Calibrating the Cross Staff

Cross staffs came with three or four transoms of varying lengths, each measuring a different range of angles. The scale for each transom would be engraved along its own side of the staff.

Calibrate your cross staff using the figures in Figure 13.10. If you have no means of measuring inches or centimetres, use five coins taped together as your base unit. Calibrating a cross staff makes it obvious that it is easiest to use for altitudes of between 30-60°.

How Use Cross Staff

Line up the top edge of the cross staff with the horizon.

Slide transom along until Polaris appears on the top edge, or in the ring, groove, short length of narrow pipe or whatever you have used as a sight.

Check top edge is still aligned with, horizon.

Check Polaris is still in the sight.

Repeat last two steps until you are satisfied all is well.

Take reading.

13.9 Using the Cross Staff The Back Staff

To avoid staring into the sun navigators fitted a small box with a slot called a Dutch Shoe to the transom, held the cross staff backwards and stood with their back to the sun. The slot threw a sharp line of sunlight on the staff when the sun was 'on'.

A variety of backwards-shooting instruments soon appeared. The best known was designed by John Davis, a 16th-century English navigator whose name is remembered in the Davis Strait. He described his design in The Seaman's Secrets. It employs the same principles as the cross staff but allows the observer to stand with his back to the sun and uses the shadows it casts to measure altitude. This also eliminated ocular parallax but, as stars cast no shadows, it could not be used at night, until the mirrored back staff was introduced. Davies' back staff remained in use for over 200 years

