The Distance to Supernova SN1987A and the Speed of LightSource
When supernova SN1987A exploded, its light soon struck a ring of gas some distance from the star and illuminated it. As viewed from Earth, the ring appeared around the supernova about a year after it exploded. Its angular size combined with the time it took for the ring to be illuminated after SN1987A was first observed allows a direct, trigonometric calculation of the distance to that supernova with an error of less than 5%.
Oddly enough, if we use the older Newtonian physics (which most creationists love because it allows them to play around with the speed of light) we find that a change in the speed of light does not affect our calculations of the distance to SN1987A! Gordon Davisson pointed out that interesting tidbit.
The distance is based on triangulation. The line from Earth to the supernova is one side of the triangle and the line from Earth to the edge of the ring is another leg. The third leg of this right triangle is the relatively short distance from the supernova to the edge of its ring. Since the ring lit up about a year after the supernova exploded, that means that a beam of light coming directly from the supernova reached us a year before the beam of light which was detoured via the ring. Let us assume that the distance of the ring from the supernova is really 1 unit and that light presently travels 1 unit per year.
If there had been no change in the speed of light since the supernova exploded, then the third leg of the triangle would be 1 unit in length, thus allowing the calculation of the distance by elementary trigonometry (three angles and one side are known). On the other hand, if the two light beams were originally traveling, say three units per year, the second beam would initially lag 1/3 of a year behind the first as that's how long it would take to do the ring detour. However, the distance that the second beam lags behind the first beam is the same as before. As both beams were traveling the same speed, the second beam fell behind the first by the length of the detour. Thus, by measuring the distance that the second beam lags behind the first, a distance which will not change when both light beams slow down together, we get the true distance from the supernova to its ring. The lag distance between the two beams, of course, is just their present velocity multiplied by the difference in their arrival times. With the true distance of the third leg of our triangle in hand, trigonometry gives us the correct distance from Earth to the supernova.
Consequently, supernova SN1987A is about 170,000 light-years from us (i.e. 997,800,000,000,000,000 miles) whether or not the speed of light has slowed down.
Sorry, you have to define a light-year as a distance in meters and then ignore the time-element for this to have any meaning whatsoever.
In other words, a 'light-year' is defined as a certain number of meters. That number of meters obviously will not change regardless of the speed of light. It is the time required to transverse this number of meters that is critical. What proponents of this argument do is define light-years in terms of distance and then use this defined distance measure as though it holds some time element.
Nice little trick, but such is the lack of critical thinking among adherents of this argument.