its a well know fact that speed of light had dropped between 1920 -1945.
A Brief History of the Speed of Light - http://www.pbs.org/wgbh/nova/blogs/physics/2015/02/brief-history-speed-light/
The speed of light keeps varying but to solve this problem a bunch of scientists fixed the speed of light by definition i.e,299 792 458 m / s.
Scientists slow the speed of light -http://www.bbc.com/news/uk-scotland-glasgow-west-30944584
Rupert Sheldrake speaks, argues that speed of light is dropping - https://whyevolutionistrue.wordpress.com/2013/03/06/tedx-talks-completely-discredited-rupert-sheldrake-speaks-argues-that-speed-of-light-is-dropping/