Facts of the Matter
Hard to tell what makes time tick
TIME IS A strange thing. It is an elusive, relative and undefinable continuum that flows from past to future. Our lives are controlled by it, and we think we understand it until we try to define it.
Nonetheless, we keep track of time with a clock that ticks away at a uniform rate regardless of our relative perception of it.
Before the adoption of time zones, local standard time was solar time as measured with a sundial and represented with a clock.
As the earth rotates from west to east, the location on the surface where the sun is directly overhead moves westward at the rate of 15 degrees per hour.
From our perspective on Earth's surface, the sun moves from east to west and crosses the local meridian approximately every 24 hours and defines the length of the solar day.
Because all locations that lie at the same longitude have the same solar time, traveling north-south presents no time problems. Traveling east-west is a different story using local standard time because clocks differ between places by an amount corresponding to the difference in their geographical longitude, which is usually not a convenient number.
In the 19th century, as the telegraph made it possible to communicate instantaneously over long distances, railroad schedules became impossibly complicated with each location having its own local time.
Although it was possible to synchronize clocks between one location and another, such a solution created one of two problems, neither of which was acceptable.
On one hand, it would be chaotically complicated to change clock time and coordinate railroad schedules between cities if each was on its own local solar time.
On the other hand, using a "universal clock" would create a situation in which the local time would differ markedly from the solar time to which people are accustomed. For example, 1 p.m. might fall in early afternoon in one place but in the darkness of mid-evening in another.
Time zones are a compromise that relaxes the complexities of geographic time while still allowing local time to approximate solar time to within a half-hour.
Time zones divide the earth into 24 segments, each segment having a width of 15 degrees longitude, the amount that the earth rotates in one hour (360 degrees per rotation divided by 24 hours per day).
The discovery that Earth's tilt and elliptical orbit affect the relationship between solar time and clock time led to the use of mean solar time, which is the average over a year of apparent solar time, as a standard. The difference of mean solar time from apparent solar time is the equation of time.
Each time zone is a region that uses the same standard clock time bordered by meridians each 15 degrees of longitude apart. Calculated standard time is based on the center of each 15-degree band.
Two observers at the same latitude who are separated by exactly 15 degrees of longitude will experience local noon exactly one hour apart.
Standard time is based on the location of the Royal Observatory at Greenwich, England, where midnight is zero hours, or 0000 UTC.
Most adjacent time zones are exactly one hour apart and compute their local time as an offset from Coordinated Universal Time (UTC), formerly called Greenwich Mean Time (GMT).
In reality, time zone boundaries are affected by political and social factors. For example, a situation where two parts of the same city were in different time zones would create problems.
It does happen that parts of some states -- and adjacent states -- might be in different time zones, so travelers must be aware of these situations to keep track of the correct local time. The switch to daylight-saving time complicates the situation when some localities set clocks ahead during the summer months while others do not.
HAWAIIAN STANDARD TIME (HST) is 10 hours behind UTC and for purposes of calculation is -10 hours. When it is 0000 UTC it is 1400 HST or 2 p.m.
The center of the Hawaiian Standard time zone is located at 150 degrees west longitude (-150 degrees), which runs through Kaunakakai, Molokai.
Sunrise and sunset times are different for locations that lie within the same time zone at different east or west longitudes. The sun rises and sets earlier in Hilo than in Honolulu. This actual difference is partly because Hilo is slightly further south than Honolulu, but mostly due to the difference in longitude.
Because of the way time is calculated, it is possible for it to be two different days at the same time at different locations on the earth's surface.
Suppose that it is one minute past midnight Sunday in Greenwich, 0001 UTC.
Moving eastward in 15-degree intervals, adding one hour at each 15-degree jump, we would find local time in Honolulu to be 2:01 p.m. Sunday.
But moving westward in 15-degree intervals, subtracting one hour at each 15-degree jump, we would find local time in Honolulu to be 2:01 p.m. Saturday.
Obviously it cannot be both Saturday and Sunday in Honolulu at the same time.
Hence the international date line represented by the 180-degree meridian, which is both 180 degrees east and 180 degrees west.
The dateline, like other time zone boundaries, zigzags to satisfy political and social needs.
By international convention, just to the east of the international dateline it is one day earlier than just to the west of the dateline.
A traveler appears to lose a day traveling westward across the dateline, and to gain a day traveling eastward.
In actuality, no time is lost or gained; it is nothing more than the undeniable fact that the earth is a spheroid. The most serious effect of the time zone compromise is jet lag.
Just in case you're thinking about Superman flying really fast around the earth eastward to travel back in time, it won't work.
Think about it.
Richard Brill, professor of science at Honolulu Community College, teaches earth and physical science and investigates life and the universe. E-mail questions and comments to email@example.com