On Tuesday, January 2, the Earth reaches its shortest distance from the Sun in all of 2024, at the same time, it moves at its highest speed than in the rest of the year.
Six months later, on Thursday, July 4, the Earth will be the furthest from the Sun in all of 2024, moving at its slowest speed of the year.
We call it perihelion, the moment when the Earth is least far from the Sun, which happens in the first days of January of each year, while we call it aphelion, the moment when the Earth is furthest from the Sun, which happens in the first days of July.
And this is normal and happens year after year.
Some define perihelion as the moment when the Earth is closest to the Sun, but 147 million km is not close, which is why we say it better: less far away.
THE ORBIT
The Earth moves around the Sun in a path we call the Earth’s orbit, which it travels in 365.25 days per year.
Let’s not confuse the Earth’s orbit with the Earth’s orbit, which is around the Earth, where the satellites, the Space Station, the Hubble Telescope and, further away, the Moon move.
The Earth’s orbit is not a circle but an ellipse, although it is not very elongated, it looks very similar to a circle.
Being an ellipse, there is a time in the year when the Earth is at its closest distance from the Sun, the perihelion, and six months later, the Earth will be at its greatest distance from the Sun, the aphelion.
During 2024, perihelion will occur on January 2 at 6:39 p.m. (Central Mexico Time -6 UT) (00:39 on January 3 UT), when the Earth reaches a minimum distance from the Sun of 147,100,632 kilometres.
While the aphelion will be on July 4, at 11:06 p.m. (Central Mexico Time -6 UT) (05:06 a.m. UT on July 5), when the Earth is 152,099,968 kilometers from the Sun.
A difference of 5 million km! 4,999,336 to be precise.
We drive the average distance from the Earth to the Sun, 150 million km.
The word aphelion comes from the Greek απο (apo) which means far away, and ηλιοσ (helios) the Sun. While perihelion comes from the Greek περι (peri) = around.
SPEED
But there is more, the Earth not only varies its distance from the Sun throughout the year, but also varies its speed both at perihelion and at aphelion.
Thanks to Kepler’s Laws, we know that the Earth moves at different speeds depending on its distance from the Sun. Thus, as its distance from the Sun decreases (the perihelion), the Earth increases its speed, while as it moves away from the Sun (the aphelion), the Earth slows down.
The difference between both speeds is minimal, being 30.29 km/s (109,044 km/h) at perihelion and 29.29 km/s (105,444 km/h) at aphelion.
SEASONS
On January 2, the Earth will be at perihelion, its shortest distance from the Sun and at its highest speed throughout 2024, but shouldn’t it be hot if we are closer to the Sun? And six months later, when the Earth is furthest from the Sun, shouldn’t it be winter?
Although it may seem illogical, it is not.
The Seasons on Earth do not depend on the distance to the Sun but on the roundness of the Earth and the inclination, which causes the Sun’s rays to fall obliquely (inclined) in one hemisphere and at the same time in the other hemisphere they fall perpendicular.
When rays fall obliquely, they must cross more atmosphere and lose more energy; when they reach the ground they do not heat up much. While when they fall perpendicular, they lose less energy as they travel through less atmosphere and when they reach the ground they heat up more.
This means that depending on the position of the Earth with respect to the Sun (the month), the rays will fall in one hemisphere or the other.
The Earth has a tilt of 23.5º. That is why if the Sun’s rays fall perpendicular in the northern hemisphere, in the southern hemisphere they fall obliquely. In the north it will be summer and in the south it will be winter. And six months later, it will be the opposite.
It is worth mentioning that throughout the year, the tilt of the Earth is always directed in the same direction, only the position of the Earth around the Sun varies.
That is, the north of the Earth’s axis of rotation points to a star, the North Star or Polaris, in the tail of the constellation Ursa Minor. And this does not vary, no matter what month of the year we are in.
However, two observations must be made.
It would seem that the inclination of the Earth does not change, but it does, the tilted Earth makes a turn similar to a top about to fall every 25,776 years, so during our lifetime and that of several generations, the Earth has varied little the direction of the inclination, it is not noticeable to us. This movement is called Precession of the Equinoxes and was discovered by the greatest astronomer of antiquity, Hipparchus of Nicaea, in the 2nd century BC! C!
It will be almost 13 thousand years from now, when the inclination of the Earth points to another star (Vega) and then, if we will see changes in the seasons.
Finally, there is a difference in temperatures in the northern and southern hemisphere, but it is for geographical reasons: in summer at the North Pole the average temperature is 0° and in summer at the South Pole it is -28.2° . In winter at the North Pole the average temperature is -40° while at the South Pole it is -60°. The South Pole is colder because it is a rock mass with a very thick layer of ice, it is the highest continent in the world, while the North Pole is frozen water surrounded by land.
Happy new year and happy perihelion!