Earth reaches its annual far point from the Sun, aphelion, on Friday, July 6, 2018. Find a brief explanation of aphelion and perihelion—and better understand how our planet Earth orbits its star!
What are Perihelion and Aphelion?
The terms perihelion and aphelion describe different points in the Earth’s orbit of the Sun. Recall that the Earth orbits the Sun in an elliptical path—which is oval, not round!
- Aphelion is the point of the Earth’s orbit that is farthest away from the Sun.
- Perihelion is the point of the Earth’s orbit that is nearest to the Sun.
The Sun is about 93 million miles away from Earth. The actual distance varies because of Earth’s elliptical orbit.
Here’s how to better remember these two terms: The words come from Ancient Greek, in which helios means “Sun,” apo means “far,” and peri means “close.”
Image credit: NASA (Note that “perihelon” should be spelled “perihelion”
Did you know that the Moon’s orbit around the Earth is also elliptical? The point in the Moon’s orbit that is closest to the Earth is called the Perigee and the point farthest from the Earth is known as the Apogee. Perigee and Apogee are often confused.
When are Aphelion and Perihelion?
- Aphelion always happens in early July. About two weeks after the June solstice, Earth is farthest from the Sun.
- Perihelion always happens in early January. About two weeks after the December Solstice, Earth is closest to the Sun.
Isn’t it interesting that the Earth is farthest from the Sun during the hot summer months and closest during the winter months?
Though this doesn’t seem to make sense, it’s the not the distance from the Sun that causes our seasons. Seasons happen because Earth’s axis is tilted at an angle. It’s because Earth orbits the Sun on a tilt that our planet gets more or less of the Sun’s direct rays at different times of the year.
Aphelion and Perihelion on Mars
Though aphelion and perihelion are terms mostly used in reference to Earth since it’s our home planet, they are also relevant to other planets orbiting the Sun.
Every planet has points in the orbits when they are farthest or closest away from their star.
For example, planet Mars has an even more elliptical orbit than Earth. In comparison, Earth’s orbit seems almost circular. Mars also has four seasons but they are twice as long because it takes about two Earth years for Mars to go around the Sun.
The southern hemisphere of Mars has a warmer, shorter spring and summer than in the north, as Mars is closest to the Sun towards the end of southern spring. The southern winter is longer because Mars is farthest away from the Sun then, moving more slowly in its elliptical orbit around the Sun. For Mars, going from a colder winter to a warmer spring can be quite dramatic.
Image credit: JPL.NASA.gov