Aphelion Meaning
We know that all planets, asteroids, and comets in the solar system revolve around the Sun in approximately elliptical orbits. Thus as a result there are points in the orbital pathways when the heavenly body is farthest and nearest to the sun. Aphelion is when a planet, an asteroid, or a comet is most distant from the sun in its orbit. This is the aphelion definition in astronomy. This answers the question of what is aphelion. All planets in the solar system have an aphelion. The perihelion condition is the exact opposite. The tilt of the axis of the planet and the elliptical orbital pathways cause aphelion and perihelion.
Perihelion and Aphelion of the Earth
Perihelion is the point in the orbital pathway of earth where it is closest to the sun. The distance is approximately 91.4 million miles or 147 million kilometers. Aphelion on the other hand is the point in the orbit when the earth is farthest away from the sun. The distance is approximately 94.5 million miles or 152million kilometers. Aphelion occurs around July 4 about two weeks after the June Solstice. Perihelion occurs around January 3 after about two weeks of the December Solstice. The aphelion and perihelion occur around the same time every year. This 2021, aphelion of Earth was observed on July 5, at 6.27 P.M (Eastern Time) and perihelion of Earth was observed on January 2, at 8,52 A.M (Eastern Time). The precession of the perihelion events is the reason why the orbit is not a simple closed curve such as an ellipse. Milankovitch Cycles occur because of these events.
Earth’s orbital pathway around the sun cannot be described as a perfect circle. The orbital path is an elliptical one with an eccentricity of 0.017. Therefore the sun will not be at the center. According to the laws of Johannes Kepler, all planets in the Solar system have elliptical orbits with the Sun at one of the focal points. Now due to this reason at times, Earth will be closer to the sun and at times it will drift far away from the sun. Thus we can confirm that the elliptical orbit of the earth around the sun is the cause for aphelion and perihelion. Another cause for this perihelion and aphelion may be the 23.5-degree tilt of the axis of Earth. This tilt results in the changing of the seasons.
When the earth is closest to the sun i.e. during aphelion, the northern hemisphere has a winter season, and the southern hemisphere experiences the summer season. Thus it can be seen that the distance between the earth and the sun does cause any noticeable effect in the changing of the seasons. The minor effects of the difference in distances are overshadowed by the oceanic southern hemisphere and the continental northern hemisphere. Hence the change of the seasons is due to the rotation of the earth around its tilted axis. The tilt is measured to be 23.5 degrees. This is the reason for winter in the northern hemisphere in December - January and summer in the southern hemisphere as the Sun is farther south during this time. The part of the earth where sun rays fall slantingly experience winter whereas the parts where the sun rays fall directly experience the summer season.
Did You Know?
The words “ perihelion” and “ aphelion” are derived from ancient Greek, where “apo” means far, “peri” means close, and “helios” stands for the sun.
All planets, comets, and asteroids in the solar system have approximately elliptical orbits and thus also have a perihelion and aphelion.
A few other similar terms used for describing the most distant points in the orbital paths of heavenly bodies moving around some center of force (apart from the sun) include “apogee”( farthest point from earth) and “aposaturnium” (farthest point from Satan).
FAQs on Aphelion
1. What are aphelion and perihelion in simple terms?
Aphelion and perihelion describe the two extreme points in the orbit of a planet or comet around the Sun. Aphelion is the point where the object is farthest from the Sun. Perihelion is the point where it is closest to the Sun. Every object in our solar system that follows an elliptical path has both an aphelion and a perihelion.
2. What is the main difference between Earth's aphelion and perihelion?
The main differences for Earth relate to distance and timing:
- Distance: At aphelion, Earth is about 152.1 million km from the Sun. At perihelion, it's about 147.1 million km away.
- Timing: Earth reaches aphelion in early July, a couple of weeks after the June solstice. It reaches perihelion in early January, a couple of weeks after the December solstice.
3. Why do aphelion and perihelion occur in the first place?
Aphelion and perihelion exist because Earth's orbit around the Sun is not a perfect circle; it's an ellipse. According to Kepler's First Law of planetary motion, the Sun isn't at the exact centre of this elliptical path but at one of its two focal points. This geometric arrangement means that as Earth travels along its orbit, its distance from the Sun naturally increases and decreases, creating the farthest (aphelion) and closest (perihelion) points.
4. If Earth is farthest from the Sun at aphelion in July, why is it summer in India?
This is a common point of confusion! Earth's seasons are caused by the 23.5-degree tilt of its axis, not its distance from the Sun. In July, the Northern Hemisphere (where India is located) is tilted towards the Sun. This tilt means we receive more direct and concentrated sunlight for longer hours, causing summer, even though the entire planet is at its farthest orbital point.
5. How does Earth's speed change when it is at aphelion?
Earth's orbital speed changes throughout the year. As per Kepler's Second Law, a planet moves slower when it is farther from the Sun and faster when it is closer. Therefore, the Earth moves at its slowest orbital speed when it is at aphelion and its fastest at perihelion.
6. Do all planets in the solar system have an aphelion?
Yes, all planets, comets, and asteroids that orbit the Sun follow an elliptical path. Because their orbits are not perfectly circular, every one of them has a point where they are farthest from the Sun (aphelion) and a point where they are closest (perihelion). The exact distances, however, vary greatly for each celestial body.
7. What is the term for the farthest point in the Moon's orbit around Earth?
The terms aphelion and perihelion are specific to orbits around the Sun (from the Greek 'helios' for Sun). For an object orbiting the Earth, the equivalent terms are apogee (farthest point) and perigee (closest point). So, the Moon is at apogee when it is farthest from Earth.
