Wednesday, May 21, 2008

Celestial Navigation (Getting Prepared)

But before you start taking altitudes of celestial objects, you must be able to find them with the sextant. Trying to find a star with the sextant on a rocking boat is not easy The eyepiece has a small angle of view and the sight is twisted by the sextant mirrors.
One way to find a star is to turn the sextant upside down, point it at the star, and bring the horizon up to the star by adjusting the arm.
It is best if you know the approximate altitudes and azimuths of the stars you are going to observe.

One of the best times to takes sights is on the 04-08 watch. Take advantage of the time of the day when the sky is in best condition for celestial navigation. In the civil twilights, the times when the sun is 6° below the horizon, it's dark enough to see the stars and planets, and light enough to see the horizon.

Take the following items to the deck of the boat:

1) Sextant.

2) Watch.

3) Pencil.

4) Paper, attach the paper to a board, so it's easy to take notes and your work will not be carried away by the wind.
Try to establish a routine to handle these items. You will be observing two numbers (altitude and time) at once, possibly on a rocking boat, so don't let these items make things difficult in the critical time. Save the documentation of your work.

When taking an observation, set your sextant to the expected altitude and point it to the expected azimuth, using a hand compass. The celestial body will probably show in your view.
Adjust the sextant to the correct instrumental altitude. Write name, time and altitude of the observed celestial object.

It's good practice to adjust the sextant micrometer drum always in the same direction. For example, put the star below the horizon and then bring it up by turning the drum in the same direction in all observations. If you go past, repeat the operation from the start. Do the same for the Index Error measurement. The sextant will give different readings, depending on the direction you adjust the drum. Using the same direction for both altitudes and index error measurements cancels this problem.

After adjusting the sextant's drum, read the watch first, because it's changing fast. Write the time. Then write the sextant altitude. Before taking altitudes, measure the Index Error,
Set the altitude to 0°00' and point to the horizon. Adjust the drum until both sides of the horizon are level. Read the Index Error and write it down.

To get good results in celestial navigation, you need to be methodic. As you know, there are many steps, and its easy to make a mistake.
Read the measurements out loud before writing your notes down. (navigators are said to speak to themselves). Make your notes in an organized format. In the header of the table, write date, assumed position, time of twilight and index error. Have a plan ready before going on deck.

But even with all the care, some errors will show. Wrong time or altitude (the so called 60 mile error). Bad star identification. Even wrong date. One of the biggest mistakes is simple adding and subtracting. The important thing is to detect mistakes.
Having good dead reckoning navigation helps a lot. If one of your lines seems out of place, you may have made a mistake. This is why it's good to take several sights.