The explanation lies in Earth’s tilt. And it’s not just the Earth — every planet in the Solar System is tilted, all at different angles.
The Earth’s axis of rotation is tilted at an angle of 23.5 degrees away from the perpendicular. This tilt — combined with factors such as Earth’s spin and orbit — leads to variations in the duration of Sunlight that any location on the planet receives on different days of the year.
On the Equator, day and night are equal. The closer one moves towards the poles, the more extreme the variation. During summer the hemisphere which is tilted towards the Sun and the polar region receives 24 hours of daylight for months. Likewise, During winter, the region is in total darkness for months.
The Earth’s the North Pole points towards the North Star over an extended period, as the Earth moves around the Sun through the course of a year.
The Northern Hemisphere spends half the year tilted in the direction of the Sun, getting direct sunlight during long summer days. During the other half of the year, it tilts away from the Sun, and the days are shorter.
Winter Solstice, December 21, is the day when the North Pole is most tilted away from the Sun. That’s on December 22 at 04:19 Universal Time (UTC). It’s when the sun on our sky’s dome reaches its farthest southward point for the year. Therefore, December 22 become the longest night in 2019.
The tilt is also responsible for the different seasons that we see on Earth. The day occurs on the side facing the Sun, and changes towards night as Earth continues to spin on its axis.
The Earth’s tilt helps define some familiar imaginary lines, which are also key to determining when a Solstice occurs. These are latitudes, which are a measure of a location’s distance from the Equator.