on.update = function(dt)
	local P = 10  -- Period in seconds
	local t = Current_time - math.floor(Current_time/P)*P
	-- t is between 0 and P
	assert(0 <= t)
	assert(t <= P)
	if t > P/2 then
		Night_blue = (P-t)/(P/2)
	else
		Night_blue = t/(P/2)
	end
	Angle = t*2*math.pi/P
	table.insert(Debug_state, Night_blue)
	if #Debug_state > 800 then
		table.remove(Debug_state, 1)
	end
end