Position of the sun when farthest north (summer solstice) or farthest south (winter solstice). The solstices occur because the rotation axis of the Earth is tilted by an angle of 23.5° from the vertical. If the Earth’s rotation axis were perpendicular to the plain of its orbit, there were be no solstice days and no seasons. The Sun attains its most northerly declination (+23.5°) around 21 June; this is the northern summer solstice and marks the longest day of the year for northern hemisphere observers. This is also the date of the southern winter solstice (shortest day of the year for southern hemisphere observers). Six months later, the Sun reaches its most southerly declination (-23.5°) and the solstices are reversed in each hemisphere.

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