I wonder how we define a "day" and when do decide it is next day now. Earth rotates/wiggles all the time and it is far from a perfect sphere or any shape. And i

I wonder how we define a "day" and when do decide it is next day now. Earth rotates/wiggles all the time and it is far from a perfect sphere or any shape. And it changes its shape as well How can we decide on such small time scales it is no longer the same day when there are so many variables.