Havoc: That is my understanding. If light can be slowed down that drastically, then one cannot state that the speed of light is necessarily constant or equal when coming from any given source compared to any other source. Between that and the red shift issue, that pretty much blows any confidence in results based on light time and leaves only Geometric projections which are of no confidence over infinite distances. The precision of the angles at base become a guess and thusly precision in general erodes.
Ummm.… Let us start with the reader knowing at least something about the electromagnetic spectrum.
Light is slowed down in different mediums such as glass or water. The ratio of the speed of light in a vacuum versus the speed in the medium such as water is called the index of refraction. This is why you see a pencil "bend" when you place it half way into a glass of water. As light crosses over from one medium to another, not only does its speed change, but its wavelength as well. Note: the frequency remains unchanged. Thusly, the index of refraction can be stated in terms of wavelength as well. Snell's Law describes this phenomenon. (You can Google Snell' Law if you want to follow the math).
What is even more interesting (I know I am being really simple here so for all you physics lurkers out there, please do not have an apoplexy) is that different frequencies travel at different velocities in a medium. This is why a prism splits the different frequencies of a white light into a rainbow due to the index of refraction of each color (frequency).
Ok, now that we got thru that, when physicist and astronomers talk about the speed of light, they are usually talking about light speed in a vacuum. In fact, it is now an invariant. In other words, its speed in a vacuum never changes. This invariant speed is 299,792.458 km/sec. This enables us to use the speed of light over time as a distance measurement more commonly known as the light year. BTW, astronomers prefer to use Parsecs instead of light years when describing distances
Note (as per your earlier posting) that SN1987A is still a long long way away, regardless of the speed of light.