Explain why the northern hemisphere has summer at the same time that the southern hemisphere has winter.

The reason why the Northern Hemisphere experiences summer while the Southern Hemisphere experiences winter is due to the tilt of the Earth's axis. Here's a step-by-step explanation:

1. The Earth's axis is tilted at an angle of approximately 23.5 degrees relative to its orbit around the Sun. This tilt is the reason we have different seasons.

2. When the Earth orbits around the Sun, this tilt does not change. Therefore, at different times of the year, different parts of the Earth receive different amounts of sunlight.

3. During the time when the Northern Hemisphere is tilted towards the Sun, it receives more direct sunlight, leading to warmer temperatures and the season we know as summer.

4. At the same time, the Southern Hemisphere is tilted away from the Sun, receiving less direct sunlight, leading to cooler temperatures and the season known as winter.

5. The situation is reversed six months later, when the Earth has moved halfway around its orbit. The Southern Hemisphere is then tilted towards the Sun and experiences summer, while the Northern Hemisphere is tilted away and experiences winter.

So, the tilt of the Earth's axis and its orbit around the Sun together result in different parts of the Earth receiving different amounts of sunlight at different times of the year, causing the seasons.