Perihelion

The perihelion is the point in the orbit of a planet, asteroid or comet where it is nearest to the sun.

The word perihelion stems from the Greek words "peri," meaning near, and "helios," meaning the Greek god of the sun. (The similar word, perigee, refers to the nearest point in some object's orbit of earth.)

All planets, comets and asteroids in our solar system have approximately elliptical (a kind of non-circular) 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.

Earth comes closest to the sun every year around January 3. It is farthest from the sun every year around July 4. The difference in distance between Earth's nearest point to the sun in January and farthest point from the sun in July is not very great. Earth is about 147.1 million kilometers from the sun in early January, in contrast to about 152.1 million kilometers in early July.

When Earth is closest to the sun, it is winter in the northern hemisphere and summer in the southern hemisphere. Thus it is possible to see that Earth's distance from the sun does not cause the season to change. Instead, Earth's seasons come and go because Earth does not orbit exactly upright with respect to the plane of our world’s orbit around the sun. Earth's axis is tilted to that plane by 23-and-a-half degrees. The Earth's tilted axis itself rotates about the notional axis orthogonal (perpendicular) to the orbital plane, almost precisely once per year. Winter falls on that part of the globe where sunlight strikes least directly. Summer falls on that part of the globe where sunlight strikes most directly.