But while higher is better from a fuel conservation perspective, there is an upper limit on how high it is safe to orbit humans for long periods.
越高耗费的燃料越少，而载人飞船能够长时间安全飞行的轨道高度就有上限。
2
Suggested causes of ice age periods include the positions of the continents, variations in the Earth's orbit, changes in the solar output, and Vulcanism.
导致冰河期形成的可能原因包括：大陆的位置，地球轨迹的变化，太阳辐射的输出，以及火山的作用。
3
Organisms were also affected by periods of glaciation that followed variations in the earth's orbit and in the output of energy by the sun.