A celestial body is farthest from the Sun during aphelion and closest during perihelion. The eccentricity of an object’s orbit determines the difference between the two points. Earth’s aphelion occurs in July and appears hottest due to Northern Hemisphere land masses. Pluto’s orbit is highly elliptical, with extreme variation in distance from the Sun. Astronomers can calculate distances and map paths of celestial objects.
An aphelion is a moment at a point in a celestial body’s orbit around the Sun when that celestial body is farthest from the Sun. The opposite of an aphelion is a perihelion, when that object is closest to the Sun. Each of the objects which orbits the Sun experiences aphelion and perihelion at different points in time, and the difference between these two points can vary radically, depending on the eccentricity of the object’s orbit.
While people often say that objects like the Earth “go around the Sun,” this terminology isn’t technically entirely correct, because objects orbiting the Sun actually have elliptical orbits, as described by Kepler in his First Law. The amount of variance from a perfectly circular orbit is known as the “eccentricity” of the orbit. The greater the eccentricity, the more elliptical the orbit. Earth has a fairly low eccentricity, around .0167, in contrast to Mercury, with an eccentricity of .2056.
In the case of Earth, aphelion occurs in the Northern Hemisphere summer, around July. The precise timing of aphelion shifts by about 30 minutes each year, following a 21,000-year cycle, meaning that ultimately aphelion for Earth will fall in the Southern Hemisphere summer. Although the Earth is at its farthest point from the Sun during aphelion, the planet appears to be at its hottest point in the Northern Hemisphere summer, because large land masses in the Northern Hemisphere heat up and give off their heat all over the planet.
At perihelion, the Earth is approximately 91 million miles (147 million kilometers) away from the Sun, in contrast to 95 million miles (152 million kilometers) away from the Sun at aphelion. This variation from distance is relatively small when compared to the orbits of many other objects that orbit the Sun. Pluto, for example, has a very elliptical orbit, with extreme variation between near and far points. Pluto’s distance from the Sun is measured in astronomical units (AU), with each AU representing the mean distance between the Sun and the Earth: at perihelion, Pluto is 30 times farther (30 AU) from the Sun than the Earth, and at aphelion, Pluto is 49 times farther (49 AU) away.
Astronomers can use an assortment of formulas to calculate the distances of various objects from the Sun at different points in their orbits, and can also map the paths that a myriad of objects will take as they move around the Sun. Numerous graphs depicting the orbits of the planets and their near and far points are found in atlases and scientific texts.
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