Chapter 25 — Time (2) | General Navigation

Conversion of Arc to Time

Since the Earth rotates 360° in 24 hours, there is a direct relationship between longitude (arc) and time. This is the fundamental link between geographic position and local time.

The Core Conversion Rule

360° of longitude = 24 hours → divide by 360:

360°

= 24 h 00 min

15°

= 1 h 00 min

1°

= 0 h 04 min

= 0 h 00 min 04 sec

30'

= 0 h 02 min

45'

= 0 h 03 min

Worked Example (Q1 from practice set): Convert 153°30' to time.

153° × 4 = 612 min = 10 h 12 min

30' × 4 sec = 120 sec = 2 min

Total = 10 h 14 min ✓ (Answer: c)

Degrees	Hours:Min	Degrees	Hours:Min	Degrees	Hours:Min
15°	1:00	90°	6:00	180°	12:00
30°	2:00	105°	7:00	270°	18:00
45°	3:00	120°	8:00	300°	20:00
60°	4:00	135°	9:00	330°	22:00
75°	5:00	150°	10:00	360°	24:00

Local Mean Time (LMT)

Local Mean Time is based on the position of the mean Sun relative to the observer's own meridian. It is the most natural form of local time.

📌 Definition: When the mean Sun transits (crosses) your meridian, it is 1200 LMT at your location.

📌 When the mean Sun crosses your anti-meridian (opposite meridian), it is 0000 LMT (midnight).

📌 Every degree of longitude represents exactly 4 minutes of LMT difference.

Figure 1 — Local Mean Time illustration — the mean Sun transiting the 45°E meridian shows 1200 LMT at all places on that meridian

LMT Calculation Rules

To convert LMT at one location to LMT at another — or to/from UTC:

➡ East of Greenwich: LMT = UTC + arc/time (East is ahead of UTC)

➡ West of Greenwich: LMT = UTC − arc/time (West is behind UTC)

Memory aid: East = Add, West = Subtract (EAWS / "Longitude East, UTC least")

Figure 2 — The relationship between LMT at different longitudes, showing how the date can change when crossing the 180° meridian

Co-ordinated Universal Time (UTC)

UTC is a single, worldwide time standard based on the mean Sun as seen from the Greenwich Meridian (0°). All aviation uses UTC (also called Zulu time) for flight plans, NOTAMs, METARs, AIPs and ATC.

✅ UTC = LMT at 0° longitude (Greenwich)

✅ No seasonal adjustments — UTC is constant year-round

✅ Used for ALL aviation operations globally

Zone Time (ZT)

Zone Time divides the Earth into 25 time zones, each approximately 15° of longitude wide (1 hour). Zone Time is the system used by ships at sea and on trans-oceanic routes.

Figure 3 — The 25 Zone Time zones — each 15° wide, labelled A through Z (excluding J). Zones east of Greenwich are Fast (+); zones west are Slow (−)

Zone Time System

Each zone is assigned a Zone Description (ZD) — the number of hours to add or subtract to get UTC:

• Zone A (+1 hour from UTC), Zone B (+2 hours), … Zone M (+12 hours)

• Zone N (−1 hour from UTC), Zone O (−2 hours), … Zone Y (−12 hours)

• Zone Z = UTC itself (Greenwich meridian area)

Formula: UTC = ZT + ZD (where ZD is negative for East zones when converting — add ZD to ZT to get UTC means subtract the fast description)

Simpler: UTC = ZT − Zone Description (where zone descriptions are +ve for East, −ve for West)

Ship/aircraft use: At Zone boundary crossings, ships advance or retard clocks by 1 hour. Aircraft on polar routes may use a specific fixed Zone Time (often UTC or the departure/arrival zone).

The International Date Line

The International Date Line (IDL) runs approximately along the 180° meridian (with deviations to keep island groups together). It is where the calendar date changes.

🔴 Crossing West to East (travelling eastwards, e.g. Pacific → Americas): go BACK one day (subtract 24 hours / repeat the day).

🔵 Crossing East to West (travelling westwards, e.g. Americas → Pacific): go FORWARD one day (add 24 hours / skip a day).

LMT boundary: At 180°E the LMT is 12 hours ahead of UTC. At 180°W the LMT is 12 hours behind UTC. These two times differ by 24 hours — hence the date must change at this meridian.

