Perihelion and aphelion
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The perihelion is the point in the orbit of a planet, minor planet, or comet, where it is nearest to its orbital focus, generally a star. It is the opposite of aphelion, which is the point in the orbit where the celestial body is farthest from its point of orbit.
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 object's 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 around the Sun. (Any single revolution of a body around its parent object is only approximately elliptical, because precession of the perihelion prevents the orbit from being a simple closed curve such as an ellipse.) Hence, an orbiting body has a closest and a farthest point from its parent object, that is, a perihelion and an aphelion, known collectively as apsides. Orbital eccentricity measures the flatness (departure from a perfect circle) of the orbit.
When Earth is closest to the Sun, it is winter in the northern hemisphere and summer in the southern hemisphere. Hence, 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 further 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 (for a table of these dates for various years, see Apsis) 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. In the southern hemisphere winter falls at the same time as aphelion. The decrease in solar radiation at aphelion, plus shorter periods of daylight cause, in general, less heat from the Sun to hit the southern hemisphere in winter than the northern hemisphere during its winter at perihelion six months later.
- "Perihelion and Aphelion". National Earth Science Teachers Association (NESTA). Retrieved 2015-09-19.
- "Introductory Astronomy: Ellipses". Washington State University.
- "Solar System Exploration: Science & Technology: Science Features: Weather, Weather, Everywhere?". NASA. Retrieved 2015-09-19.