II Scan the text to match the clauses together to make sentences.

A

1 Human navigators use

2 The peak elevation angle occurs

3 Thus, at local noon or midnight, latitude can be calculated by

4 When time began to be broadcast to vehicles in the 1930s, off-meridian

observations of theelevation angles of two or more celestial bodies became

5 These fixes were

6 In the 1930s, hand-held bubble-level sextants were built

7 The human navigator observed

8 The accuracy of celestial fixes was

9 The first automatic star trackers, built in the late 1950s,

B

a measured the azimuth and elevation angles of stars relative to a gyroscopically

stabilized platform.

b 5–50 miles in the air, limited by the uncertainty in the horizon and the inability to make precise

angular measurements on a pitching, rolling vehicle.

c sun and stars through an astrodome on top of the aircraft.

d to measure the elevation of celestial bodies from an aircraft without the need to

see the horizon.

e hand-calculated using logarithms, then plotted on charts by a navigator.

f possible at any known time of night (cloud cover permitting).

g simple arithmetic from a table of declination (the angle of the sun or star above

the Earth’s equatorial plane).

h at local noon or midnight:elevation angle (degrees) = 90 – latitude + declination.

i sextants to measure the elevation angle of celestial bodies above the visible

horizon.