Why Does Earth's Perihelion Occur in Winter for One Hemisphere?
The Earth's closest approach to the Sun, known as perihelion, takes place on January 3, 2026. This event marks a significant point in our planet's orbit, where it is approximately 2.5 million kilometers closer to the Sun than at aphelion, the point of maximum distance. This might seem counterintuitive, especially for those in the Northern Hemisphere, as one might expect winter to occur when Earth is closest to the Sun. However, this phenomenon is primarily due to the Earth's axial tilt and the shape of its orbit.
The Earth's axial tilt of 23.5 degrees is the primary cause of seasons. When the Northern Hemisphere is tilted towards the Sun, it experiences summer with more sunlight. Conversely, when it is tilted away, it enters winter with less sunlight. The Earth's orbit around the Sun is not a perfect circle but an ellipse, influenced by the gravitational pull of gas giants like Jupiter and Saturn over hundreds of thousands of years. This results in variations in the distance between Earth and the Sun, known as eccentricity.
Eccentricity is the reason why our seasons have slightly different lengths. Currently, the Northern Hemisphere's summers are about 4.5 days longer than winters, and springs are about three days longer than falls. As eccentricity decreases, the length of seasons will gradually even out. The current low eccentricity of 0.0167 makes Earth's orbit nearly circular, leading to almost equal season lengths, though not perfectly.
The Earth's orbit is divided into four quadrants, and the seasons are defined by the quadrant Earth is passing through. The duration spent in each quadrant varies, with less time at perihelion. According to NASA, the line joining a planet and the Sun sweeps out equal areas in equal time intervals, meaning planets don't move at a constant speed. This results in perihelion being the point of closest approach and aphelion the point of greatest separation, with the planet moving fastest at perihelion.
The timing of aphelion and perihelion is a coincidence, as they are currently close to the solstices. However, this won't always be the case. In 1,000 years, Northern Hemisphere summers will be around six hours longer than today. The timing of these events shifts due to the Earth's calendar not perfectly aligning with its orbit. In 6,430 CE, perihelion will coincide with the March equinox, marking a significant change in the relationship between Earth's orbit and its seasons.
