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The Earth revolves around the Sun like this.
An apsis is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. Within our current understanding of celestial mechanics, this focus is also the center of attraction and the center of mass of a system. In geocentric systems, the apsis is measured from the center of the Earth.
Periapsis is the term used to refer to the point when two bodies are the closest together. Apoapsis is the point when they are the farthest apart. A straight line drawn through the periapsis and apoapsis is the line of apsides and is the major axis of the ellipse of system. Other terms are derived from these: perigee and apogee to orbits around the Earth and perihelion and aphelion to refer to orbits around the Sun.
When you think about the Earth’s orbit around the Sun, the time of apsis is most easily expressed by the seasons. That will help you understand the contribution of the elliptic orbit to seasonal variations; especially in insolation at the top of the atmosphere. This mechanism is primarily controlled by the annual declination of the Sun, which is a consequence of the tilt of the Earth’s rotation axis relative to the plane of the orbit. Earth’s perihelion occurs about 14 days after the December solstice, which puts it right about January 4. At that time, Earth is147,098,074 km. The Earth comes to aphelion in July when it is 152,097,701 km from the Sun. The dates of perihelion and aphelion slowly change in a cyclical manner, making one complete cycle in 22,000 to 26,000 years.
Apsis, or a derivative of it, plays an important role in our understanding of many planetary systems and, as such, is a term you must be familiar with to understand astronomy.
We have written many articles about apsis for Universe Today. Here’s an article about perihelion, and here’s an article about elliptical orbit.
If you’d like more info on Apsis, check out Wikipedia, and here’s a link to NASA’s Solar System Simulator.
We’ve also recorded a series of episodes of Astronomy Cast about every planet in the Solar System. Start here, Episode 49: Mercury.
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