During which season in the northern hemisphere is the Earth closest to the Sun?
According to Kepler's First Law of Planetary Motion, all planets, comets and asteroids in our solar system have approximately elliptical orbits. (Any single revolution of a body around the Sun is only approximately elliptical, because the phenomenon known as precession of the perihelion prevents the orbit from being a simple closed curve such as an ellipse.) Thus, they all have a closest and a farthest point from the Sun: a perihelion and an aphelion, known collectively as apsides. Orbital eccentricity measures the flatness (departure from a perfect circle) of the orbit.
Earth comes closest to the Sun every year around January 3. It is farthest from the Sun every year around July 4. (For a table of these dates for various years, see Apsis.)
When Earth is closest to the Sun, it is winter in the northern hemisphere and summer in the southern hemisphere.