To clarify this even further, as someone who professionally plays an instrument that is traditionally placed at the back of an orchestra, you absolutely cannot play by ear: you MUST play by watching a combination of the stick in the conductor’s hand and the bow of the first violinist and cellist at the front. If you play what sounds in sync to you, the conductor and audience will hear you too late; the round trip from the front to the back of a stage, plus the sound traveling through the brass tubes of your instrument, plus the trip from the rear of the stage to the first row of the audience simply takes so long that it will sound noticeably wrong. The same is true for the far sides of an orchestra pit underneath the stage of a musical or opera. It only takes 20 meters/yards to become an issue.