction tables give you Hc and Zn for the AP without any trigonometry.