The seasonal variations in the amount of solar energy that any place on the Earth receives are influenced by

Correct — D, the inclination of the Earth about its axis. The Earth's axis is tilted about 23.5 degrees from the perpendicular to its orbital plane, and it keeps pointing the same way in space as the planet revolves round the Sun. That fixed tilt is what makes the noon Sun high over one hemisphere and low over the other at different times of year, changing both the angle at which the rays strike a place and the number of hours of daylight it receives. Both effects alter the solar energy arriving per unit area, and together they are the seasons. Without the tilt there would be no seasonal variation anywhere on Earth.

• (a)the speed of rotation of the Earth — Rotation produces day and night, and its speed sets their length at twenty-four hours. It is essentially constant through the year and cannot generate a seasonal pattern.
• (b)pollution — Aerosols and particulate matter do cut the solar radiation reaching the ground and can vary with season in a given city, but they are a local and variable factor, not the reason any place on Earth has seasons.
• (c)cloudiness — Cloud cover strongly affects how much sunlight reaches the surface on a given day, and the monsoon makes it a seasonal factor in India. But cloudiness is itself a consequence of the seasonal heating pattern, not its cause, and it does not explain seasons in cloudless deserts or at the poles.

The Earth revolves round the Sun in about 365.25 days along a nearly circular orbit, and its axis is inclined at roughly 23.5 degrees to the perpendicular of the orbital plane. Because the axis stays parallel to itself throughout the orbit, first one hemisphere and then the other leans towards the Sun. Around 21 June the northern hemisphere is tilted towards it and the overhead Sun is at the Tropic of Cancer; around 22 December the southern hemisphere leans in and the overhead Sun is at the Tropic of Capricorn; at the two equinoxes, about 21 March and 23 September, the Sun is overhead at the equator and day and night are equal everywhere.

A common misconception is that summer comes when the Earth is nearest the Sun. In fact the Earth is at perihelion in early January, when the northern hemisphere has its winter, and the two hemispheres have opposite seasons at the same moment — which distance alone could never explain. The tilt does, because it changes the angle of incidence of the rays and the length of the day together. Cloudiness and pollution modify how much of that energy actually reaches the ground, but they cannot be the cause of a pattern that is identical in date every year across the whole planet. This card marks a different letter from the answer we hold on file.

• The Earth's axis is inclined about 23.5 degrees to the perpendicular of the orbital plane and stays parallel to itself as the Earth revolves.
• Seasons arise from the changing angle of the Sun's rays and the changing length of day, both consequences of that tilt.
• The Sun is overhead at the Tropic of Cancer around 21 June and at the Tropic of Capricorn around 22 December.
• At the equinoxes the Sun is overhead at the equator and day and night are equal.
• The Earth is closest to the Sun in early January, so distance cannot explain the northern summer.

• Believing summer occurs when the Earth is nearest the Sun.
• Choosing cloudiness because it obviously affects sunshine on any particular day.
• Confusing rotation with revolution; rotation gives day and night, revolution with tilt gives the seasons.

A cause-of-seasons item with two locally plausible distractors, so it rewards distinguishing what modifies insolation from what causes its annual cycle.

• practice — not a real PYQ

  On 21 June the Sun's rays fall vertically over the

  • (a)Equator
  • (b)Tropic of Cancer
  • (c)Tropic of Capricorn
  • (d)Arctic Circle

  Answer(b) Tropic of Cancer — the northern hemisphere is tilted towards the Sun, giving it the summer solstice.

• practice — not a real PYQ

  If the Earth's axis were perpendicular to the plane of its orbit, the most direct consequence would be

  • (a)no day and night
  • (b)no seasons
  • (c)no tides
  • (d)a longer year

  Answer(b) no seasons — day length and the angle of the Sun's rays at any place would stay the same all year.