The moment at which a body in the solar system has an ecliptic longitude differing from that of the sun by 180 , so that it lies opposite the sun in the sky and crosses the meridian at about midnight 在地球观测，太阳系内天体的黄经和太阳黄经相差180时，称为冲。内行星没有冲的现象，当一天体冲的时候，在地球上是观测这天体的良好机会，因除了整夜可见这天体外，天体会于各地子夜时分经过各地的子午圈。
The angular distance between the sun and a planet or the moon . it is also the difference in ecliptic longitude between the sun and other celestial objects . when an inferior planet follows the sun in its daily motion , appearing east of the sun in the evening , it is in eastern elongation 距角是太阳和太阳系内天体的角距，亦即是太阳和它们相差的黄经，以在日落后见于西方的内行星来说，这时内行星在处于太阳的东面，若这时的角距是在这段时间内最大的，便是
The ecliptic coordinate system is defined as one which uses the ecliptic and the vernal equinox as the zero point . the ecliptic longitude of a body is the angular distance from 0 to 360 from the vernal equinox , measured eastwards along the ecliptic to the intersection of the body s circle of longitude 黄道是太阳在天球运行的轨道黄道座标系统是以黄道为基础的座标系统：黄道经度是以春分点为零度的大圆圈，向东起计共360度，而天体的黄道经度是天体的经圈与春分点的角距离。
However , the moon s path is tilted about 5 to the ecliptic , making the alignment of the sun , the earth and the moon in a straight line a relatively rare case . only when the sun and the moon are close enough to the nodes crossing points of the ecliptic and the moon s paths during full moon , a lunar eclipse can occur 要注意的是，由于太阳和月球在天空的轨道称为黄道和白道并不在同一个平面上，而是有约5度的交角，所以只有太阳和月球分别位于黄道和白道的两个交点附近，才有机会连成一条直线，产生月食。
The orbit of venus , for example , is inclined 3 . 4 degrees to earth ' s orbit , so even when venus is in the same direction as the sun ( “ in conjunction , ” as astronomers say ) , most of the time it is too far above or below the ecliptic to cross the sun ' s face [ see top illustration in box on opposite page ] 例如金星轨道和地球轨道有3 . 4度的夹角，因此即使金星和太阳在同一个方位（天文学家称之为合） ，大部份时候金星都远在黄道的上方或下方，而不是横过太阳表面（见右页金星凌日的几何学） 。
