(a) Draw a diagram to show the four major positions of the earth during its revolution around the sun (b) describe the effects of the revolution of the eart...

(a) Diagram showing the four major positions of the earth during its revolution round the sun

The sun is drawn at the centre with the earth shown at four points on its orbit. Throughout the year the earth's axis stays tilted at \(23\tfrac{1}{2}^\circ\) from the vertical (that is, \(66\tfrac{1}{2}^\circ\) to the plane of the orbit) and always points in the same direction in space. The four cardinal positions are the two solstices and the two equinoxes:

• 21 June - Summer Solstice: the North Pole is tilted toward the sun, the overhead (vertical) sun is at the Tropic of Cancer (\(23\tfrac{1}{2}^\circ\)N); longest day in the Northern Hemisphere.
• 23 September - Autumnal Equinox: the axis is sideways to the sun, the overhead sun is at the Equator; day and night are equal everywhere.
• 22 December - Winter Solstice: the North Pole is tilted away from the sun, the overhead sun is at the Tropic of Capricorn (\(23\tfrac{1}{2}^\circ\)S); longest day in the Southern Hemisphere.
• 21 March - Vernal (Spring) Equinox: the axis is again sideways to the sun, the overhead sun is back at the Equator; day and night are equal everywhere.

(b) Effects of the revolution of the earth

1. Change of seasons: Because the tilted axis keeps a fixed direction, each hemisphere is inclined toward the sun for part of the year and away from it for another part, producing the succession of spring, summer, autumn and winter.
2. Varying length of day and night: Away from the Equator the days and nights change in length through the year - long days and short nights in summer, the reverse in winter - while at the Equator they remain nearly equal all year.
3. Changing altitude and apparent migration of the midday sun: The overhead noon sun appears to move to and fro between the Tropic of Cancer and the Tropic of Capricorn, so the noon sun stands higher in the sky in summer and lower in winter.
4. Occurrence of the equinoxes and solstices: The revolution fixes the four cardinal dates (21 March, 21 June, 23 September, 22 December) on which the equinoxes and solstices fall.
5. Periods of continuous day or night near the poles: Beyond the Arctic and Antarctic Circles there are spells of unbroken daylight (midnight sun) and unbroken darkness (polar night).
6. It fixes the length of the year: One complete revolution takes \(365\tfrac{1}{4}\) days; the accumulated quarter-days give an extra day (leap year) every fourth year.