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There are two extremes to any obit: perihelion and aphelion. Perihelion is when a celestial body is at its closest point to another celestial body. Aphelion is when a celestial body is at its farthest point from another celestial body. Both are examples of an apsis.
An apsis(apsides for plural usage) is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. This focus is also the center of attraction, which is usually the center of mass of the system. In geocentric models, an apsis is measured from the center of the Earth.
The point of closest approach is called the periapsis or pericenter. The point of farthest excursion is called the apoapsis, apocentre or apapsis. A straight line drawn through the periapsis and apoapsis is the line of apsides. This is the major axis of the ellipse. Other terms, derived from these, are used to identify the body being orbited. The most common are perigee and apogee, referring to orbits around the Earth, as well as perihelion and aphelion, which refer to orbits around the Sun.
During the Earth’s orbit around the Sun, perihelion and aphelion can be somewhat related to the seasons. Earth’s orbit has changed over the millenia, but, currently, perihelion occurs about 14 days after the December solstice. On average perihelion occurs on January 4 each year. At that time, Earth is 147,098,074 km or 0.98328989 astronomical units from the Sun. Aphelion occurs nearly an exact six months later, so Earth is at aphelion in July. At that time it is 152,097,701 km or 1.01671033 astronomical units from the Sun. The dates of perihelion and aphelion change over time and make one complete cycle every 22,000 to 26,000 years.
Perihelion and Aphelion are the terms used to express the apsides of Earth. There are many variations depending on which celestial body you are talking about.
We’ve also recorded a series of episodes of Astronomy Cast about every planet in the Solar System. Start here, Episode 49: Mercury.