Distance from Eye in Inches

Distance from Eye in Centimetres

Distance from Eye in Inches

Distance from Eye in Centimetres

6 inch

7 inch

8 inch

9 inch

15 cm

18cm

20cm

agrees

Transom

Transom

Transom

Transom

tangent

Degrees

Transom

Transom

Transom

10

34.0

39.7

45.4

51.0

0.1763

10

85.1

102.1

113.4

11

30.9

36.0

41.2

46.3

0.1944

11

77.2

92.6

102.9

12

28.2

32.9

37.6

42.3

0.2126

12

70.6

84.7

94.1

13

26.0

30.3

34.7

39.0

0.2309

13

65.0

78.0

86.6

14

24.1

28.1

32.1

36.1

0.2493

14

60.2

72.2

80.2

15

22.4

26.1

29.9

33.6

0.2679

15

56.0

67.2

74.6

16

20.9

24.4

27.9

31.4

0.2867

16

52.3

62.8

69.7

17

19.6

22.9

26.2

29.4

0.3057

17

49.1

58.9

65.4

18

18.5

21.5

24.6

27.7

0.3249

18

46.2

55.4

61.6

19

17.4

20.3

23.2

26.1

0.3443

19

43.6

52.3

58.1

20

16.5

19.2

22.0

24.7

0.3640

20

41.2

49.5

54.9

21

15.6

18.2

20.8

23.4

0.3839

21

39.1

46.9

52.1

22

14.9

17.3

19.8

22.3

0.4040

22

37.1

44.6

49.5

23

14.1

16.5

18.8

21.2

0.4245

23

35.3

42.4

47.1

24

13.5

15.7

18.0

20.2

0.4452

24

33.7

40.4

44.9

25

12.9

15.0

17.2

19.3

0.4663

25

32.2

38.6

42.9

26

12.3

14.4

16.4

18.5

0.4877

26

30.8

36.9

41.0

27

11.8

13.7

15.7

17.7

0.5095

27

29.4

35.3

39.3

28

11.3

13.2

15.0

16.9

0.5317

28

28.2

33.9

37.6

29

10.8

12.6

14.4

16.2

0.5543

29

27.1

32.5

36.1

30

10.4

12.1

13.9

15.6

0.5774

30

26.0

31.2

34.6

31

10.0

11.6

13.3

15.0

0.6009

31

25.0

30.0

33.3

32

9.6

11.2

12.8

14.4

0.6249

32

24.0

28.8

32.0

33

9.2

10.8

12.3

13.9

0.6494

33

23.1

27.7

30.8

34

8.9

10.4

11.9

13.3

0.6745

34

22.2

26.7

29.7

35

8.6

10.0

11.4

12.9

0.7002

35

21.4

25.7

28.6

36

8.3

9.6

11.0

12.4

0.7265

36

20.6

24.8

27.5

37

8.0

9.3

10.6

11.9

0.7536

37

19.9

23.9

26.5

38

7.7

9.0

10.2

11.5

0.7813

38

19.2

23.0

25.6

39

7.4

8.6

9.9

11.1

0.8098

39

18.5

22.2

24.7

40

7.2

8.3

9.5

10.7

0.8391

40

17.9

21.5

23.8

41

6.9

8.1

9.2

10.4

0.8693

41

17.3

20.7

23.0

42

6.7

7.8

8.9

10.0

0.9004

42

16.7

20.0

22.2

43

6.4

7.5

8.6

9.7

0.9325

43

16.1

19.3

21.4

44

6.2

7.2

8.3

9.3

0.9657

44

15.5

18.6

20.7

45

6.0

7.0

8.0

9.0

1.0000

45

15.0

18.0

20.0

46

5.8

6.8

7.7

8.7

1.0355

46

14.5

17.4

19.3

47

5.6

6.5

7.5

8.4

1.0724

47

14.0

16.8

18.7

48

5.4

6.3

7.2

8.1

1.1106

48

13.5

16.2

18.0

49

5.2

6.1

7.0

7.8

1.1504

49

13.0

15.6

17.4

50

5.0

5.9

6.7

7.6

1.1918

50

12.6

15.1

16.8

51

4.9

5.7

6.5

7.3

1.2349

51

12.1

14.6

16.2

52

4.7

5.5

6.3

7.0

1.2799

52

11.7

14.1

15.6

53

4.5

5.3

6.0

6.8

1.3270

53

11.3

13.6

15.1

54

4.4

5.1

5.8

6.5

1.3764

54

10.9

13.1

14.5

55

4.2

4.9

5.6

6.3

1.4281

55

10.5

12.6

14.0

13.10 Calibrating a Cross Staff (continued overleaf )

56

4.0

4.7

5.4

6.1

1.4826

56

10.1

12.1

13.5

57

3.9

4.5

5.2

5.8

1.5399

57

9.7

11.7

13.0

58

3.7

4.4

5.0

5.6

1.6003

58

9.4

11.2

12.5

59

3.6

4.2

4.8

5.4

1.6643

59

9.0

10.8

12.0

60

3.5

4.0

4.6

5.2

1.7321

60

8.7

10.4

11.5

61

3.3

3.9

4.4

5.0

1.8040

61

8.3

10.0

11.1

62

3.2

3.7

4.3

4.8

1.8807

62

8.0

9.6

10.6

63

3.1

3.6

4.1

4.6

1.9626

63

7.6

9.2

10.2

64

2.9

3.4

3.9

4.4

2.0503

64

7.3

8.8

9.8

65

2.8

3.3

3.7

4.2

2.1445

65

7.0

8.4

9.3

66

2.7

3.1

3.6

4.0

2.2460

66

6.7

8.0

8.9

67

2.5

3.0

3.4

3.8

2.3559

67

6.4

7.6

8.5

68

2.4

2.8

3.2

3.6

2.4751

68

6.1

7.3

8.1

69

2.3

2.7

3.1

3.5

2.6051

69

5.8

6.9

7.7

70

2.2

2.5

2.9

3.3

2.7475

70

5.5

6.6

7.3

71

2.1

2.4

2.8

3.1

2.9042

71

5.2

6.2

6.9

72

1.9

2.3

2.6

2.9

3.0777

72

4.9

5.8

6.5

73

1.8

2.1

2.4

2.8

3.2709

73

4.6

5.5

6.1

74

1.7

2.0

2.3

2.6

3.4874

74

4.3

5.2

5.7

75

1.6

1.9

2.1

2.4

3.7321

75

4.0

4.8

5.4

76

1.5

1.7

2.0

2.2

4.0108

76

3.7

4.5

5.0

77

1.4

1.6

1.8

2.1

4.3315

77

3.5

4.2

4.6

78

1.3

1.5

1.7

1.9

4.7046

78

3.2

3.8

4.3

79

1.2

1.4

1.6

1.7

5.1446

79

2.9

3.5

3.9

80

1.1

1.2

1.4

1.6

5.6713

80

2.6

3.2

3.5

81

1.0

1.1

1.3

1.4

6.3138

81

2.4

2.9

3.2

82

0.8

1.0

1.1

1.3

7.1154

82

2.1

2.5

2.8

83

0.7

0.9

1.0

1.1

8.1443

83

1.8

2.2

2.5

84

0.6

0.7

0.8

0.9

9.5144

84

1.6

1.9

2.1

85

0.5

0.6

0.7

0.8

11.4301

85

1.3

1.6

before it was eventually displaced by the octant and is the first navigational instrument whose inventor is known.

Some of these remarkably simple instruments can give acceptable results. The Kamal is the simplest. A shadow astrolabe allows you take noon sights without staring into the sun. A sun shadow board confirms your latitude, but you may obtain better results using a quadrant or a cross staff. However, the instrument that gives you the best results is the one you are most comfortable using.

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  • Ruby
    How to use cross staff?
    1 year ago

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