The Earth is at perihelion on January 3 and at aphelion on July 4. Because of precession, in 15,000 A.D. the amount of sunlight in summer will be more than at present in the northern hemisphere but less than at present in the southern hemisphere. Explain why.

The journey of Earth around the Sun is fundamental to how we experience time and seasons. Imagine Earth's orbit as a slightly squashed circle, known as an ellipse. While the distance between the Earth and the Sun changes throughout the year, this variation doesn't cause the seasons. Instead, the seasons are a result of our planet’s axial tilt—a constant angle of about 23.5 degrees relative to its orbital plane.

As Earth revolves around the Sun, different parts of the planet receive varying amounts of sunlight. This variation in sunlight, combined with the axial tilt, leads to the familiar cycle of winter, spring, summer, and autumn in different parts of the world. Our calendars are based on this orbital journey, completing what we call a year. However, due to the subtle but persistent force of axial precession, the specifics of this orbital path slowly transform over millennia, leading to significant changes in the distribution of sunlight on Earth's surface.

Every year, Earth experiences two key points in its orbit related to its distance from the Sun: perihelion and aphelion. Perihelion is the point where Earth is nearest to the Sun—this happens around January 3. Conversely, aphelion occurs when Earth is at its farthest from the Sun, around July 4. From our perspective, it might seem logical to assume that Earth would be warmer during perihelion and cooler during aphelion due to proximity. However, the reality is not so straightforward.

The Earth's axial tilt plays a much more crucial role in determining the seasons than its distance from the Sun. That means that despite being closer to the Sun at perihelion, the northern hemisphere can be in the depths of winter, while the southern hemisphere basks in summer warmth. Reciprocally, during aphelion, the northern hemisphere can enjoy hot summer weather. It's a quirky consequence of our tilted Earth that we experience such contrasts, regardless of our current address in the elliptical lane around our stellar host.

Seasonal changes are a direct result of Earth's axial tilt, as different regions tilt towards or away from the Sun at different times of the year. Currently, in the northern hemisphere, summer occurs when the Earth is actually farther from the Sun (at aphelion), and winter when it is closest (at perihelion). But there's a slowly unfolding shift known as axial precession which can alter this rhythm over long periods of time.

Due to axial precession, the orientation of Earth's axis slowly gyrates, completing a cycle approximately every 26,000 years. So, fast forward to 15,000 AD, and Earth's summer in the northern hemisphere will coincide with perihelion, receiving more direct sunlight and experiencing hotter summers than we do now. Conversely, the southern hemisphere's summer will align with aphelion, seeing milder summers. This is a profound reminder that our planet’s dance with the Sun is an intricate ballet, where even the slightest shift can change the character of our seasons across thousands of years.