MPSC 2023 Prelims Paper-I · Question 27 of 100

Correct — option (4), '22 December'. The single word that decides this question is 'southern'. The Earth's axis is inclined at about 23.5 degrees to the perpendicular of its orbital plane and holds that direction fixed as the Earth travels round the Sun, so as the year passes, first one hemisphere and then the other is tilted towards the Sun. A solstice is the moment at which that tilt reaches its extreme, and it is marked by the Sun's vertical rays falling on a tropic rather than on the equator. On about 22 December the Sun is directly overhead at the Tropic of Capricorn, at 23.5 degrees south. The southern hemisphere is then turned towards the Sun: it receives the year's most direct radiation, its day is the longest and its night the shortest of the year, the whole area within the Antarctic Circle has continuous daylight, and the season there is summer. That is the summer solstice of the southern hemisphere, and option (4) is the answer. The same date is simultaneously the winter solstice of the northern hemisphere — the shortest day in India — which is exactly why the question is worth asking and exactly how it is failed. Indian textbooks, written for the northern hemisphere, usually give 21 June as 'the summer solstice' without qualification, and a candidate who has stored the phrase without the hemisphere attached will read the stem, recognise the words 'summer solstice' and mark 21 June. The two solstice dates and the two equinox dates are not four separate facts about the Earth; they are two positions in the orbit, each of which carries opposite names in the two hemispheres. Once that is grasped, the question becomes a matter of naming the position rather than recalling a date: maximum southern tilt means the Sun over Capricorn, which means late December. It is worth noting that these dates shift by a day either way through the leap-year cycle, so the solstice may fall on 21 or 22 December; the paper uses the conventional textbook date.

• (1)21 March — 21 March is an equinox, not a solstice, so it is disqualified before hemispheres are even considered. On that date the Sun's vertical rays fall on the equator, neither hemisphere is tilted towards it, and day and night are of nearly equal length everywhere on the Earth — which is what the word equinox means. It is called the spring or vernal equinox in the northern hemisphere and the autumnal equinox in the southern, so it too carries opposite names on either side of the equator, but no equinox is a solstice under any name. A candidate who chooses it has generally kept the four dates as an undifferentiated list without recording which two are the extremes of tilt and which two are the moments of balance between them.
• (2)21 June — This is the trap the question is built around, and it is the wrong answer that costs the most marks. 21 June is indeed a solstice, and in the northern hemisphere it is the summer solstice: the Sun stands overhead at the Tropic of Cancer, India has its longest day, and the region within the Arctic Circle has continuous daylight. But on that same date the southern hemisphere is tilted away from the Sun, so it is having its shortest day and its winter solstice. The stem asks specifically about the southern hemisphere, and the answer therefore has to be the opposite solstice. This option catches anyone who has memorised 'summer solstice — 21 June' as a fixed pair, which is how the fact is usually printed in books written for readers in the north, and who then reads past the word 'southern' in a stem whose grammar is already awkward.
• (3)23 September — 23 September is the second of the two equinoxes, when the Sun's vertical rays again fall on the equator and day and night are nearly equal everywhere. In the northern hemisphere it is called the autumnal equinox and in the southern the spring equinox — the mirror of 21 March. Like the March date it is a moment of balance rather than of extreme tilt, so it cannot be a solstice for either hemisphere. Choosing it usually means the candidate has correctly reasoned that the answer must be a date associated with the southern hemisphere's warming season and has then picked the September date because spring in the south begins around then. Spring is not summer, and an equinox is not a solstice; the stem asks for the extreme, which comes three months later.

The seasons are produced by two facts working together: the Earth revolves round the Sun in a year, and its axis is tilted about 23.5 degrees from the perpendicular to the plane of that orbit while remaining parallel to itself throughout the journey. Because the tilt does not swing to follow the Sun, the hemisphere leaning towards it changes continuously through the year, and with it the angle at which sunlight strikes the ground and the number of hours of daylight. Four positions in the orbit are given names. At the two solstices the tilt is at its maximum: on about 21 June the Sun is overhead at the Tropic of Cancer and on about 22 December at the Tropic of Capricorn, which is why the tropics lie where they do — they are the extreme limits of the Sun's apparent annual migration. At the two equinoxes, about 21 March and 23 September, the Sun is overhead at the equator, the circle of illumination passes through both poles, and day and night are of nearly equal length everywhere. The polar circles at 66.5 degrees follow from the same tilt, being the latitudes beyond which the Sun fails to set at one solstice and fails to rise at the other. It is worth destroying one persistent misconception here: the Earth is actually nearest the Sun in early January and farthest in early July, so distance plainly cannot be what makes January cold in Maharashtra. The tilt does all the work.