IDL Deviations: Fiji, Tonga and other islands have the IDL bent around them to maintain the same calendar date within each island group, even though some straddle the 180° line.

Standard Time (ST)

Standard Time is the legal time kept by a country, established by government legislation. It is usually based on Zone Time but can differ by half-hours or non-standard amounts.

Country/Region	Standard Time	Offset from UTC
Iraq	UTC+3	+3 hours
Libya	UTC+1	+1 hour
Tonga	UTC+13	+13 hours
Canada (Labrador)	UTC−4	−4 hours
New York (USA)	UTC−5	−5 hours
Ghana	UTC (no offset)	0 hours
Hong Kong	UTC+8	+8 hours
Australia (NSW/QLD)	UTC+10	+10 hours
New Zealand	UTC+12	+12 hours
India	UTC+5:30	+5 h 30 min

Summer Time (Daylight Saving Time / DST): Many countries advance clocks by 1 hour during summer. Countries marked * in the Air Almanac may keep summer time. When DST is in effect, add 1 hour to Standard Time for the offset.

Practice Questions

Set 1 — Multiple Choice

Q1. Without using the Air Almanac, convert 153°30' of arc of longitude into time.

a) 10 hrs 24 min
b) 10 hrs 22 min
c) 10 hrs 14 min
d) 10 hrs 8 min

Answer: (c) — 153° × 4 min = 612 min = 10 h 12 min. Plus 30' × 4 sec = 120 sec = 2 min. Total = 10 h 14 min.

Q2. The definition of Local Mean Time (LMT) is:

a) time based upon the average movement of the Earth around the Sun.
b) when the mean Sun is transiting your meridian, it is 1200 hrs LMT.
c) when the mean Sun is transiting your anti-meridian, it is 0000 hrs LMT (2400 hrs LMT, previous day).
d) all of the above.

Answer: (d) — All three statements correctly define or describe properties of Local Mean Time. LMT is based on the mean (average) Sun, it is 1200 at meridian transit, and 0000 at anti-meridian transit.

Q3. Local Mean Time (LMT) always changes by a day when crossing:

a) the Greenwich Meridian
b) 180°E/W
c) the International Date Line
d) the Equator

Answer: (b) — LMT (unlike Standard Time) changes by a full day at the mathematical 180° meridian. The International Date Line (IDL) follows this meridian but deviates for political reasons. The question asks specifically about LMT, which changes at the geometric 180°E/W line.

Q4. Zone Time (ZT) is used:

a) by aircraft on trans-oceanic routes
b) as legal time in all countries
c) by ships at sea
d) in polar regions

Answer: (c) — Zone Time is the system used specifically by ships at sea. Aircraft use UTC. Countries use Standard Time (not necessarily Zone Time). Polar regions often use UTC or a designated Standard Time.

Q5. A ship at longitude 83°E observes sunrise at a Zone Time of 0500 ZT on Zone Date 15th May. What is the UTC?

a) 2300 UTC 14th May
b) 1100 UTC 15th May
c) 2328 UTC 14th May
d) 1032 UTC 15th May

Answer: (a) — At 83°E, the zone description is +6 (nearest full zone: 83÷15 = 5.5 → round to +6). UTC = ZT − Zone Description = 0500 − 0600 = 2300 on the previous day (14th May). Note: the zone boundary is at 82.5°E, so 83°E falls in Zone +6.

Q6. On Mid-summer Day in the Southern hemisphere, the Sun will be overhead:

a) 66½°S
b) 23½°N
c) 23½°S
d) the Equator

Answer: (c) — SH Midsummer = December 21 (NH Winter Solstice). The Sun is overhead the Tropic of Capricorn at 23½°S.

Q7. On Mid-winter Day in the Northern hemisphere, the Sun will be overhead:

a) 66½°S
b) 23½°N
c) 23½°S
d) the Equator

Answer: (c) — NH Midwinter = December 21. The Sun is overhead the Tropic of Capricorn at 23½°S. (Same solar position as Q6 — different hemisphere perspective, same answer.)

Set 2 — Calculation Problems

Q1. At 48°30'N 103°15'E the time is 1045 LMT on 17th May. What is the LMT and local date at 48°30'S 007°15'E?

