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As you probably know, the Earth’s axis is tilted by 23.5 degrees away from the Sun/Earth plane. This is why we have seasons. When the northern hemisphere is tilted towards the Sun, we get summer in the north. And then, six months later, the northern hemisphere is tilted away from the Sun, and we get winter in the north (and the opposite for the southern hemisphere).
Declination is a measurement of the angle between the Sun’s rays and the Earth’s equatorial plane.
The declination of the Sun changes over the course of the year. During the spring equinox, the declination of the Sun is zero. And then it reaches its maximum declination of 23.5° during the summer solstice. It shrinks back down to zero for the fall equinox, and then it drops into the negative, reaching -23.5° during the winter solstice.
During the summer solstice, a region of the Earth north of the Arctic Circle, experiences sunlight 24 hours a day. And Antarctica receives 24 hours of darkness.
We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.