Celestial Navigation

Question 1

What is twilight?

Answer 1

The period when the suns centre is below the celestial horizon but the observer is still receiving light reflected and scattered by the upper atmosphere

Question 2

What are the three phases of twilight?

Answer 2

Astronomical twilight - when the sun is 18 degrees below the celestial horizon

Nautical twilight - when the sun is 12 degrees below the celestial horizon

Civil twilight - when the sun is 6 degrees below the celestial horizon

Question 3

What is the celestial horizon?

Answer 3

A great circle, the plane of which is 90 degrees at all points from the observers zenith

Question 4

What is the theoretical horizon?

Answer 4

The visible horizon with all the refractive elements of the atmosphere removed

Question 5

What is the Greenwich Hour Angle?

Answer 5

It is the angular distance “celestial longitude” given as degrees west of the Greenwich meridian

Question 6

What is declination?

Answer 6

It is the angular distance (celestial latitude) North or South of the celestial equator

Question 7

What is the Local Hour Angle?

Answer 7

It is the angular distance of the observed body to the west of the observers meridian

Question 8

What is Dip?

Answer 8

The difference between the terrestrial horizon and the visible horizon due to the observers height of eye

Question 9

What are the errors of the sextant?

Answer 9

1. Perpendicularity error
2. Side Error
3. Index Error
4. Instrument error

Question 10

What is sextant perpendicularity error and how do you correct it?

Answer 10

The index mirror is not perpendicular to the plane of the instrument.

Holding the sextant horizontally with the index arm approx 60 degrees, the reflected image and the true arc should join seamlessly. If they do not, use the screw on the back of the index mirror to correct.

Question 11

What is sextant side error and how do you correct it?

Answer 11

When the horizon mirror is not perpendicular with the plane of the instrument.

With the sextant held vertically, do the true and reflected image and the true image overlap? If they appear side by side, side error is present. Use the side screw of the horizon mirror to correct.

Question 11

What is sextant index error and how do you correct?

Answer 11

The index mirror and the horizon mirror are not parallel when the index arm is at zero degrees.
True image and reflected image should appear side by side. If there is any step, index error is present.
Correct using the top screw of the horizon mirror or if small, apply to the sextant altitude.

Question 11

What are the 4 sextant instrument errors?

Answer 11

1. Centering error - the index arm pivots off centre
2. Prismatic error - The two faces of the mirror are not parallel
3. Shade error - the shade faces are not parallel to each other
4. Graduation error - error of the arc or vernier scales

Question 12

What is the minimum height above horizon that sights should be taken?

Answer 12

10 degrees due to refraction in the atmosphere causing errors

Question 13

How do you correct sextant altitude to true altitude?

Answer 13

1. Apply any index error to the sextant altitude
2. Subtract the dip correction
3. Apply the altitude correction for refraction, semidiameter and parallax

Question 14

Why is a noon sight important?

Answer 14

At local noon, the sun is due North or South on the observers meridian. Calculating the suns GHA at this time, gives the observers longitude. More importantly, the suns altitude +/- its declination at this time, gives the observers latitude.
Used with other position lines, it can give a position fix.

Question 15

What is the procedure for taking a noon sight to find latitude.

Answer 15

1. Calculate meridional passage (When the sun crosses the observers meridian)
2. Record the time and sextant altitude when the sun is at its highest point.
3. Apply sextant index error to give observed altitude. 90 degrees - sextant altitude = zenith distance between the observer zenith and the body.
4. Find the suns declination (angular distance between the body and the celestial equator) in the nautical almanac.
5. Using the ZD and the declination, latitude can be calculated.

Question 16

What information can be obtained from a noon fix?

Answer 16

1. Latitude (position line)
2. Azimuth to find compass error
3. Azimuth and intercept to give a position line
4. Local Noon

Question 17

What are the five methods of calculating an azimuth of a heavenly body?

Answer 17

1. A, B, C Method by tables
2. A, B, C method using calculation
3. By formula
4. Sight reduction tables
5. Amplitude

Question 18

How do you find the azimuth of a heavenly body by tables to find the true bearing?

Answer 18

Information required - LHA, Declination and Latitude

Enter table A with LHA and latitude to give a value for A
(named opposite to latitude unless LHA is between 90 and 270 degrees)

Enter table B with LHA and declination to give a value for B
(value is always the same as declination)

C = A+/-B
(A and B same sign +, A and B Different sign -)

C takes the sign of the greater of A and B

Enter table C with c value and latitude to find the azimuth

When LHA < 180 - Azimuth is West (western hemisphere)
When LHA > 180 - Azimuth is East (Eastern hemisphere)

Azimuth is added or subtracted from either North or South depending on whether it is Easterly or Westerly to give the true bearing.

Question 19

How do you find the azimuth of a heavenly body by A, B, C by calculation?

Answer 19

Information required - LHA, Declination and latitude

A = Tan Lat / Tan LHA
B = Tan Dec / Sin LHA
C = A +/- B

C is named the greater of A and B

Az = Tan-1 (1 / C x Cos Lat)

LHA < 180 Azimuth is West (Western Hemisphere)
LHA > 180 Azimuth is East (Eastern hemisphere)

Question 20

How do you find azimuth of a heavenly body using the formula method?

Answer 20

Information required - Latitude, LHA and Declination

tan Az = (Sin -LHA)
————
(cos +/- Lat x tan +/- Dec) - (Sin +/- Lat x Cos - LHA)

Lat N = +, S = -
Dec N = +, S = -

LHA 000-180 180-360

+Az S Az W N Az E
-Az N Az W S Az E

Azimuth added or subtracted from N or S depending whether it is E or W

Question 21

When can an amplitude be calculated?

Answer 21

At sunrise or sunset

Question 22

At what point does the bearing of the sun and time have to be taken for an amplitude?

Answer 22

When the sun is half a diameter above the horizon

Question 23

What are the two methods of calculating azimuth by amplitude?

Answer 23

1. By equation
2. By using the tables

Question 24

How do you calculate the azimuth by amplitude calculation?

Answer 24

Information i need - Declination, Latitude

1. Take a bearing of the sun and note the exact time in GMT
2. Use the formula

Sin Az = Sin Dec / Cos Lat

Az named same N/S as declination

Body rising = named E
Body setting = named W

Add or subtract from E or W depending on whether it is N or S

Question 25

How do you calculate amplitude by using tables?

Answer 25

Enter the amplitude tables with latitude and suns declination to give the bearing of rising or setting sun

Question 26

How do you take a sun sight and use the sight reduction tables to find a position line?

Answer 26

1. Take a sextant sun sight and correct to give an observed altitude
2. Convert the time to GMT
3. Enter nautical almanac and get GHA and declination, applying increments and corrections
4. Use the closest longitude to your DR with minutes and seconds the same as the GHA. Calculate LHA (should be a whole number)
5. Choose a whole number latitude closest to your DR. Now have an assumed lat and long
6. Enter sight reduction tables with LHA and latitude to get azimuth and calculated altitude
7. Mark the line of azimuth from the assumed position
8. Compare the observed altitude to calculated altitude to get the intercept either towards or away from the observed body and draw perpendicular to the azimuth line to get the fix.