(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 Equ...

Answer Details

(a)(i) The Equinoxes:

An equinox occurs twice a year, on about 21 March (spring/vernal equinox) and 23 September (autumn equinox). At these times the sun is directly overhead at the Equator (the sun's vertical rays fall on \(0^\circ\) latitude). Because the circle of illumination passes through both poles, day and night are equal in length (12 hours each) all over the world. The word equinox means "equal night". Neither the North nor the South Pole is tilted towards the sun.

(a)(ii) Great circles:

A great circle is any circle drawn on the globe whose plane passes through the centre of the Earth, dividing the globe into two equal halves (hemispheres). It is the largest circle that can be drawn on the Earth's surface. The Equator is a great circle, and every pair of opposite meridians of longitude together forms a great circle. The parallels of latitude other than the Equator are small circles, because their planes do not pass through the centre of the Earth. The shortest distance between any two points on the globe lies along the arc of a great circle, which is why aircraft and ships follow great-circle routes.

(b) Why twilight lasts longer at the Arctic Circle than at the Equator:

Twilight is the period of half-light before sunrise and after sunset, caused by the atmosphere scattering the sun's rays while the sun is still below the horizon. Its length depends on the angle at which the sun sinks below the horizon. At the Equator the sun's daily path is almost vertical (perpendicular) to the horizon, so the sun drops quickly and steeply below the horizon, passing through the twilight zone in a short time; twilight is therefore brief. Near the Arctic Circle the sun moves in a path that is greatly slanted (oblique) to the horizon; it sinks very gradually and travels a long, shallow path just below the horizon, so its rays continue to light the sky for a much longer period. Hence twilight lasts far longer in high latitudes than at the Equator.