Perihelion and aphelion
The perihelion is the point in the orbit of a planet, minor planet, or comet, where it is nearest to the Sun. It is the opposite of aphelion, which is the point in the orbit where the celestial body is farthest from the Sun.
The word perihelion stems from the Ancient 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 refer to the nearest and furthest points in some objects' orbit around the Earth.)
According to Kepler's First Law of Planetary Motion, all planets, comets and asteroids in the 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. Thus Earth's distance from the Sun does not significantly 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 of 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 the Earth's distance from the Sun.
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. Because of the increased distance at aphelion, only 93.55% of the solar radiation from the Sun falls on a given square area of land than at perihelion. As winter also falls in the southern hemisphere at the same time as aphelion, this decrease in solar radiation due to the aphelion plus shorter periods of daylight causes, in general, less heat from the Sun to hit the southern hemisphere in winter than solar radiation hitting the northern hemisphere during its winter at perihelion six months later.
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