Daylight saving time (abbreviated DST), also sometimes erroneously referred to as daylight savings time, is the practice of advancingclocks during summer months so that evening daylight lasts longer, while sacrificing normal sunrise times. Typically, regions that use daylight saving time adjust clocks forward one hour close to the start of spring and adjust them backward in the autumn to standard time.[1]
American inventor and politician Benjamin Franklin proposed a form of daylight time in 1784. He wrote an essay "An Economical Project for Diminishing the Cost of Light" to the editor of The Journal of Paris, suggesting, somewhat jokingly, that Parisians could economize candle usage by getting people out of bed earlier in the morning, making use of the natural morning light instead.[2] New Zealander George Hudson proposed the idea of daylight saving in 1895.[3] The German Empire and Austria-Hungary organized the first nationwide implementation, starting on April 30, 1916. Many countries have used it at various times since then, particularly since theenergy crisis of the 1970s.
The practice has both advocates and critics.[1]Some early proponents of DST aimed to reduce evening use of incandescent lighting, once a primary use of electricity.[4] Today's heating and cooling usage patterns differ greatly, and research about how DST affects energy use is limited and contradictory.[5]
DST clock shifts sometimes complicate timekeeping and can disrupt travel, billing, record keeping, medical devices, heavy equipment,[6] and sleep patterns.[7] Computer software often adjusts clocks automatically, but policy changes by various jurisdictions of DST dates and timings may be confusing.[8]
Industrialized societies generally follow a clock-based schedule for daily activities that do not change throughout the course of the year. The time of day that individuals begin and end work or school, and the coordination of mass transit, for example, usually remain constant year-round. In contrast, an agrarian society's daily routines for work and personal conduct are more likely governed by the length of daylight hours[9][10] and by solar time, which change seasonally because of the Earth's axial tilt. North and south of the tropics daylight lasts longer in summer and shorter in winter, with the effect becoming greater as one moves away from the tropics.
By synchronously resetting all clocks in a region to one hour ahead of standard time(one hour "fast"), individuals who follow such a year-round schedule will wake an hour earlier than they would have otherwise; they will begin and complete daily work routines an hour earlier, and they will have available to them an extra hour of daylight after their workday activities.[11][12] However, they will have one fewer hour of daylight at the start of each day, making the policy less practical during winter.[13][14]
While the times of sunrise and sunset change at roughly equal rates as the seasons change, proponents of Daylight Saving Time argue that most people prefer a greater increase in daylight hours after the typical "nine to five"workday.[15][16] Supporters have also argued that DST decreases energy consumption by reducing the need for lighting and heating, but the actual effect on overall energy use isheavily disputed.
The manipulation of time at higher latitudes (for example Iceland, Nunavut or Alaska) has little impact on daily life, because the length of day and night changes more extremely throughout the seasons (in comparison to other latitudes), and thus sunrise and sunset times are significantly out of phase with standard working hours regardless of manipulations of the clock.[17] DST is also of little use for locations near the equator, because these regions see only a small variation in daylight in the course of the year.[18] The effect also varies according to how far east or west the location is within its time zone, with locations farther east inside the time zone benefiting more from DST than locations farther west in the same time zone.[19]
Although they did not fix their schedules to the clock in the modern sense, ancient civilizations adjusted daily schedules to the sun more flexibly than DST does, often dividing daylight into twelve hours regardless of daytime, so that each daylight hour was longer during summer.[20] For example, Roman water clocks had different scales for different months of the year: at Rome's latitude the third hour from sunrise, hora tertia, started by modern standards at 09:02 solar time and lasted 44 minutes at the wintersolstice, but at the summer solstice it started at 06:58 and lasted 75 minutes (see alsoRoman timekeeping).[21] After ancient times, equal-length civil hours eventually supplanted unequal, so civil time no longer varies by season. Unequal hours are still used in a few traditional settings, such as some monasteries of Mount Athos[22] and allJewish ceremonies.[23]
During his time as an American envoy to France, Benjamin Franklin, publisher of the old English proverb "Early to bed, and early to rise, makes a man healthy, wealthy and wise",[24][25]anonymously published a letter suggesting that Parisians economize on candles by rising earlier to use morning sunlight.[26] This 1784satire proposed taxing window shutters, rationing candles, and waking the public by ringing church bells and firing cannons at sunrise.[27] Despite common misconception, Franklin did not actually propose DST; 18th-century Europe did not even keep precise schedules. However, this soon changed as rail transport and communication networks came to require a standardization of time unknown in Franklin's day.[28]
Modern DST was first proposed by the New Zealand entomologist George Hudson, whoseshift work job gave him leisure time to collect insects and led him to value after-hours daylight.[3] In 1895 he presented a paper to the Wellington Philosophical Societyproposing a two-hour daylight-saving shift,[11]and after considerable interest was expressed in Christchurch, he followed up in an 1898 paper.[29] Many publications credit DST proposal to the prominent English builder and outdoorsman William Willett,[30] who independently conceived DST in 1905 during a pre-breakfast ride, when he observed with dismay how many Londoners slept through a large part of a summer day.[16] An avid golfer, he also disliked cutting short his round at dusk.[31] His solution was to advance the clock during the summer months, a proposal he published two years later.[32] The proposal was taken up by the Liberal Member of Parliament (MP) Robert Pearce, who introduced the first Daylight Saving Bill to theHouse of Commons on February 12, 1908.[33]A select committee was set up to examine the issue, but Pearce's bill did not become law, and several other bills failed in the following years. Willett lobbied for the proposal in the UK until his death in 1915.
William Sword Frost, mayor of Orillia, Ontario, introduced daylight saving time in the municipality during his tenure from 1911 to 1912.[34]
Starting on April 30, 1916, the German Empireand its World War I ally Austria-Hungary were the first to use DST (German: Sommerzeit) as a way to conserve coal during wartime. Britain, most of its allies, and many European neutrals soon followed suit. Russia and a few other countries waited until the next year, and the United States adopted it in 1918.
Broadly speaking, daylight saving time was abandoned in the years after the war (with some notable exceptions including Canada, the UK, France, and Ireland). However, it was brought back for periods of time in many different places during the following decades and commonly during World War II. It became widely adopted, particularly in North America and Europe, starting in the 1970s as a result of the 1970s energy crisis.
Since then, the world has seen many enactments, adjustments, and repeals.[35] For specific details, an overview is available atDaylight saving time by country.
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