The summer solstice occurs today-the longest day of summer in the northern hemisphere, when the sun is directly above the Tropic of Cancer. This day marks the beginning of summer in the northern hemisphere and the beginning of winter in the southern hemisphere. During the year, there will be two such astronomical events-the other is the winter solstice on December 21. In these events, the earth reaches a point whose inclination is the greatest angle to its orbital plane. Therefore, one hemisphere receives more sunlight than the other.

On Monday, to commemorate the summer solstice, NASA shared an image showing the position of the earth relative to the axis of the sun during these two events. The axis of the earth is an imaginary pole that passes through the center of our planet from “top” to “bottom”. The earth rotates around this pole, causing the day and night to alternate.

But the axis is always inclined (23.5 degrees with respect to the sun-earth line). NASA said in a blog post that because of this tilt and the planet’s orbit around the sun, we have seasons. When the earth is closer to the sun, the areas closer to it are hot and humid day and night; when the earth is farther from the sun, the farther away from the sun, the colder the area.

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During the summer solstice, the North Pole faces the sun more directly than at any other time of the year, while the South Pole is far away from the sun. This results in longer exposure to sunlight in the northern hemisphere (more than 12 hours), and less exposure to areas south of the equator.