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美国科学家预测 六亿年后将再无日全食
2017-08-23 06:52:20 中国青年网
新华社北京8月22日新媒体专电
美国《大众机械》月刊网站8月18日发表文章称,在21日的“美国超级日全食”期间,月球的阴影将形成一条横贯美国东西海岸、宽度约为70英里(约合113公里)的日全食带。这将是一次令人震撼的天文事件,下一次能看到横贯美国东西海岸的日全食将是7年以后。
而且,终有一天,日全食将不复存在。
之所以存在日全食,是因为在合适的条件下,我们在天空中看到的月亮的直径大于太阳的直径,在地球的某些地方,所有直射的太阳光都将被月球遮挡。但终有一天,在你我都离开人世很久很久以后,天空中的月亮将不再能完全遮挡直射的太阳光。
月球正在以每年3.8厘米的速度远离地球。自几十亿年前诞生以来,月球一直在逐渐远离地球。在月球诞生时,从地球上看,天空中的月亮应该比现在更近、更大。因为月球一直在远离地球,而且我们在地球上看到的月亮越来越小,终有一天,月亮将小到无法遮挡整个太阳。
这一天大约是在6亿年以后。美国国家航空航天局发表的论文预测,日全食将在大约5.63亿年以后不复存在。但英国天文学家琼·梅乌斯在其数理天文学著作中说,因为月球和地球轨道的微小变化,6.2亿年以后,日全食还将断断续续地出现,最后一次日全食将出现在距今12亿年以后。
但最后一次日全食的持续时间将非常短暂,大约只有几秒,甚至更短。随着月球逐渐远离地球,它能遮住太阳光的频率将下降,持续时间也将缩短。
由于地球、月球和太阳的轨道是椭圆形的,所以这些星球有时会靠近,有时又会远离彼此。每年在北半球的冬季,地球都会抵达距离太阳最近的点,即近日点;而在北半球的夏季,地球将抵达距离太阳最远的点,即远日点。月球几乎每月都会发生类似的情况。
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