Earth is not an upright planet, it leans a little on its side and is tilted relative to the plane of its orbit around the Sun. The amount of tilt has varied throughout geologic time from about 21.5º to 24.5º. It is presently inclined at 23.5º and its tilt is decreasing by a small amount annually.

But how did the tilt originate? The most widely accepted hypothesis is that the tilt was caused by the same massive impact that gave rise to our Moon. The tilt of the Earth’s axis is the reason we have seasons, so if this explanation is correct, the legacy of this impact that occurred more than 4.6 billion years ago is still felt by all of Earth’s inhabitants.

Because Earth’s tilt is 23.5º, on one day each year the Sun shines directly overhead at 23.5ºN latitude. This day is June 21st, and the latitude is the Tropic of Cancer. On this day, all localities in the northern hemisphere have maximum daylight (summer), while all localities in the southern hemisphere have their longest nights (winter). On December 21st, the situation is reversed and the Sun shines directly overhead at 23.5ºS latitude, which is the Tropic of Capricorn. In the six months between these two cardinal dates, the Sun shines most directly on differing positions between these latitudes, gradually shifting over a six-month period from one tropic to the other. From June 21st until December 21st, the length of daylight in the northern hemisphere progressively decreases to a minimum, while it increases to a maximum in the southern hemisphere.

This means that in any year, there are only two occasions when the Sun shines directly on the equator. These two occasions occur on March 20th and September 22nd, the days that are known as the equinoxes. On these two days, all areas on Earth have twelve hours of daylight and twelve hours of darkness.

Earth is hottest in the tropics and coldest near the poles because the Sun’s rays strike Earth most directly in the tropical region and least directly at the poles. In turn, this influences the amount of solar energy that reaches the surface. Solar energy reaching Earth’s surface in higher latitudes is less intense than it is in lower latitude equatorial regions because the radiation travels through a thicker layer of atmospheric gases, which absorb, reflect, and scatter it.

If Earth’s axis were not tilted, the amount of solar energy reaching its surface at any particular location would remain the same throughout the year and we would not experience seasons. The familiar cycle of spring, summer, fall and winter, which we all take for granted, may therefore owe its origin to a chance impact very early in Earth’s history.

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The tilt of Earth’s axis relative to the Sun is 23.5º, so the boundary between night and day (AB), is inclined at an angle of 23.5º to Earth’s axis. The northern hemisphere is tilted towards the sun in June, and experiences more direct and longer hours of sunlight. However, the sunlight that reaches Earth’s surface in the far northern latitudes passes through a thicker layer of atmosphere and so is less intense than the sunlight shining on equatorial latitudes. On June 21st (left), the Sun shines directly on latitude 23.5ºN, which defines the northern limit of the tropics (the Tropic of Cancer). In this orientation, areas south of the Antarctic Circle receive no sunlight. On December 21st (right), the northern hemisphere is tilted away from the Sun and experiences longer hours of darkness. Locations north of the Arctic Circle receive no daylight at all.

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Earth revolves around the Sun with the northern hemisphere pointing towards the Sun during the northern hemisphere summer and away from it during the northern hemisphere winter. Because Earth’s axis is tilted at 23.5º, the Sun shines directly on latitude 23.5ºN (the Tropic of Cancer) on June 21st. This date marks the summer solstice in the northern hemisphere when the day is longest. On December 21st (the winter solstice), the Sun shines directly on latitude 23.5ºS (the Tropic of Capricorn) and the northern hemisphere day is shortest. On the equinoxes of March 20th and September 22nd, day and night are the same length (12 hours) everywhere on Earth. The diagrams are not to scale.