What Is the Summer Solstice?何为夏至？

史蒂芬·施耐德 陈栋

The summer solstice1 marks the official start of summer. It brings the longest day and shortest night of the year for the 88 percent of Earth’s people who live in the Northern Hemisphere2. People around the world observe3 the change of seasons with bonfires and festivals and Fête de la Musique4 celebrations.

Astronomers can calculate an exact moment for the solstice， when Earth reaches the point in its orbit where the North Pole is angled closest to the sun. That moment will be at 10：54 a.m. Eastern Time on June 21 this year. From Earth， the sun will appear farthest north relative to the stars. People living on the Tropic of Cancer5， 23.5 degrees north of the Equator6， will see the sun pass straight overhead at noon. Six months from now the sun will reach its southern extreme and pass overhead for people on the Tropic of Capricorn7， and northerners will experience their shortest days of the year， at the winter solstice.

The sun’s angle relative to Earth’s equator changes so gradually close to the solstices that， without instruments， the shift is difficult to perceive for about 10 days. This is the origin of the word solstice， which means “solar standstill8.”

This slow shift means that June 21 is only about 1 second longer than June 20 at mid-northern latitudes. It will be about a week before there’s more than a minute change to the calculated amount of daylight.

Monuments9 at Stonehenge in England， Karnak in Egypt， and Chankillo in Peru reveal that people around the world have taken note of the sun’s northern and southern travels for more than 5，000 years. From Stonehenge’s circle of standing stones， the sun will rise directly over an ancient avenue leading away to the northeast on the solstice. We know little about the people who built Stonehenge， or why they went to such great effort to construct it—moving multi-ton stones from rock outcrops10 as far as 140 miles away. All this to mark the spot on the horizon where the sun returns each year to rest for a while before moving south again. Perhaps they， like us， celebrated this signal of the coming change of seasons.

夏至標志着夏天的正式开始。地球上88%的人生活在北半球，这一天对他们而言是一年中白昼最长、夜晚最短的一天。世界各地的人们会通过篝火、节庆和音乐节等方式庆祝这一季节变换。

天文学家可以计算出夏至点的精确时间，届时地球将沿轨道运行至北极点和太阳夹角最小的位置。今年的夏至点将发生在6月21日美国东部时间上午10点54分。从地球上看，相对于其他行星，这一刻太阳将出现在最北的位置。生活在北回归线（赤道以北23.5度）的人们在正午时将看到太阳直射。再过6个月，太阳将到达它的南部极点并从南回归线上的人头顶掠过，而北半球的居民将在冬至经历他们一年当中最短的白天。

太阳相对于地球赤道的夹角在至点附近变化十分缓慢，以至于如果没有仪器，这种移动大约10天之内都难以察觉。这也是“至点”这个词的由来——意思就是“太阳停滞”。

这一缓慢的移动意味着北半球中纬度地区6月21日白天只比6月20日长出约一秒。约一周之后，白昼的计算量才会有一分钟以上的变化。

英格兰的巨石阵、埃及的卡纳克神庙、秘鲁的查基洛观测台，这些历史遗迹揭示了5000多年前，世界各地的人们便已经在记录太阳的南北向之旅。夏至这天，从巨石阵那伫立的石圈看去，太阳会从一条通向东北方向的古道上方升起。我们对建造巨石阵的人知之甚少，也不了解为何他们要跋山涉水从140英里外的露头区搬来这些数吨重的巨石来建造它。所有这一切都为了标记出地平线上每年太阳都会回归并作短暂停留的那个位置，随后太阳便重又向南移动。也许他们也像我们一样，在庆祝这一季节变化的标志。

（译者 为“《英语世界》杯”翻译大赛获奖选手）