Part I · Foundations · Page 4 of 4
Crossing the date line
Zone descriptions run from +12, just east of 180°, to −12, just west of it. The two zones meet at the 180th meridian, and they are a whole day apart. So a boat crossing 180° keeps the same time of day on the ship’s clock but changes the date: sailing west, the calendar jumps a day forward; sailing east, the same date comes round twice.
Predict Sailing west, you cross 180° at 1400 on Tuesday. What should the ship's clock read just after?
- UT
- 02:00, Wednesday, July 8
- Zone description
- +12
- Ship's clock (zone time)
- 14:00
- Ship's calendar
- Tuesday, July 7
East of 180°, zone time = UT − 12 h: the day before UT's.
UT is the same on both sides: work it out first, then zone time = UT − the new ZD
1400 Tuesday, July 7, ZD +12: UT = 1400 + 12 h = 0200 Wednesday, July 8.
After the crossing, ZD −12: zone time = 0200 + 12 h = 1400 Wednesday, July 8.
At sea the zone changes at 180° itself. The civil date line on maps bends round some island groups, such as Kiribati and Tonga, but a ship’s zone description follows the meridian. The almanac never changes: it is always entered with UT, on the UT date.