乌尔姆市政厅的天文钟, 德国 Astronomical clock at Town Hall of the City of Ulm, Germany (© Tomekbudujedomek/Getty Images)

乌尔姆市政厅的天文钟, 德国 Astronomical clock at Town Hall of the City of Ulm, Germany (© Tomekbudujedomek/Getty Images)

节约日光的艺术 The art of saving daylight

夏令时

每年三月，美国大部分地区开始实行夏令时，时钟拨快以最大限度地利用日光。这一想法可以追溯到1784年，当时本杰明·富兰克林曾幽默地建议人们早起以节省蜡烛的使用。美国在第一次世界大战期间，于1918年正式采用夏令时，作为一项节能措施。经过多年执行不一之后，1966年的《统一时间法案》确立了全国范围内季节性时钟调整的标准。虽然如今大多数州都实行夏令时，但夏威夷州和亚利桑那州（纳瓦霍族保留地除外）不实行，理由是日光变化幅度很小，且出于气候方面的考虑。此外，美国的一些属地全年也保持标准时间。

今天的图片展示的是德国乌尔姆市政厅上的天文钟。这座天文钟安装于16世纪，它追踪的是太阳和月亮的周期，而不是法定时间，提醒我们日光本身是由天文规律决定的，而不是由根据现代时间表调整的时钟决定的。

Daylight Saving Time

Every March, Daylight Saving Time begins in most of the United States, shifting clocks forward to maximize daylight. The idea can be traced back to 1784, when Benjamin Franklin humorously suggested earlier rising to conserve candle use. The US adopted Daylight Saving Time much later in 1918 as an energy-saving measure during World War I. After years of uneven observance, the Uniform Time Act of 1966 established a nationwide standard for seasonal clock changes. While most states observe Daylight Saving Time today, Hawaii and Arizona (with the exception of the Navajo Nation) do not, citing minimal daylight variation and climate concerns. Several US territories also remain on standard time year-round.

Today's image shows the astronomical clock on Ulm's Town Hall in Germany. Installed in the 16th century, it tracks solar and lunar cycles rather than legal time, reminding us that daylight itself is governed by astronomy—not by clocks adjusted for modern schedules.