Solstice and equinox dates are a standing item in the MPSC geography section because they are compact, unambiguous and easy to set, and the commission's usual method of raising the difficulty is not to ask for a harder fact but to change the hemisphere. That single substitution converts a question answerable by rote into one that requires the underlying model, and it explains why a candidate can know all four dates and still lose the mark. The defence is to store the four dates as two orbital positions rather than as four labels: June solstice, Sun over Cancer, summer in the north and winter in the south; December solstice, Sun over Capricorn, summer in the south and winter in the north; and the two equinoxes as the balance points between them, each also carrying opposite names. Then any question of this family is answered by locating the position first and naming it afterwards. The same discipline serves the related questions the commission asks around this topic — where the midnight sun is seen and when, why the tropics and the polar circles lie at the latitudes they do, and why the length of daylight varies more with latitude than the noon temperature does.

• The Earth's axis is inclined about 23.5 degrees to the perpendicular of its orbital plane and stays parallel to itself throughout the orbit, so the hemisphere tilted towards the Sun changes through the year; this, and not any change of distance, produces the seasons.
• On about 21 June the Sun's vertical rays fall on the Tropic of Cancer: it is the summer solstice in the northern hemisphere, with the longest day there, and simultaneously the winter solstice in the southern hemisphere.
• On about 22 December the Sun's vertical rays fall on the Tropic of Capricorn: it is the summer solstice in the southern hemisphere, with the longest day and continuous daylight within the Antarctic Circle, and the winter solstice in the northern hemisphere.
• On about 21 March and 23 September the Sun is overhead at the equator and day and night are of nearly equal length everywhere; these are the equinoxes, and each is named spring in one hemisphere and autumn in the other.
• The Earth is closest to the Sun in early January and farthest in early July, which by itself disproves the idea that the seasons are caused by the Earth's varying distance from the Sun.

• Storing 'summer solstice' as a fixed pair with 21 June, when the phrase names a different date in each hemisphere and the hemisphere is what the stem is testing
• Reading past a single qualifying word such as southern in a stem, particularly when the surrounding grammar is awkward and invites skimming
• Confusing an equinox with a solstice, or treating the four dates as an unstructured list rather than as two orbital extremes and two balance points
• Explaining the seasons by the Earth's distance from the Sun, when the Earth is in fact nearest the Sun during the northern winter

The Earth's motions and their consequences are asked in MPSC papers in a small set of dependable shapes: the date of a named solstice or equinox, the latitude over which the Sun is vertical on a given date, the place at which the midnight sun can be observed and when, the reason for the seasons, and the distinction between rotation and revolution and what each produces. The commission's favourite complication is the hemisphere switch used here, and its second favourite is asking for the consequence rather than the date, as in which latitude has the longest day on a given date. Both are answered from the same small model, so the efficient preparation is to hold the model rather than the list: the tilt, the fixed direction of the axis, and the four positions in the orbit with what is happening in each hemisphere at each.

No directly related past PYQ was found.

• practice — not a real PYQ

  On about 22 December the vertical rays of the Sun fall on which of the following parallels of latitude ?

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

  Answer(c) The Tropic of Capricorn — at the December solstice the southern hemisphere is tilted towards the Sun to its maximum extent, and the overhead Sun reaches its southern limit at 23.5 degrees south. The Sun is overhead at the equator only at the two equinoxes, at the Tropic of Cancer at the June solstice, and never at the Arctic Circle, which lies far outside the belt of the Sun's apparent annual movement.

• practice — not a real PYQ

  The occurrence of seasons on the Earth is caused mainly by which of the following ?

  • (a)The changing distance of the Earth from the Sun during its orbit
  • (b)The inclination of the Earth's axis combined with its revolution round the Sun
  • (c)The rotation of the Earth on its own axis once in twenty-four hours
  • (d)Periodic variation in the quantity of heat given out by the Sun

  Answer(b) The inclination of the Earth's axis combined with its revolution round the Sun — because the axis stays tilted in a fixed direction while the Earth orbits, each hemisphere leans towards the Sun for part of the year and away from it for the rest, changing both the angle of the incoming rays and the length of daylight. Distance cannot be the cause, since the Earth is nearest the Sun in early January; rotation produces day and night rather than the seasons.