Watch: Earth's wobble casts a weird shadow called twilight zone

One picture taken every day of the year starting September 2023 through to September 2024 provides a pretty fascinating perspective of the twilight zone transit due to the Earth's 23.5-degree axial tilt as it orbits around the Sun.

If we didn't have this tilt on Earth, we wouldn't have seasonal changes and the length of the day would just be the same year-round. Instead, we have two equinoxes a year – one in March and one in September – when the Sun is directly above the equator, making day and night almost equal around the world.

In the Northern Hemisphere, the winter solstice follows three months after the September equinox, when the North Pole is at its maximum tilt away from the Sun, receiving the least amount of sunlight per day and giving us in the north our longest night. Near the North Pole around this time, the Sun can go weeks without ever rising above the horizon, called Polar Night.

(Quick editor note: the Tweet above says December 2023-September 2024, when it should read September 2023-September 2024)

The summer solstice in June for the Northern Hemisphere is exactly the opposite, leaning into the Sun. Near the Arctic Circle, places like northern Alaska or Norway experience a phenomenon called the Midnight Sun, where the Sun does not set at all and simply makes an elliptical path around the sky without ever dipping below the horizon. The further north you are, the higher the Sun will be in the sky during this extended day. At the North Pole itself, the Sun won't set for six months as the Sun makes its continuous loops.

Our Earth's rotational axis isn't perfect. It has a slight wobble with a cycle of about 26,000 years. That means that over centuries, the dates of solstices and equinoxes shift slightly before shifting back.

Early civilizations took notice of the Earth's rotation around the Sun. Stonehenge, which no one is entirely sure who built or exactly when, aligns perfectly with the sunrise on the summer solstice.

The Great Sphinx of Egypt – built some 5,000 years ago – also directly faces the rising sun on the spring (vernal) and autumn equinoxes. As was the case with many cultures, the chief deity of the ancient Egyptians was a sun god, Ra, who was worshipped as the creator of everything.

El Castillo was built nearly 2,000 years ago by the Mayans in the Yucatán Peninsula in Mexico. The Mayans were known for having incredibly precise calendars. El Castillo was built perfectly so that during spring and autumn equinoxes, the sunlight casts a shadow along the pyramid's staircases to give an illusion of Kukulcán, the "Serpent of Light", slithering down to meet the stone-carved head of the reptile at the bottom of the staircase. It's all quite precise, considering the Mayans used several different calendars rather than our modern-day Gregorian calendar – though, technically, one of the Mayan calendars – Haab' – did have 365 days in it.

It's pretty cool seeing a time lapse of Earth from a geostationary perspective, but it's also really cool seeing it from the International Space Station.

Source: EUMESTAT second-generation weather satellite