金星的轨道及其运动(第1页)
金星的轨道及其运动
金星的轨道接近正圆,在距离太阳1.08亿千米处运行,金星绕太阳一周需要不到225天,准确来说是224天16小时49分钟。
金星的轨道位于地球轨道内部。就像我们所看到的水星的运动过程一样,也就是说,我们能看到金星从太阳的一侧到另一侧的运动过程。在这个问题上,我们不再赘述已经给出过的一般性解释,下面仅限于明确金星运动的重要性。
就金星的轨道半径来说,它离太阳的距角比水星远得多,出于这个原因,金星可以跟着太阳或先行太阳4个多小时;所以我们在黄昏之后仍能看到金星,或者在天边出现曙光之前看到金星。也就是说,金星与水星不同,它可以在夜间发光,但确实这时候的金星是紧贴在地平线上的。
金星在其轨道上的运行速度高于地球,但差得并不太多,结合二者的运动,它们相对的位置只做缓慢的改变,因此我们的邻居金星的可视时间比水星间隔更长,也持续得更久,同时,我们所看到的金星的不同相位接替地更慢——金星相位经历一个周期需要584天。
我们可以看到,金星也会像水星那样从太阳前方经过,但这一现象更为罕见,其间隔周期的情况相当复杂。例如,金星第一次在某一天经过太阳,第二次要等8年之久,第三次则是在121.5年后,第四次距第三次又是8年,第五次回到起点,得等上105.5年。然后这套间隔周期重新开始:8年,121.5年,8年,105.5年,依此类推。现代时期观测到的最近两次分别发生在1874和1882年,下一次只能等到2004年(1)。我们不得不对这种情况的罕见感到遗憾,因为像这样的现象有助于我们进行各种各样意义重大的研究,我们将进一步强调这一点。
金星轨道与地球轨道之间的关系。图中标出的角度是指金星在太阳两侧的最大距角,位置S对应晚上的距角,位置M对应早上的距角。在C1(上合点)位置时金星在太阳背面,此时距离地球最远;在C2(下合点)位置时与地球距离最近。这些不等的距离解释了金星视直径的变化。OrbitedelaTerre:地球轨道;OrbitedeVénus:金星轨道;Terre:地球
(1) 即金星凌日。2004年,金星凌日发生于6月8日,2012年6月再次发生,下一次金星凌日则要等到2117年。
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