Principles of Celestial Navigation

A Day in the Life of a Navigator » Three-Star Fix, Evening » Exercises

A partially completed strip form is available for printing here. Fill in the values on the form, then enter your results in the boxes below. If you do not have a Nautical Almanac, you can view an altitude correction table here.

Enter the Ho for each star here:




Altair 21° 23.3'

Spica 39° 56.4'

Dubhe 41° 28.8'

Using the U.S. Naval Observatory web site, find Hc and Zn for each star. Go to to get this information. You’ll need to enter the date and time (17 July 2018, 23:10:25 UT) and the assumed position (32° 23.5' N, 55° 44.3' W).

Enter the computed altitude (Hc) and azimuth (Zn) for each star:

Altair     Hc     Zn

Spica     Hc     Zn

Dubhe     Hc     Zn

Altair Hc 21° 19.8'     Zn 92.9°

Spica Hc 39° 45.2'     Zn 214.1°

Dubhe Hc 41° 33.9'     Zn 326.1°

Next, find the intercept for the line of position of each star based on difference between Ho and Hc. Enter your answers in the boxes below.

For each star, enter the distance and direction to intercept:




Altair 3.5 minutes toward

Spica 11.2 minutes toward

Dubhe 5.1 minutes away

If you did not get these values, you may wish to recheck your work and make sure you understand each step before proceeding. You can compare your completed strip form with the filled-in version (available as pdf).

Plot the Evening Three-Star Fix

The differences, and Ho-Mo-To, are used to plot the three lines of position. Print the pdf of the plotting form to work on drawing the corresponding lines of position. When you have finished, continue to the next page to view Chief Sheedy correctly plotting the three-star evening fix.

printable version of Universal Plotting Sheet with dead reckoning marked for beginning to plot three-star evening fix.