今日為一年一度的夏至，為北(南)半球所有地區日照最長(短)的日子。下圖為今天香港時間 16:57 的晝夜平分線 :
Today is the day of the summer solstice for the northern hemisphere. It means that every place in the northern hemisphere has the longest sunlight duration of the year and every place in the southern hemisphere has the shortest sunlight duration of the year. The following is a plot of the terminator at 16:57 Hong Kong Time today:
由上圖可見，北極全日有日光，而南極則整天在黑夜裡。這情況會維持至 9 月尾秋分附近為止。
As you can see from the above plot, the North Pole receives sunlight for 24 hours a day while the South Pole receives nothing, and this will continue until the autumnal equinox that is in late September.
天文學上，"至" 為太陽的赤緯 (Declination)之絕對值達到最高的一天。請再次觀看上圖 - 今天太陽的赤緯達到 23.44 度，這也是黃道與地球赤道的夾角 (黃赤交角):
Astronomically, the solstices are also the time when the Sun reaches the greatest declination in absolute value. Refer again to the above plot - you see that the declination of the Sun is 23.44 degrees - which is equal to the angle between the eliptic plane and the Earth's equatorial plane, also known as the obliquity of the ecliptic:
大致上太陽的赤緯為太陽直射地球的緯度，由冬至的 -23.44度逐步北移至夏至的23.44度，這代表今天過後太陽直射點將逐漸南移回赤道，繼而到達南半球。
Roughly speaking, the declination of the Sun is the latitude at which the Sun strikes on Earth, which varies from 23.44 degrees South (i.e. -23.44 degrees) on December solstice and 23.44 degrees North on June solstice. So this means that after today, the sunsolar point (i.e. the point that the sun strikes) will be on its journey towards the equator and back to the Southern hemisphere later this year.
那麼你可能會問為甚麼最熱的時間不在夏至? 原因是總熱量還沒有到達頂點。雖然今天北半球所接收的熱量理論上為全年最高，但總熱量於今天過後還在上升，因為太陽仍然在赤道以北一段距離。7月下旬後，當太陽回到較南位置時，地面所失的熱量將會比接收的為多，所以平均氣溫在 7 月尾才開始下跌 (當然，香港於七月受高壓脊覆蓋而長期天晴，自然亦會有較多能量到達)。
So you may ask, "why the hottest time of the year is not on the June solstice?" This is because the total heat received has not reached its highest point. Although theoretically today is the day that the Northern Hemisphere receives the most heat, the "cumulative heat content" over the hemisphere is still rising after today because we are still absorbing more heat than we are losing after today. Only after late July is the Sun sufficiently south that we start to receive less heat than we lose; and thus the hottest time is usually in July (of course, July is the hottest month in HK not only because of this reason - HK is often affected by ridges in July which bring fine weather to us, allowing more solar heat to enter).
我們可以看看香港今天見到的天球，焦點放在太陽身上:
It's also constructive to look at the celestial sphere as seen in Hong Kong today, focussing on the Sun:
太陽於香港天頂以北經過，代表香港的緯度 (22.3 N) 比今天太陽的赤緯 (23.44 N) 要南；但北京的情況就不同：
The Sun actually passes north of zenith (directly overhead) of Hong Kong. This means that Hong Kong is at a location South of that the Sun strikes (or the declination) today, which is true because our latitude is 22.3 degrees North compared to the solar declination of 23.44 degrees North. The same isn't true for Beijing, for example:
在北京，今天太陽於天頂以南經過，亦代表北京的緯度 (39.9 N) 比今天太陽的赤緯 (23.44 N) 要北。
As we see that the Sun culminates (i.e. at the highest altitude) South of zenith in Beijing. This confirms with the observation that Beijing is North of the declination of the Sun today - its latitude is 39.9 degrees North.
從天文台的網頁可見，今天日出為於 5 時 40 分，而日落為 19 時 10 分，日光時間達 13.5 小時之久，這亦是香港最長的日光時間，從明日開始將會逐漸下跌直到本年 12 月 21 日 (冬至)。
Referring to the Observatory's website, the Sun rises at 5:40 and sets at 19:10 HKT today, for a sunlight duration of 13.5 hours. This is the maximum that we can get and it will start to diminish all the way until the winter solstice on Dec 21, 2009.
今天先說到這裡，並祝各位父親節快樂。
So much for now and, by the way, happy father's day!
浪卡路徑圖
