2021-10-15 天文距离单位 | 弧度 radian (二）

@ AU：

# 天文单位距离AU(Astronomical Unit: au, A)

+ 通常将地球绕太阳公转轨道的半长轴作为一种度量长度的单位

+ 称为天文单位，记为AU

+ 1 AU = 1.495 978 7 x 10^11 m

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@Radian

radian | ˈreɪdɪən | noun
Geometry a unit of measurement of angles equal to about 57.3°, equivalent to the angle subtended at the centre of a circle by an arc equal in length to the radius. [2]

【數學】 弧度

A radian is an angular measurement. If we draw two straight lines that cross at the centre of a circle such  that the length of the circumference between them (an arc) equals the  radius, then the angle between these lines is 1 radian. The angular circumference of a circle is 2π radians,  or 360 degrees, so that:

1 radian = 360/2π degrees = 5717’44.8”.

In the image[4] above, the arc length, L, between A and B is the same length as the radius, R, when the angle AOB is 1 radian.

相关阅读 & Reference 文献：

 [1].  2021-03-02 《天体物理学》|绪论|宇宙概观|太阳系

 [2]. Oxford Dictionary of English (British English). Copyright © 2010, 2017 by Oxford University Press.

[3]. 譯典通英漢雙向字典

[4]. https://astronomy.swin.edu.au/cosmos/R/Radian

以上。