1.The stars which we see glittering in the sky on a dark and cloudless night are almost certainly the suns of other solar systems more or less like our own， but they are so far away in space that it is unlikely that we shall ever get to know very much about them.
2.The earth is， however， very different from the moon， which the American astronauts have found to be without life or vegetation， whereas our earth is very much alive in every respect.
whereas = while 然而
alive意为“有活力的”，在句中作表语。类似的形容词还有afraid， alone， alike， asleep， awake， aware等
in every respect 在每一个方面
3.In most of the oceans and seas some of the water is found to be flowing in a particular direction - that is to say， from one part towards another part of the ocean or sea concerned.
to be flowing不定式作主语补语
concerned 是分词作定语，修饰the ocean or sea
4.There are many thousands of currents in the waters of the oceans and seas ，but only certain of the stronger and better marked currents are specially named and of great importance.
Thousands/hundreds of + 名词复数
Eg.Hundreds of meters
5.The circumstances under which different people live make a great difference between the way in which they live and the way in which we live， and it ought to be our business to try to understand those different circumstances so that we can better understand people of other lands.
三个定语从句，由under which 和 两个in which 引导，
第一个：under which different people live 修饰 circumstances
第二个：in which they live 修饰 the way
第三个：in which we live 修饰 the way
ought to = should 应该
it是形式主语，真正主语是to try to understand those different circumstances
other lands 其它国家