What Is the Summer Solstice?何為夏至？

史蒂芬·施耐德 陳棟

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.

夏至標志著夏天的正式開始。地球上88%的人生活在北半球，這一天對他們而言是一年中白晝最長、夜晚最短的一天。世界各地的人們會通過篝火、節慶和音樂節等方式慶祝這一季節變換。

天文學家可以計算出夏至點的精確時間，屆時地球將沿軌道運行至北極點和太陽夾角最小的位置。今年的夏至點將發生在6月21日美國東部時間上午10點54分。從地球上看，相對于其他行星，這一刻太陽將出現在最北的位置。生活在北回歸線（赤道以北23.5度）的人們在正午時將看到太陽直射。再過6個月，太陽將到達它的南部極點并從南回歸線上的人頭頂掠過，而北半球的居民將在冬至經歷他們一年當中最短的白天。

太陽相對于地球赤道的夾角在至點附近變化十分緩慢，以至于如果沒有儀器，這種移動大約10天之內都難以察覺。這也是“至點”這個詞的由來——意思就是“太陽停滯”。

這一緩慢的移動意味著北半球中緯度地區6月21日白天只比6月20日長出約一秒。約一周之后，白晝的計算量才會有一分鐘以上的變化。

英格蘭的巨石陣、埃及的卡納克神廟、秘魯的查基洛觀測臺，這些歷史遺跡揭示了5000多年前，世界各地的人們便已經在記錄太陽的南北向之旅。夏至這天，從巨石陣那佇立的石圈看去，太陽會從一條通向東北方向的古道上方升起。我們對建造巨石陣的人知之甚少，也不了解為何他們要跋山涉水從140英里外的露頭區搬來這些數噸重的巨石來建造它。所有這一切都為了標記出地平線上每年太陽都會回歸并作短暫停留的那個位置，隨后太陽便重又向南移動。也許他們也像我們一樣，在慶祝這一季節變化的標志。

（譯者 為“《英語世界》杯”翻譯大賽獲獎選手）