Theory question 202
The Earth and Solar System(a) Write explanatory notes on the (i) Equinoxes (ii) Great circles
(b) Explain why the period of twilight is longer along the Arctic Circle than at the Equator
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(a)(i) Equinoxes: 21 March and 23 September, when the midday sun is overhead at the equator; day and night are equal (12 hours) everywhere; mark the start of spring and autumn
(ii) Great circles: circles on the globe whose plane passes through the earth's centre, dividing it into two equal halves, e.g. the equator and any pair of opposite meridians; the shortest route between two points lies along a great circle (used by ships and aircraft)
(b) Twilight is the period when the sun is just below the horizon (up to 18°) and its light is scattered by the atmosphere; at the equator the sun rises and sets vertically, so it passes quickly through this zone (short twilight); at the Arctic Circle the sun's path is oblique/slanting to the horizon, so it stays just below the horizon for a long time – longer twilight
Twilight length depends on the angle at which the sun crosses the horizon.