A new class of conformal invariants for a given spacetime M is introduced exploiting the conformal geometry of any light ray $\mathrm{\Gamma }$. Each congruence of light rays passing through a given point p defines the sky S(p) of such point. The new conformal invariants are defined on the bundle of skies of the spacetime M, being called sky invariants accordingly. The natural conformal covariant derivative defined on a light ray and its associated covariant calculus allows us to show the existence of a natural conformal invariant differential of arc that, together with the restriction of the curvature of the conformal covariant derivative, can be used to construct a sky invariant that will be called the sky curvature. An algorithm, that can be implemented on any symbolic manipulation software system, to compute the sky curvature will be discussed and the main ideas and the explicit computation of the sky curvature are illustrated in Schwarzschild spacetime.

利用任何光线的共形几何，引入了给定时空M的一类新共形不变量$\mathrm{\Gamma }$. 通过给定点p的光线的每个全等定义了该点的天空S ( p )。新的共形不变量定义在时空M的天空束上，因此被称为天空不变量。在光线上定义的自然共形协变导数及其相关的协变演算使我们能够证明弧的自然共形不变量微分的存在，连同对共形协变导数的曲率的限制，可用于构造天空不变量，称为天空曲率。将讨论可以在任何符号操作软件系统上实现的计算天空曲率的算法，并在 Schwarzschild 时空中说明天空曲率的主要思想和显式计算。