地球自转一周的时间(地球自转一周并不是24小时……)

4.22世界地球日

每年的4月22日是世界地球日（Earth Day），是一个专为世界环境保护而设立的节日。现今，地球日的活动已发展至全球192个国家，每年有超过10亿人参与其中，使其成为世界上最大的民间环保节日之一。

了解地球

先从那些你不知道的地球冷知识开始吧~

01

地球不是圆的

许多人认为地球是一个球体。事实上，其形状类似于球体，但两极是扁平的，赤道是凸出的，这个凸起是由于地球的自转造成的。这意味着从一极到另一极的测量距离大约比地球穿过赤道的直径小43公里。

Many people tend to think that the Earth is a sphere. In fact, its shape is similar to a sphere, but where the poles are flattened and the equator bulges. This bulge is due to our planet's rotation. This means that the measurement from pole to pole is about 43 km less than the diameter of Earth across the equator.

02

死海是陆地的最低点

陆地上的最低点相对容易到达，即约旦、以色列和约旦河西岸之间的死海。这个超咸湖的表面低于海平面1388英尺(423米)。

The lowest point on land is relatively accessible. It's the Dead Sea between Jordan, Israel and the West Bank. The surface of this super-salty lake is 1,388 feet (423 m) below sea level.

03

地球曾经是紫色的

马里兰大学的微生物遗传学家Shil DasSarma认为，今天地球上的生命是绿色的，但在早期可能是紫色的。他说，古代的微生物可能使用了叶绿素以外的分子来利用太阳光线，而正是这种分子赋予了生物体紫色的色彩。

It used to be purple … well, life on early Earth may have been just as purple as it is green today, suspects Shil DasSarma, a microbial geneticist at the University of Maryland. Ancient microbes, he said, might have used a molecule other than chlorophyll to harness the sun's rays, one that gave the organisms a violet hue, he suggests.

045

地球的大气层能延伸到一万公里

地球的大气层在距离地表50公里以内是最厚的，但它实际上达到了1万公里。它由对流层、平流层、中间层、热层和外层五层组成。通常，空气压力和密度会随着高度的升高而逐渐降低。

Earth's atmosphere is thickest within the first 50 km from the surface or so, but it actually reaches out to about 10,000 km into space. It is made up of five main layers – the Troposphere, the Stratosphere, the Mesosphere, the Thermosphere, and the Exosphere. As a rule, air pressure and density decrease the higher one goes into the atmosphere and the farther one is from the surface.

05

地球自转一周不需要24小时

实际上，地球自转一周需要23小时56分4秒，天文学家称之为恒星日。但同时，地球绕太阳公转。每一天，太阳与背景恒星之间的移动幅度约为1°——与天空中月亮的大小差不多。由于地球也绕着太阳转，如果把来自太阳的小运动加起来，总共是24小时，这就是所谓的太阳日。太阳日是太阳连续两次经过同一子午线的时间间隔。

It actually takes 23 hours, 56 minutes and 4 seconds for the Earth to rotate once completely on its axis, which astronomers refer to as a Sidereal Day. But the Earth orbits around the Sun. Every day, the Sun moves compared to the background stars by about 1° – about the size of the Moon in the sky. And so, if you add up that little motion from the Sun that we see because the Earth is orbiting around it, as well as the rotation on its axis, you get a total of 24 hours.This is what is known as a Solar Day, which – contrary to a Sidereal Day – is the amount of time it takes the Sun to return to the same place in the sky.

06

南极洲是一个极端的大陆

南极大陆是一个极端的地方，尽管它只是第五大大陆，但其南极冰盖包含了地球上70%的淡水和90%的冰。或许你不知道，大家往往认为南极洲是沙漠，其内陆地区每年的降水量只有2英寸(50毫米)(当然，通常是雪)。

The southern continent is a place of extremes, with the Antarctic ice cap containing some 70 percent of Earth's fresh water and about 90 percent of its ice, even though it is only the fifth largest continent. Did you know Antarctica is actually considered a desert? Inner regions get just 2 inches (50 millimeters) of precipitation a year (typically as snow, of course).

07

珊瑚礁是最庞大的生物结构

珊瑚礁是地球上物种生存密度最高的生态系统，可以与雨林相媲美。美国国家海洋和大气管理局表示，虽然珊瑚礁是由微小的珊瑚虫组成，但它们是地球上最庞大的生物结构——一个由相互联系的有机体组成的群落——有些甚至可以从太空中看到。

Coral reefs support the most species per unit area of any of the planet's ecosystems, rivaling rain forests. And while they are made up of tiny coral polyps, together coral reefs are the largest living structures on Earth — a community of connected organisms — with some visible even from space, according to NOAA.

编辑：高启辉 侯俊杰

实习生：白洁