Perihelion and aphelion phenomenon - what are they?

Perihelion and aphelion phenomenon - what are they?

The Earth's orbital motion, that is, its year-long rotation around the Sun, is extremely important for our planet. Seasons depend on it and also the ability to observe through a telescope more constellations that change their position. The orbital motion causes changes in the appearance of the night sky, the Sun's altitude, and aphelion and perihelion. These two Greek-derived terms are directly related to the way the Earth orbits the Sun. We are about to examine what they mean, when aphelion and when perihelion occur, and why they depend on the shape of the Earth's orbit.

What does the Earth's orbit look like?

The Earth circles around the Sun in a specific orbit, which has the shape of an ellipse, and it is this shape that allows us to distinguish aphelion and perihelion. The Earth's orbit around the Sun lasts 365 days 6 hours 9 minutes 9.54 seconds, and during all this time it moves closer or further away from its star. So when we read that the distance of the Earth from the Sun in light years is 0.000016077 ly, which equals 149,600,000 kilometers, it is worth realizing that this is an averaged conventional value, which makes it easier to make calculations. The Earth's distance from the Sun varies over the course of the year, and this means that we can determine exactly when it is closest to the Sun and when it is furthest from it.

aphelium and peryhelium scheme

1. aphelion; 2. peryhelion; 3. the Sun
Source: Wikipedia

Aphelion - what is this moment in the Earth's circular motion?

Let's start with an explanation of the term aphelion. This is a word of Greek origin (apo - "from" and helios - "sun") meaning the place in the orbit of a body revolving around the Sun that is farthest from the star. So if we talk about the Earth's aphelion, it is the point at which it is farthest from the Sun - during its annual orbital motion. We can talk about the outermost point of the orbit only in the case of elliptical orbits, along which planets, comets or asteroids orbit - circular orbits do not have it.

Aphelion - when will the Earth be farthest from the Sun?

At the moment of aphelion, the Earth is 152.1 million kilometers away from the Sun. It occurs at very regular intervals - every orbital year and always takes place around July 4. Aphelion of 2023 took place on July 6 at 21:06 - BST

What is perihelion and when does it occur?

Perihelion is the point of the orbit that is opposite to aphelion, and thus marks the moment when the Earth is closest to the Sun - at a distance of 147.1 million kilometers. This term also comes from Greek - from the words peri "at" and Helios "Sun", and it always happens at the beginning of the year - between January 2 and 4. Thus, this orbit point is also crossed at equal intervals and occurs only for objects with a stable elliptical orbit.

Mercury's perihelion is very interesting, during which it approaches the Sun at a distance of 46 million kilometers, and at the moment of greatest distance it is as far as 69.8 million kilometers from our star. Here the differences are already very large, and they are influenced by Mercury moving in an orbit of the greatest eccentricity. Also specific for Mercury is the fact that its perihelion undergoes precession, so there are some perturbations in the orbit of this planet. Since the orbits of planets are mainly affected by other planets, it was assumed in the 19th century that there must be an unknown planetary object in our solar system - between the Sun and Mercury - perturbing its orbit. This, of course, was not confirmed, and the mystery of perihelion precession was explained in 1916 by Albert Einstein's general theory of relativity - it is caused by perturbations from other Solar System bodies (mainly Earth, Jupiter and Venus), relativistic gravitational effects resulting from the general theory of relativity, as well as the flattening of the Sun.

The distance of the Sun from the Earth and its impact on our planet

Aphelion and perihelion alone do not affect the Earth and its conditions in any way. Although our planet receives 6.5% more solar energy at perihelion than when it is farthest from the Sun, these differences are too small to affect Earth's temperature and climate. In contrast, the points marking the Earth's maximum proximity/distance to/from the Sun affect the length of the seasons. During perihelion, the Earth moves slower, resulting in a 5-day shorter winter in the northern hemisphere - compared to summer.

Earth and Sun

On the other hand, the Earth's orbital motion and its degree of inclination to the ecliptic, which is 66°33', are very important. They affect the changing seasons in the southern and northern hemispheres, and the occurrence of polar nights and days depends on them. They cause differences in the degree of illumination of the Earth, which is why our planet has different climate zones, zones of illumination of the Earth and the length of day and night varies. At different times the Sun rises and sets, and we can observe the Sun's changing position against the backdrop of the Zodiac, and enjoy the changing night sky, which is certainly important for all astronomical observation enthusiasts. If it were not for the Earth's rotation, the sky would always look the same, and the telescope would serve us to see the same constellations over and over again.


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