AstroNavigation

Plotting Exercise

Now that you have completed the AstroNav course, you can practice the fundamentals of plotting using the AstroNav technique with the example of the lower limb of the Sun from the course, along with two additional sights. This is called a 3-star fix.

Assuming that we have been anchored since we have taken the Sun sight, we wait until twilight to take two more sights. The second will be Diphda, the most brilliant star in the constellation Cetus the Whale, and the third will be Capella, the brightest star in the constellation Auriga and third brightest in the northern celestial hemisphere.

The first step is to set up the longitude and latitude lines on the plotting paper and marking the dead reckoning position N 30° 12′ and W 60° 10′ .This was illustrated for the Sun example during the Plotting a Line of Position video. Remember to correctly scale the longitude lines.

Use the data in the table below to plot the three sights (all three sights use the same dead reckoning position because the boat is anchored):

Celestial Body Sun Diphda Capella
Time (GMT) 13:35:09 21:57:05 22:02:25
Total Observed Latitude (ho) 26° 28.2′ 41° 27.1′ 31° 42.3′
Assumed Latitude N 30° N 30° N 30°
Assumed Longitude W 60° 10′ W 60° 31.0′ W 60° 28.8′
Total Computed Altitude (hc) 25° 52.4 41° 28.6′ 31° 1.5′
Azimuth (Zn) 141.9° 169.8° 51.5°
Note that you must calculate the intercept from hc and ho. You may wish to use  a blank SRF for each of these two new sights.

Here are some useful plotting guidelines:

1. Mark the Assumed Position (AP) for your Celestial Body (CB)
2. Determine the direction of the Celestial Body (CB):
  • Use the compass at center of paper, mark the Azimuth (Zn) of the Celestial Body (CB), clockwise from north.
  • With a ruler, connect the center of the compass with the Azimuth (Zn) mark.
  • Move that ruler to the AP using a rolling ruler making sure to keep the ruler parallel to the original direction.
  • Draw the azimuth line and an arrowhead in the direction of the CB and label.
3. Mark the intercept distance from the AP, towards or away from the CB
4. Draw a perpendicular line to the azimuth line at the intercept distance; label it with the GMT time and name of the Celestial Body (CB).

 

The three LOP’s intersect and form a triangle. At the center of that triangle is the highest probability of your position, the 3-star fix. This is what the 3-star fix example should look like.

Data for the examples are taken from the U.S. Naval Observatory’s Celestial Navigation Data website.