It come as a surprise when we learnt that during the Apollo program, the astronauts had no way of knowing that their rocket engine was ready to ignite. We learned this as we were browsing through an article about First Man and we extracted from the article and quoted the statement below.
For example, in one scene, the filmmakers were looking for how the astronauts would know that a rocket engine was ready to ignite. In reality, the spacecraft had no such indicator.
“I said, ‘Put a light in the cockpit.’ It is not supposed to be there, but put a light in the cockpit to make sure that the audience knows that the engine is going to start firing,” said Worden.
Thus, a light indicator was placed for making the movie as a way to help the audience to know that the rocket engine is ready for ignition. This small diversion from the actual history was based on the advice by Al Worden. He is a former Apollo 15 astronaut and is serving as a technical adviser for the movie First Man.
The small diversion is placing the indicator lights when making the movie.
In making this movie, Damien Chazelle, screenwriter Josh Singer and their production team are trying to get things right and to be as historically accurate as possible.
Apparently they were doing a good job at that since Neil Armstrong’s son Rick Armstrong quoted (also from the same article):
“Probably people in this room and people in the space community think Hollywood can’t do this. They can’t accurately represent history because they don’t care about it, they are not interested in it. But when you come to understand the level of work that they have done to be as accurate as they can, you’ll be impressed,”
And Neil’s other son Mark Armstrong who had a cameo in a scene of Mission Control had this to say:
“Since I was in that scene in Mission Control, that was one of the things that really struck me as over the top as to attention to detail and made me feel really good about the film,”
On the whole we can safely conclude that this movie may go down as one of the most accurate depiction of a historical event. So that little indicator light is not a major issue. The movie is slotted to be released in early October 2018.