Khoa học nghiên cứu vũ trụ trong tổng thể của nó gọi là vũ trụ học, một lĩnh vực kết hợp giữa vật lý và thiên văn. Vũ trụ học, về cuối thế kỷ 20, được phân làm hai nhánh chính là thực nghiệm và lý thuyết | CLIMATE CHANGE At that time in Cambridge Nicholas Shackleton was measuring as Emiliani had done the proportion of heavy oxygen in forams from seabed cores. But he picked out just the small animals that originally lived at the bottom of the ocean. When there s a lot of ice in the world locked up ashore the heavy oxygen in ocean water increases. With his bottom-dwelling fossils Shackleton thought he was measuring the changing volumes of ice during the ice ages and warmer interludes. In the seabed core used by Shack leton Neil Opdyke of Columbia detected a reversal in the Earth s magnetic field about 700 000 years ago. That result in 1973 gave the first reliable dating for the ice-age cycles and the various climatic stages seen in the cores. It was by then becoming obvious to the experts concerned that the results of their researches were likely to mesh beautifully with the Milankovitch Effect. When the snow lies all summer Milutin Milankovitch was a Serbian civil engineer whose hobby was the climate. In the 1920s he had refined a theory of the ice ages from prior ideas. Antarctica is always covered with ice sheets so the critical thing is the coming and going of ice on the more spacious landmasses of the northern hemisphere. And that depends on the warmth of summer sunshine in the north. Is it strong enough to melt the snows of winter The Earth slowly wobbles in its orbit over thousands of years. Its axis swivels affecting the timing of the seasons. The planet rolls like a ship affecting the height of the Sun in the sky. And over a slower cycle the shape of the orbit changes putting the Earth nearer or farther from the Sun at different seasons. Astronomers can calculate these changes and the combinations of the different rhythms for the past few million years. Sometimes the Sun is relatively high and close in the northern summer and it can blast the snow and ice away. But if the Sun is lower in the sky and farther away the winter snow fails to melt. It lies all .
