ASA 107 – CELESTIAL NAVIGATION EXAM PRACTICE
QUESTIONS AND CORRECT ANSWERS (VERIFIED
ANSWERS) PLUS RATIONALE 2026 Q&A
1–20: Time, Longitude and Chronometer
1. What is the fundamental relationship between longitude and time used in celestial
navigation?
A) 15° of longitude corresponds to 1 hour of time
B) 10° of longitude corresponds to 1 hour of time
C) 20° of longitude corresponds to 1 hour of time
D) 30° of longitude corresponds to 1 hour of time
Correct Answer: A) 15° of longitude corresponds to 1 hour of time
Rationale: Earth rotates through 360° in approximately 24 hours. Dividing 360° by 24 gives
15° per hour, or 1° every four minutes. This relationship allows a navigator to convert
longitude differences into time differences. It is fundamental when converting between
local time, zone time, and Greenwich Mean Time.
2. A vessel is at 45° W longitude. What is the approximate time difference from
Greenwich?
A) 2 hours
B) 3 hours
C) 4 hours
D) 5 hours
Correct Answer: B) 3 hours
Rationale: Longitude is converted to time by dividing degrees by 15. At 45° west, 45 ÷ 15
equals 3 hours. Because the position is west of Greenwich, local mean time is
approximately three hours behind Greenwich time. The direction of longitude determines
whether the time difference is added or subtracted.
,3. What happens to local time as a vessel travels westward?
A) It becomes earlier
B) It remains unchanged
C) It becomes later
D) It changes only at midnight
Correct Answer: C) It becomes later
Rationale: Earth rotates from west to east, so locations farther west experience local solar
time later than locations farther east when comparing the same instant. Consequently,
traveling westward requires adding time when converting longitude differences. This
relationship is essential when determining local hour angle and converting observed times
to GMT.
4. A navigator needs to convert a known GMT to zone time. Which information is most
directly required?
A) Vessel speed
B) Compass deviation
C) Sextant index error
D) The vessel’s time-zone description or longitude-based zone
Correct Answer: D) The vessel’s time-zone description or longitude-based zone
Rationale: Zone time is related to GMT through the vessel’s assigned time zone. A navigator
must know the applicable zone correction and its sign to convert correctly. This is separate
from sextant corrections, compass deviation, or vessel speed. Accurate time conversion is
especially important because celestial calculations depend strongly on precise time.
5. How many degrees of longitude correspond to four minutes of time?
A) 1°
, B) 4°
C) 15°
D) 60°
Correct Answer: A) 1°
Rationale: Since Earth rotates 15° per hour, it rotates 1° every four minutes. This provides a
convenient mental conversion for celestial navigation calculations. For example, a
longitude difference of 7°30′ corresponds to 30 minutes of time. Remembering this
relationship helps reduce errors during manual navigation calculations.
6. A chronometer gains 2 seconds per day. After five days, approximately how much
will it gain?
A) 2 seconds
B) 5 seconds
C) 10 seconds
D) 20 seconds
Correct Answer: C) 10 seconds
Rationale: A daily rate describes the amount by which a watch or chronometer gains or
loses each day. A gain of two seconds per day over five days produces a total gain of 10
seconds. The navigator must apply this accumulated error when determining accurate
observation time.
7. What is the primary purpose of determining chronometer error?
A) To correct the vessel’s compass heading
B) To establish the difference between indicated and correct time
C) To calculate the height of eye
D) To determine atmospheric pressure
Correct Answer: B) To establish the difference between indicated and correct time
, Rationale: Chronometer error represents the difference between the time indicated by the
instrument and the correct reference time. Because celestial observations depend on
accurate time, even a relatively small time error can produce a significant positional error.
Daily rate allows the navigator to update a previously determined chronometer error.
8. A watch loses 3 seconds per day. After four days, what is the accumulated loss?
A) 3 seconds
B) 7 seconds
C) 9 seconds
D) 12 seconds
Correct Answer: D) 12 seconds
Rationale: The daily rate is multiplied by the number of elapsed days. A loss of three
seconds per day for four days gives 3 × 4 = 12 seconds. The navigator must account for the
accumulated loss rather than applying only the single-day rate when determining
observation time.
9. Which reference time is the fundamental basis for celestial navigation time
calculations?
A) Greenwich time
B) Local apparent time only
C) Ship’s bell time
D) Sunrise time
Correct Answer: A) Greenwich time
Rationale: Greenwich-based time provides a universal reference for celestial calculations.
The Nautical Almanac gives celestial information referenced to Greenwich, so
observations must be related accurately to that reference. Local or ship’s time can still be
used operationally, but it must be correctly converted to the required reference time.