LMT @ 103°15'E 17th 10:45
Arc/Time 103°15'E − 6:53 (103°×4min + 15'×4sec = 412+60 = 472min = 6h52m → 6:53 rounded)
UTC 17th 03:52
Arc/Time 007°15'E + 0:29 (7°×4 = 28min + 15'×4sec = 60sec = 1min → 29min)
LMT @ 007°15'E 17th 04:21

Answer: 04:21 LMT on 17th May at 007°15'E.

Q2. When LMT at 46°N 008°30'W is 2300 on 9th May, what is the LMT at (a) 46°N 108°30'W and (b) 46°N 108°30'E?

LMT @ 008°30'W 9 May 23:00

a) Diff to 108°30'W = 100°00' → arc/time = 6:40
− 6:40
LMT @ 108°30'W 9 May 16:20

b) Diff from 008°30'W to 108°30'E = 117°00' → arc/time = 7:48 (note direction change: first to UTC then to East)
From LMT 008°30'W → UTC: +0:34
UTC 9 May 23:34
Arc/Time 108°30'E +7:14 (108°×4 + 30'×4sec = 432+120sec = 432+2 = 434min = 7h14min)
LMT @ 108°30'E 10 May 06:48

Answers: (a) 16:20 LMT 9 May | (b) 06:48 LMT 10 May

Q3. At 1300 UTC on 1st April, find LMT and local date at (a) 54°30'N 007°15'E, (b) 16°27'S 107°43'W, (c) 18°46'N 168°35'E.

UTC = 1300 on 1 April

a) 007°15'E Arc/Time +0:29
LMT 13:29 on 1 April

b) 107°43'W Arc/Time −7:11 (107×4=428, 43'×4sec=172sec=2m52s ≈ 7:11)
LMT 05:49 on 1 April

c) 168°35'E Arc/Time +11:14 (168×4=672, 35'×4sec=140sec=2m20s → 11:14:20)
LMT 00:14 on 2 April (crosses midnight → next day)

Answers: (a) 13:29 LMT 1 Apr | (b) 05:49 LMT 1 Apr | (c) 00:14 LMT 2 Apr

Q4. At 30°N 046°30'E the time is 0300 LMT. What is the time at 60°N 016°15'W in (a) UTC and (b) LMT?

LMT @ 046°30'E 03:00
Arc/Time 046°30'E − 3:06
UTC 23:54 previous day

a) UTC = 23:54

Arc/Time 016°15'W − 1:05 (16×4=64, 15'×4sec=60sec=1min → 1:05)
b) LMT @ 016°15'W = 22:49 previous day

Answers: (a) UTC = 23:54 | (b) LMT @ 016°15'W = 22:49 (previous day)

Q5. At 1400 UTC on 6th November, what is the Standard Time at Iraq, Libya, Tonga, Canada (Labrador), New York, and Ghana?

UTC = 1400 on 6 November

Iraq +3 = 1700 ST 6 Nov
Libya +1 = 1500 ST 6 Nov
Tonga +13 = 0300 ST 7 Nov
Labrador −4 = 1000 ST 6 Nov
New York −5 = 0900 ST 6 Nov
Ghana UTC = 1400 ST 6 Nov

Q6. In position 48°30'N 063°57'W the LMT is 1430 on 10th February. What is the Standard Time in Hong Kong (22°20'N 114°07'E)?

LMT @ 063°57'W 10 Feb 14:30
Arc/Time 063°57'W + 4:16 (63×4=252, 57'×4sec=228sec=3m48s → 4:15:48 ≈ 4:16)
UTC 10 Feb 18:46
ST correction HK + 8:00
ST Hong Kong 11 Feb 02:46

Answer: 02:46 ST on 11th February in Hong Kong.

Q7. Wellington NZ (41°10'S 174°54'E) to Samoa (13°50'S 171°45'W), flight time 6h 15min. ETA Samoa is 2350 LMT 8th September. Find departure ST from Wellington.

Arrival Samoa (LMT) 8 Sep 23:50
Arc/Time 171°45'W +11:27 (171×4=684, 45'×4sec=180sec=3min → 11:27)
Arrive Samoa (UTC) 9 Sep 11:17
Flight time − 6:15
Depart Wellington (UTC)9 Sep 05:02
ST correction NZ +12:00
Depart Wellington (ST)9 Sep 17:02

Answer: Depart Wellington at 17:02 ST on 9th September. Note: by flying east across the IDL, you effectively gain a day — Samoa LMT 2350 on 8th September is UTC 1117 on 9th September.

