Our planet is at perihelion: it is closest to the Sun on January 4, 2023. However, it is winter in the northern hemisphere. Why are we cold at this time, if the Earth is closer to the star?
Here we are closer to the Sun than we will ever be in 2023. This Wednesday, January 4, the Earth passes perihelion. There is nothing special to observe. You can simply remember that today our planet is at its minimum distance on its trajectory with the star of the solar system. During perihelion, the Earth is about 147 million km from the Sun.
One might be surprised that the Earth’s perihelion occurs during winter, the season when it is coldest. Why are we cold when we are closer to the Sun? Because the seasons are not explained by the distance from the Earth to the Sun. Certainly, the orbit of the Earth around the star does not draw a perfect circle, but an ellipse. Hence the existence of perihelion and its opposite, aphelion (when the Earth is about 152 million km from the Sun). The perihelion occurs during the winter and the aphelion during the summer… But, be careful, this is only valid in the northern hemisphere.
The seasons are related to the axis of rotation of the Earth, tilted
For the southern hemisphere, it is the opposite: the perihelion takes place during the summer, and the aphelion during the winter. We, therefore, understand that the distance between the Earth and the Sun is not a satisfactory explanation for the seasons, since the situation is different in the two terrestrial hemispheres. The origin of the seasons actually comes from the Earth’s inclination
The Earth’s axis of rotation is tilted. // Source: Canva
The Earth is in rotation around an axis (it turns on itself), an imaginary line connecting the North pole to the South pole of the planet. Our days correspond to a complete turn on this axis. However, this axis is inclined, by 23.4°, with respect to the plane of the Earth’s orbit around the Sun (we also speak of the ecliptic plane).
The axis is always tilted in the same direction, while the Earth rotates on this axis while orbiting around the Sun. Therefore, for part of the year, the northern hemisphere of the planet is tilted more toward the Sun; another part is the southern hemisphere which is more inclined towards the star.
The Sun heats more the hemisphere inclined towards him: it is summer in this part of the globe and winter in the other. As for spring and winter, they correspond to the times when the two hemispheres receive comparable amounts of solar heat. We thus distinguish the equinoxes (spring, autumn) from the solstices (summer, winter).