试说明地球自转的方向、周期和速度。其自转的速度有什么变化?
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在现阶段,我国社会的主要矛盾是人民日益增长的美好生活需要和()之间的矛盾。
太阳系以19.6km/s的速度向武仙座方向运动,一年内飞过多少天文单位?
Also known as "digital marketing" and "e-marketing," social media marketing has purpose-built data analytics tools that allow marketers to track how successful their efforts are.()