The shortest day length that occurs in the Northern hemisphere is on
- (a)21st March
- (b)23rd September
- (c)22nd November
- (d)22nd December
Correct — D, 22nd December. The earth's axis is tilted about 23 and a half degrees from the perpendicular to its orbit, and the tilt keeps the same direction in space all year. Around 22 December the North Pole is leaning as far away from the sun as it ever does, the sun's overhead point has reached the Tropic of Capricorn, and every place in the northern hemisphere gets its shortest day and longest night of the year. This is the winter solstice for the north and, on the same day, the summer solstice for the southern hemisphere, where the day is the longest.
- (a)21st March — Around 21 March the sun is overhead at the equator and neither pole leans towards it, so day and night are of nearly equal length everywhere. This is the spring or vernal equinox for the northern hemisphere, not its shortest day.
- (b)23rd September — Around 23 September the sun is again overhead at the equator on its way south, giving the autumn equinox and once more roughly equal day and night everywhere. Days in the north are shortening at this point, but they have not yet reached their minimum.
- (c)22nd November — Late November falls between the September equinox and the December solstice, so northern days are short and still getting shorter, but the minimum is a month away. The date is chosen for this option because it looks like 22 December with the month changed.
Seasons and unequal day lengths follow from the tilt of the earth's axis together with its revolution round the sun, not from any change in distance. Four dates mark the cycle. Around 21 March and 23 September the sun is overhead at the equator and day equals night. Around 21 June it is overhead at the Tropic of Cancer, giving the northern hemisphere its longest day, with continuous daylight at and beyond the Arctic Circle. Around 22 December it is overhead at the Tropic of Capricorn, giving the northern hemisphere its shortest day, with continuous darkness at and beyond the Arctic Circle.
A common misreading is to link winter to the earth being farther from the sun. In fact the earth is nearest the sun in early January, during the northern winter; it is the tilt, and the resulting angle at which sunlight strikes the surface and the number of hours it does so, that make the season. Note too that the solstice and equinox dates shift by a day either way from year to year, because the year is not a whole number of days, which is why textbooks write these dates as approximate.
- The earth's axis is tilted about 23 and a half degrees to the perpendicular of its orbital plane.
- Around 22 December the sun is overhead at the Tropic of Capricorn and the northern hemisphere has its shortest day.
- Around 21 June the sun is overhead at the Tropic of Cancer and the northern hemisphere has its longest day.
- Around 21 March and 23 September the sun is overhead at the equator and day and night are nearly equal everywhere.
- The 22 December solstice is midwinter for the northern hemisphere and midsummer for the southern hemisphere at the same moment.
The North Pole leans furthest from the sun around 22 December — option (d).
- Attributing the seasons to the earth's changing distance from the sun rather than to the axial tilt.
- Reversing the hemispheres — 22 December is midsummer in Australia and Argentina.
- Reading 22 November as the answer because the day is already short then; the shortest day is a month later.
NDA and UPSC both set solstice and equinox items, usually asking which latitude has the longest day, or where the sun is overhead on a given date.
In the northern hemisphere, the longest day of the year normally occurs in the :
- (a) First half of the month of June
- (b) Second half of the month of June
- (c) First half of the month of July
- (d) Second half of the month of July
Answer(b) Second half of the month of June
The mirror image of this question — the longest northern day is the June solstice around 21 June, six months and half an orbit away from the shortest one on 22 December.
On 21st June, the Sun
- (a) does not set below the horizon at the Arctic Circle
- (b) does not set below the horizon at the Antarctic Circle
- (c) shines vertically overhead at noon on the Equator
- (d) shines vertically overhead at the Tropic of Capricorn
Answer(a) does not set below the horizon at the Arctic Circle
Works the same geometry at the June solstice, and shows its extreme consequence — continuous daylight at the Arctic Circle, which becomes continuous darkness there on 22 December.
- practice — not a real PYQ
On about 22 December the sun is vertically overhead at
- (a)the Equator
- (b)the Tropic of Cancer
- (c)the Tropic of Capricorn
- (d)the Arctic Circle
Answer(c) the Tropic of Capricorn — which is why the northern hemisphere has its shortest day then.
- practice — not a real PYQ
Day and night are of nearly equal length all over the world on
- (a)21 June and 22 December
- (b)21 March and 23 September
- (c)1 January and 1 July
- (d)22 November and 22 May
Answer(b) 21 March and 23 September — the two equinoxes, when the sun is overhead at the equator.