1 . How big is space? That is a question man has been asking since the beginning of time. When your grandparents were in school, it was ________that the universe was very________perhaps only 5, 000 light years________. 500 years ago people considered that the universe was only a little bit________than the earth. In modern times with the________of technology we are finally starting to________the immense（无边的）size of the universe, and it is much bigger than anyone________have ever imagined.

Scientists cannot stretch（伸展）a measuring tape from the earth to a nearby star, so________do they know how far away they are? Scientists use something________parallax（视差）. Parallax means________that they look at how much the stars move in the sky________throughout the year as the earth circles the sun. This is not something________to you. In fact if you stretch out your hand in front of you and________your thumb, while________one eye, and then the other, your thumb will________to move back and forth. Stars do the________thing, but our eyes are too close to see the________. However, if we take a picture while on one side of the earth's orbit, and then take another when we get to the________side of the orbit, then we have a large enough________that we can see the stars' parallax, and________how far away they really are.