Perihelion and aphelion

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Main article: Apsis
1. Planet at aphelion 2. Planet at perihelion 3. Sun

The perihelion is the point in the orbit of a planet, asteroid, comet or other sun-orbiting body where it is nearest to the sun. It is the opposite of aphelion, which is the point where the orbit of the object is farthest from the sun.

The word perihelion stems from the Greek words "peri," meaning near, and "helios," meaning the sun. Aphelion derives from the preposition apo meaning away, off, apart (The similar words, perigee and apogee, 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, 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.) The difference in distance between Earth's nearest point to the sun in January and farthest point from the sun in July is about 5 million kilometers (3.1 million miles). Earth is about 147.1 million kilometers (91.4 million miles) from the sun at perihelion in early January, in contrast to about 152.1 million kilometers (94.5 million miles) at aphelion in early July.

When Earth is closest to the sun, it is winter in the northern hemisphere and summer in the southern hemisphere. Thus Earth's distance from the sun does not affect what season occurs. Instead, Earth's seasons come and go because Earth does not rotate with its axis exactly upright with respect to the plane its orbit around the sun. Earth's axial tilt is 23.4 degrees. This puts the sun farther south in December and January, so the north has winter and the south has summer. Thus winter falls on that part of the globe where sunlight strikes least directly, and summer falls where sunlight strikes most directly, regardless of Earth's distance from the sun.