10. If 30° of longitude corresponds to a time difference, what is that difference?
QUESTIONS AND CORRECT ANSWERS (VERIFIED
ANSWERS) PLUS RATIONALE 2026 Q&A
1–20: Time, Longitude and Chronometer
1. What is the fundamental relationship between longitude and time used in celestial
navigation?
A) 15° of longitude corresponds to 1 hour of time
B) 10° of longitude corresponds to 1 hour of time
C) 20° of longitude corresponds to 1 hour of time
D) 30° of longitude corresponds to 1 hour of time
Correct Answer: A) 15° of longitude corresponds to 1 hour of time
Rationale: Earth rotates through 360° in approximately 24 hours. Dividing 360° by 24 gives
15° per hour, or 1° every four minutes. This relationship allows a navigator to convert
longitude differences into time differences. It is fundamental when converting between
local time, zone time, and Greenwich Mean Time.
2. A vessel is at 45° W longitude. What is the approximate time difference from
Greenwich?
A) 2 hours
B) 3 hours
C) 4 hours
D) 5 hours
Correct Answer: B) 3 hours
Rationale: Longitude is converted to time by dividing degrees by 15. At 45° west, 45 ÷ 15
equals 3 hours. Because the position is west of Greenwich, local mean time is
approximately three hours behind Greenwich time. The direction of longitude determines
whether the time difference is added or subtracted.
,3. What happens to local time as a vessel travels westward?
A) It becomes earlier
B) It remains unchanged
C) It becomes later
D) It changes only at midnight
Correct Answer: C) It becomes later
Rationale: Earth rotates from west to east, so locations farther west experience local solar
time later than locations farther east when comparing the same instant. Consequently,
traveling westward requires adding time when converting longitude differences. This
relationship is essential when determining local hour angle and converting observed times
to GMT.
4. A navigator needs to convert a known GMT to zone time. Which information is most
directly required?
A) Vessel speed
B) Compass deviation
C) Sextant index error
D) The vessel’s time-zone description or longitude-based zone
Correct Answer: D) The vessel’s time-zone description or longitude-based zone
Rationale: Zone time is related to GMT through the vessel’s assigned time zone. A navigator
must know the applicable zone correction and its sign to convert correctly. This is separate
from sextant corrections, compass deviation, or vessel speed. Accurate time conversion is
especially important because celestial calculations depend strongly on precise time.
5. How many degrees of longitude correspond to four minutes of time?
A) 1°
, B) 4°
C) 15°
D) 60°
Correct Answer: A) 1°
Rationale: Since Earth rotates 15° per hour, it rotates 1° every four minutes. This provides a
convenient mental conversion for celestial navigation calculations. For example, a
longitude difference of 7°30′ corresponds to 30 minutes of time. Remembering this
relationship helps reduce errors during manual navigation calculations.
6. A chronometer gains 2 seconds per day. After five days, approximately how much
will it gain?
A) 2 seconds
B) 5 seconds
C) 10 seconds
D) 20 seconds
Correct Answer: C) 10 seconds
Rationale: A daily rate describes the amount by which a watch or chronometer gains or
loses each day. A gain of two seconds per day over five days produces a total gain of 10
seconds. The navigator must apply this accumulated error when determining accurate
observation time.
7. What is the primary purpose of determining chronometer error?
A) To correct the vessel’s compass heading
B) To establish the difference between indicated and correct time
C) To calculate the height of eye
D) To determine atmospheric pressure
Correct Answer: B) To establish the difference between indicated and correct time
, Rationale: Chronometer error represents the difference between the time indicated by the
instrument and the correct reference time. Because celestial observations depend on
accurate time, even a relatively small time error can produce a significant positional error.
Daily rate allows the navigator to update a previously determined chronometer error.
8. A watch loses 3 seconds per day. After four days, what is the accumulated loss?
A) 3 seconds
B) 7 seconds
C) 9 seconds
D) 12 seconds
Correct Answer: D) 12 seconds
Rationale: The daily rate is multiplied by the number of elapsed days. A loss of three
seconds per day for four days gives 3 × 4 = 12 seconds. The navigator must account for the
accumulated loss rather than applying only the single-day rate when determining
observation time.
9. Which reference time is the fundamental basis for celestial navigation time
calculations?
A) Greenwich time
B) Local apparent time only
C) Ship’s bell time
D) Sunrise time
Correct Answer: A) Greenwich time
Rationale: Greenwich-based time provides a universal reference for celestial calculations.
The Nautical Almanac gives celestial information referenced to Greenwich, so
observations must be related accurately to that reference. Local or ship’s time can still be
used operationally, but it must be correctly converted to the required reference time.
10. If 30° of longitude corresponds to a time difference, what is that difference?
