【英语时差8，16】丈量地球

 How big a stick would you need to measure the planet?   A thousand miles long? A million? How about three feet? That ought to be enough to do it, as an ancient Alexandrian man named Eratosthenes figured out.

Eratosthenes was a multi-talented man who lived in the third century B.C. He was puzzled one day to read that at noon on the 21st of June, pillars in the Egyptian town of Syrene cast no shadows. There is nothing so odd about this -- anything sticking vertically¹ out of the ground will cast no shadow when the sun is standing² straight overhead. What was odd, he thought, was that when he tried the experiment in Alexandria at the same time of day, a stick held vertically did cast a shadow.

How is this possible? If the earth is flat, all shadows should be the same. There would be no difference between noon in Syrene and noon in Alexandria. It's only if the earth is curved, Eratosthenes correctly guessed, that the shadows change lengths depending on how far north or south you are.

To visualize³ this, you can imagine a sheet of paper with two toothpicks stuck in it, several inches apart, and a lamp shining down from overhead. Neither toothpick casts a shadow. Bend the paper at one end, though, and the tilted⁴ toothpick begins to have one. The more you bend it, the longer the shadow.

In fact, once he had this insight into the shape of the earth, Eratosthenes was also able to correctly calculate its circumference⁵. Can you figure out how he did it? Tune⁶ in next time for the answer. 

1 vertically   

adv.垂直地

参考例句：

• Line the pages for the graph both horizontally and vertically.在这几页上同时画上横线和竖线，以便制作图表。
• The human brain is divided vertically down the middle into two hemispheres.人脑从中央垂直地分为两半球。

2 standing   

n.持续，地位；adj.永久的，不动的，直立的，不流动的

参考例句：

• After the earthquake only a few houses were left standing.地震过后只有几幢房屋还立着。
• They're standing out against any change in the law.他们坚决反对对法律做任何修改。

3 visualize   

vt.使看得见，使具体化，想象，设想

参考例句：

• I remember meeting the man before but I can't visualize him.我记得以前见过那个人，但他的样子我想不起来了。
• She couldn't visualize flying through space.她无法想像在太空中飞行的景象。

4 tilted   

v. 倾斜的

参考例句：

• Suddenly the boat tilted to one side. 小船突然倾向一侧。
• She tilted her chin at him defiantly. 她向他翘起下巴表示挑衅。

5 circumference   

n.圆周，周长，圆周线

参考例句：

• It's a mile round the circumference of the field.运动场周长一英里。
• The diameter and the circumference of a circle correlate.圆的直径与圆周有相互关系。

6 tune   

n.调子；和谐，协调；v.调音，调节，调整

参考例句：

• He'd written a tune,and played it to us on the piano.他写了一段曲子，并在钢琴上弹给我们听。
• The boy beat out a tune on a tin can.那男孩在易拉罐上敲出一首曲子。