You have a rock sample and analyze it for the presence of radioactive isotopes in order to determine when it was formed. You fin
d an isotope that has a half-life of 100 million years. After you analyze it, you determine that it has equal amounts of radioactive parent isotope and stable daughter product. A. How many half lives have gone by.
B. How old is it?
Scientists have studied eclipses since ancient times. Aristotle observed that the Earth's shadow has a circular shape as it moves across the moon. He posited that this must mean the Earth was round. Another Greek astronomer named Aristarchus used a lunar eclipse to estimate the distance of the Moon and Sun from Earth