Set 3 — Extended Problems

Q1. Determine the LMT in Wellington, New Zealand (174°46'E) at 0730 UTC.

UTC 0730
Arc/Time 174°46'E +11:39 (174×4=696, 46'×4sec=184sec=3m04s → 11:39:04 ≈ 11:39)
LMT Wellington 19:09

Q2. LMT in Boston, Massachusetts (071°08'W) is 1200. What is the UTC?

LMT Boston 12:00
Arc/Time 071°08'W + 4:45 (71×4=284, 8'×4sec=32sec → 4:44:32 ≈ 4:45)
UTC 16:45

Q3. For 1335 UTC, find LMT in Honolulu (157°52'W) and Standard Time in Sydney NSW (151°13'E).

UTC = 1335

a) Honolulu −10:31 (157×4=628, 52'×4sec=208sec=3m28s → 10:31:28 ≈ 10:31)
LMT Honolulu 03:04

b) Sydney NSW ST correction = +10:00
ST Sydney 23:35

Q4. Aircraft departs Brisbane QLD (153°02'E) at 2200 UTC on 4th May, lands Manila Philippines (120°58'E) 6h 45min later. Find LMT and ST at landing.

Departure 2200 UTC 4 May
Flight time +6:45
Arrival UTC 04:45 UTC 5 May
Arc/Time 120°58'E +8:04
LMT Manila 12:49 5 May
ST Philippines +8:00
ST Manila 12:45 5 May

Q5. By how much is LMT Bangkok (101°00'E) later than LMT San Francisco (122°25'W)?

Bangkok 101°E is ahead of UTC by: 6h 44min
San Francisco 122°25'W is behind UTC by: 8h 10min
Total difference: 14h 54min

Bangkok LMT is 14h 54min LATER than San Francisco LMT.

Q6. Aircraft departs New York (075°W) at 0915 New York Summer Time for Vancouver BC (123°20'W), flight time 5h 25min. Find ETA in Vancouver Standard Time.

Departure 0915 New York Summer Time (DST = UTC−4)
ST correction +4:00 (Summer time → subtract 1 hour from normal ST correction of −5 → effective UTC offset is −4)
Departure UTC 13:15
Flight time +5:25
Arrival UTC 18:40
Vancouver ST −8:00
ETA Vancouver ST 10:40

Q7. Rome (012°30'E) to Hong Kong (114°00'E), 21h 40min flight. Must land HK at 1835 ST. What is the Standard Time of departure from Rome?

Arrival HK ST 18:35
HK ST correction −8:00
Arrival UTC 10:35
Flight time −21:40
Departure UTC 12:55 previous day
Rome ST +1:00
Departure Rome ST 13:55 previous day

Q8. Sydney NSW to Honolulu (157°52'W), 9h 50min flight. Departs Sydney at 0930 ST on 8th May. Find LMT and local date of arrival in Honolulu.

Departure Sydney ST 0930 8 May
ST correction NSW −10:00
Departure UTC 23:30 7 May
Flight time +9:50
Arrival UTC 09:20 8 May
Arc/Time 157°52'W −10:31
Arrival LMT Honolulu 22:49 7 May

Answer: 22:49 LMT on 7th May. The aircraft crosses the IDL travelling eastward, going back one calendar day despite flying forward in physical time.

Q9. Honolulu to Sydney NSW, 10h 20min flight. Departs Honolulu at 2300 ST on 10th May. Find ST of arrival at Sydney.

Departure Honolulu ST 2300 10 May
Hawaii ST +10:00
Departure UTC 09:00 11 May
Flight time +10:20
Arrival UTC 19:20 11 May
NSW ST +10:00
Arrival ST Sydney 05:20 12 May

Q10. New York (073°45'W) to Madrid (003°33'W), departs 2000 ST 16th February, flight time 7h 45min. Find ST and date of arrival at Madrid.

Departure New York ST 2000 16 Feb
NY ST correction +5:00
Departure UTC 01:00 17 Feb
Flight time +7:45
Arrival UTC 08:45 17 Feb
Madrid ST +1:00
Arrival Madrid ST 09:45 17 Feb