Sextant and Cel Nav

Question 1

Uses of a Sextant?

Answer 1

• Altitude of a celestial body
• Horizontal sextant angle
• Vertical sextant angle

Question 2

What are the Correctable Errors?

Answer 2

• Perpendicularity
• Side Error
• Index Error

Question 3

What are the Non-correctable Errors?

Answer 3

• Prismatic
• Worm and Rack
• Graduation
• Shade Error
• Collimation
• Centering

Question 4

What is the difference between a great circle and a rhumb line?

Answer 4

Rhumb line

• Is a line on the Earth’s surface that cuts all meridians at the same angle
• It is a straight line on a Mercator chart and will be the ship’s course

Great Circle

• The shortest distance between two points on the Earth’s surface on a plane through the centre of the Earth
• It is a straight line a gnomonic chart
• Meridians are great circles so if you were steering north or south it would be a rhumb line
• Consider a great circle of a distance over 500 miles, however, it might not be appropriate in coastal areas

Plotting a Great Circle Route on a Mecator Chart
- Take a set of Lat and Long’s every 5° of longitude and this will give you a set of waypoints and a series of rhumb lines to steer.

Question 5

What could you use for a cel nav position fix?

Answer 5

• Mer Pass
• Sun, Run, Sun
• Star fix

Question 6

What calc would you do instead at sunrise and sunset when taking an azimuth?

Answer 6

• Amplitude

Question 7

Explain how you would calculate a mer’ pas’?

Answer 7

• Taken when the sun is the highest in the sky
• Workout time for mer’ pas’ in the Nautical Almanac, this will be in local time
• Bring the sun to the horizon and watch for the sun to stop rising and before it starts falling that is your altitude
• Apply corrections (index error, the height of eye, altitude correction)
• Latitude = (90° - Sextant Angle) +/- Declination of the Sun (depending on which hemisphere you’re in)
• Longitude = GHA