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| | Z-Time - by Harold F. Maybeck |
 | | When he came to the International Date Line at 180° longitude, realizing that although the time was the same in each 7.5° segment each side of 180° longitude...they were different days. |
 | | The first zone centered on 15° West longitude became time zone "N", the next time zone west centered on 30° West longitude became time zone "O", etc, etc. Approaching the International Date Line from the east, Bowditch designated the remaining "half zone" from 172.5° West longitude to 180° longitude as time zone "Y". |
 | | This notation, "Z", is appended to times on most navigational, meteorological, and astronomical charts today to indicate that the time of the chart is in "Greenwich Mean Time". |
| www.maybeck.com /ztime (1045 words) |
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