A radiation that is an after-effect of the Big Bang still around today. It was actually discovered accidentally by two Arno Penzias and Robert Wilson in 1965. They noticed a static that appeared to be coming from every direction at the same strength. Later scientists were able to trace out a Black Body curve for this background radiation, shown below.
Essentially, at very early point in the beginnings of the universe, everything cooled down enough to where atoms could form and this radiation was formed. The temperature to which it has cooled, estimated to be around -270 K, scientists believe to be consistent with a universe that started at a very high temperature and in a very dense state, the Big Bang. Therefore, this cosmic radiation is very strong evidence in favor of the Big Bang. 
Even though astronomers use scales such as arcseconds and arcminutes to measure certain distances between stars, the main tool of celestial navigation, the sextant, is only able to measure arcminutes. The sextant uses reflection in order to find the angle of celestial objects. Traditionally, the sextant has two mirrors. The horizon mirror is stationary and keeps half of what the observer sees to be the horizon line. The second is called the index mirror. This mirror pivots until the celestial object comes into view and meets the horizon line. It is then necessary to rotate the sextant in order to ensure the correct angle has been found. The celestial object should move in a “u” shape from side to side as the sextant is rotated side to side. After the correct angle is found, the observer may read the angle above the horizon the celestial body is at on the bottom of the incidence bar, which is connected to the incidence mirror and pivots with it. The diagram below illustrates the basic operation of a traditional sextant. 
Light travels at a velocity incredibly difficult for us to comprehend other then as just a very, very large number. To be exact, light travels at a constant velocity of 299,792,458 m/s within a vacuum. This is not the same for light traveling through other conditions. When light does travel through things other then a vacuum, there exists something called the refractive index. This index represents the amount that light is slowed. It represents this as the ration between the speed of light in vacuum and the speed at which the light travels within the medium, which is always greater then one since light can never travel faster then when it’s in a vacuum. What’s interesting is that this equation is also defined a different way. Instead of being the ration between two velocities it is defined as sin(i)/sin(r), where i is equal to the angle of incidence and r is the angle of refraction. Since this ration is always greater then 1, then it means that everything light passes through bends it in some way. This diagram from Encyclopedia Britannica represents a visual of the how light travels through a medium.