Earth behaves like a gyroscope in that throughout its journey around the Sun the direction of its spin axis remains constant relative to the plane of the ecliptic. The direction of Earth’s spin axis relative to the Sun is therefore continuously changing during this time. At any instant in time, one half of Earth is in sunlight and the other half is in darkness. The edge dividing daylight from night is called the circle of illumination.
On c.21 December in the northern hemisphere, the winter solstice, the shortest day; the North Pole is pointing directly away from the Sun, the Arctic Circle has twenty-four hours of darkness, and at noon the Sun is vertically over latitude ≈23.45° South, the Tropic of Capricorn.
Six months later Earth reaches the opposite side of the Sun. On c.21 June in the northern hemisphere, the summer solstice, the longest day; Earth’s North Pole points directly towards the Sun, the Arctic Circle has twenty-four hours of sunshine, and at noon the Sun is vertically over latitude ≈23.45° North, the Tropic of Cancer.
Three months either side of these two extremes Earth’s spin axis is tilted neither towards nor away from the Sun but is tangential to the path of Earth’s orbit around the Sun; the circle of illumination passes through the North and South poles, and at all locations Earth’s day and night are of equal length. In the northern hemisphere on the c.21 March, the vernal equinox, and on the c.21 September, the autumnal equinox, the Sun is directly over the equator.
Earth’s axial tilt thus causes the seasons – different latitudes receive varying amounts of sunlight during the year as Earth journeys around the Sun. In the tropics the Sun travels almost directly overhead. In the northern hemisphere the setting position of the Sun oscillates between the northwest (midsummer) and the southwest (midwinter). In midsummer the days are long, but thereafter the Sun’s rising and setting points move steadily further south and the days get shorter and colder.
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