Decide
Question 1/1
What is 052°30′ of longitude in time?
The lesson in plain words
- Arc and time
- The Earth turns 360° in 24 hours: 15° = 1 hour, 1° = 4 minutes, 15′ = 1 minute, 1′ = 4 seconds.
- The almanac has an arc-to-time table, but the arithmetic is easy.
- For 027°45′W: 27° = 1h 48m, 45′ = 3m, total 1h 51m.
- Local mean time
- Local mean time (LMT) is mean solar time on your own meridian: LMT = UT − west longitude in time, or + east longitude in time.
- It is not zone time, which is the time of the zone's central meridian (Navigation: Polaris and time).
- At 027°45′W, with ZD +2, LMT runs 9 minutes ahead of zone time.
- Local noon
- The almanac's Mer. Pass. is the LMT of the Sun's meridian passage.
- It changes by only seconds a day, so it holds at any longitude to the minute.
- So UT of local noon = Mer. Pass. + west longitude in time, or − east longitude in time.
- Worked: 26 November 2026
- DR at noon 21°10′N 027°45′W. Mer. Pass. 1147.
- Longitude in time 1h 51m, west: add. Noon is at 1338 UT.
- With ZD +2 the ship's clock reads 1138.
- Use the longitude you expect at noon: each 15′ of longitude made westward makes noon 1 minute later.
- Longitude from noon
- Turned round, the time of noon gives longitude: noon at 1338 UT with Mer. Pass. 1147 means 1h 51m west, 027°45′W.
- But near noon the Sun's altitude hardly changes for minutes, so the timing is rough: 1 minute of time is 15′ of longitude.
- Better longitude comes from timed sights (Celestial: sight reduction).
Key data
- Arc to time
- 15° = 1 h; 1° = 4 min; 15′ = 1 min; 1′ = 4 s
- LMT
- UT − W long in time, or + E long in time
- UT of local noon
- Mer. Pass. + W long in time (− E)
- Worked noon
- 1147 + 1h 51m = 1338 UT; ZT 1138 at 027°45′W
- Timing and longitude
- 1 min of time = 15′ of longitude
- Spanish terms
- Hora civil del lugar, paso del Sol por el meridiano
Key points
- 15° = 1 h, 1° = 4 min, 15′ = 1 min, 1′ = 4 s.
- LMT is your own meridian's mean time; zone time is the zone's.
- UT of noon = Mer. Pass. + W long in time (− E).
- Use the DR longitude expected at noon.
- Longitude from the time of noon is rough: 1 min = 15′.
Check what you have read
Question 1/3
Mer. Pass. is 1214 and your longitude is 060°00′E. What is the UT of local noon?
True or false?
Decide each statement before you see the answer.
Subtract west longitude in time from Mer. Pass. to get UT.
Longitude from the time of noon is rough: 1 min = 15′.
LMT is your own meridian's mean time; zone time is the zone's.
Local noon is at 1200 zone time.
Practice case & exam keys
Situation
Mer. Pass. is 1156. You are now at 045°00′W but expect 045°30′W at noon, keeping ZD +3. A crew member predicts noon at 1456 UT using your present longitude. Accept the plan?
Show the answer
Use the expected noon longitude. 045°30′W equals 3h 02m, so noon is about 1458 UT, or 1158 zone time. The original forecast is two minutes early because it uses the present longitude.
Exam keys
- Arc to time: 15° = 1 h; 1′ = 4 s.
- LMT = UT − W long in time (+ E).
- UT of local noon = Mer. Pass. + W long in time (− E); then ZT = UT − ZD.
- Worked 26 Nov 2026 at 027°45′W: 1338 UT, 1138 ZT.