Can someone please help this brainlet out by telling me what this guy means by theta naught?
Here's a link to the paper https://www.researchgate.net/publication/2174251_An_accurate_formula_for_the_period_of_a_simple_pendulum_oscillating_beyond_the_small-angle_regime

I really need this for a project I have to submit tomorrow.
I know that theta represents the angular displacement but I can't for the life of me figure what theta naught means.
If for example I displace the pendulum by pi/3 rads so that theta=pi/3, what would theta naught be?