How long does it take for the disc of the Sun to disappear?

 

I was stood on the beach at Croyde hoping to see the Green Flash at sunset. It occurred to me to time how long it took from the bottom of the disc touching the water to the top of the disc finally disappearing. The Sun subtends an angle of 0.5 degrees at the eye. 360 degrees of rotation in 24 hours means 15 degrees per hour or 1 degree of movement every 4 minutes. (The true answer needs to take into account that the Earth is moving around the Sun as well as spinning on its axis. I have simplified it down) Anyway, that meant the Sun should disappear in 2 minutes. I timed 3 min 30 seconds. That couldn't be explained by the reason given above. Looking it up online, it says that the time depends on how far away from the Equator you are and what time of year. It seems to be that 2 minutes would be fine on the Equator at a time when the Sun is going down perpendicular to the horizon. But that isn't the case in the UK so the Sun must be moving diagonally across. I need to think about how that makes the angular movement different for a circular object.