单项选择题
The surface of the Earth is the shore of the cosmic ocean. From it we have learned most of what we know about space. Recently, we have waded a little out to sea, enough to dampen our toes or, at most, wet our ankles. The water seems inviting. The ocean calls.
The dimensions of the cosmos are so large that using familiar units of distance, such as meters or miles, chosen for their utility on Earth, would make little sense. Instead we measure distance with the speed of light. In one second a beam of light travels 186,000 miles, nearly 300,000 kilometers, or seven times around the Earth. In eight minutes it will travel from the Sun to the Earth. We can say the Sun is eight light-minutes away. In a year, it crosses nearly ten trillion kilometers, about six trillion miles, of intervening space. That unit of length, the distance light goes in a year, is called a light year. It measures not time but distances, enormous distances.
The Earth is a place. It is by no means the only place. It is not even a typical place. No planet or star or galaxy can be typical because most of the cosmos is empty. The only typical place is within the vast, cold universal vacuum, the ever-lasting night of intergalactic space, a place so strange and desolate that by comparison, planets and stars and galaxies seem achingly rare and lovely. If we were randomly inserted into the cosmos, the chance that we would find ourselves on or near a planet would be less than one in a billion trillion. Worlds are precious.
A. An hour.
B. A year.
C. Eight minutes.
D. Seven days.