Question

When the earth is closest to the sun, we have winter in the northern hemisphere. Explain why. Also explain why we have summer in the northern hemisphere when the earth is farthest away from the sun.

Short answer

Answer: The seasons in the Northern Hemisphere are caused by a combination of Earth's axial tilt and elliptical orbit around the Sun. Although Earth is closer to the Sun during winter (perihelion) and farthest during summer (aphelion), the more significant factor is the axial tilt. This tilt affects the distribution of sunlight, with the Northern Hemisphere being tilted away from the Sun during winter and towards the Sun during summer. This results in cooler temperatures and winter conditions when Earth is closest to the Sun and warmer temperatures and summer conditions when Earth is farthest from the Sun.

Step-by-step solution

Understand Earth's axial tilt

Earth rotates around an imaginary line called the axis. The axis is tilted at an angle of approximately 23.5 degrees with respect to the plane of Earth's orbit around the Sun. Due to this tilt, different parts of Earth receive different amounts of sunlight at different times of the year, resulting in the change of seasons.

Learn about Earth's elliptical orbit

Earth's orbit around the Sun is not a perfect circle, but slightly elliptical. This means that Earth's distance from the Sun varies throughout the year. The part of the orbit where Earth is closest to the Sun is called perihelion, and the part where it is farthest away is called aphelion.

Realize the effect of axial tilt on sunlight distribution

Due to Earth's axial tilt, the Northern Hemisphere is tilted toward the Sun during the summer months, which causes more direct sunlight to reach the surface, leading to warmer temperatures. Conversely, during the winter months, the Northern Hemisphere is tilted away from the Sun, causing sunlight to spread over a larger area and arrive at a shallower angle, which results in cooler temperatures.

Connect perihelion with winter in the Northern Hemisphere

Perihelion occurs around January 3rd each year, which corresponds with winter in the Northern Hemisphere. Though Earth is closer to the Sun at this point, the axial tilt's effect on sunlight distribution is more significant. Since the Northern Hemisphere is tilted away from the Sun during this time, the sunlight is more spread out and arrives at a shallower angle, causing cooler temperatures and winter conditions.

Connect aphelion with summer in the Northern Hemisphere

Aphelion occurs around July 4th each year, coinciding with summer in the Northern Hemisphere. Although Earth is farthest from the Sun at this point, the axial tilt makes the Northern Hemisphere tilted towards the Sun. As a result, sunlight is more concentrated and arrives at a steeper angle, leading to warmer temperatures and summer conditions. In conclusion, the distance between Earth and the Sun does not determine the seasons directly; instead, it is the combination of Earth's axial tilt and elliptical orbit that causes the variation in sunlight distribution and temperature, leading to the different seasons in the Northern Hemisphere.

Key concepts

Axial Tilt

The Earth's axial tilt is a crucial factor in creating the seasons. Imagine Earth's axis as an invisible line through the North and South Poles. Now, picture this line leaning slightly to one side at an angle of about 23.5 degrees. This lean is not towards or away from the Sun, but rather along the orbit, causing different parts of Earth to receive varied sunlight during the year.

This tilt causes the Sun's rays to hit Earth at different angles, leading to variations in temperature and day length. For the Northern Hemisphere:

• During winter, it tilts away from the Sun, causing the Sun's rays to spread out, leading to cooler temperatures.
• In summer, it tilts toward the Sun, allowing more direct rays and thus warmer temperatures.

Essentially, axial tilt dictates how much sunlight any given hemisphere receives, not the distance from the Sun.

Elliptical Orbit

Earth's orbit isn't a perfect circle. Instead, it's more of an oval shape, known as an elliptical orbit. Due to this shape, the distance between Earth and the Sun changes throughout the year.

• Closest approach to the Sun, known as perihelion, happens in January.
• When Earth is farthest from the Sun, it's called aphelion, occurring in July.

But, despite these distance changes, the elliptical shape isn't what causes seasonal temperature changes. While we might be closer to the Sun during Northern Hemisphere winters, it's the angle of sunlight determined by Earth's axial tilt that's most impactful. So seasons stem more from tilt rather than Earth's proximity to the Sun.

Perihelion and Aphelion

Perihelion and aphelion might sound complex, but they simply represent Earth's changing distances from the Sun during its orbit. At perihelion, Earth is approximately 147 million kilometers away, closer than at any other time of the year. On the flip side, at aphelion, Earth sits about 152 million kilometers away, its farthest distance.

These points affect the duration of seasons slightly, but not their intensity.

• Perihelion occurs in January, aligning with winter in the Northern Hemisphere.
• Aphelion happens in July, matching Northern Hemisphere summer.

Yet, it's important to remember: summer isn't warmer because Earth is farther (aphelion) or winter colder because it's closer (perihelion). Instead, Earth's axial tilt ensures that sunlight distribution and angles dominate how temperatures feel on Earth. It's fascinating how these astronomical distances subtly interact with our everyday climate.