Why is earth closest to the sun in January?
Because of elliptical orbit.
Johannes Kepler was the one who understood the actual orbits of the planets around the Sun. He suggested tha the orbit of each planet around the sun is an ellipse with the Sun being at one of the focii. So, at one time of the year the Earth should be closer to the Sun than the other. When the planet is closer to the Sun it is called Perihelion and when it is away from the planet it is called Aphelion.