Summer solstice

Topic: Solar Radiation, Heat Balance and Temperature · NCERT: Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"

Meaning

The summer solstice (for the Northern Hemisphere) falls around 21 June. On this day the noon Sun is directly overhead at the Tropic of Cancer (23.5°N), the furthest north it ever reaches. The Northern Hemisphere gets its longest day, and every place north of the Arctic Circle has 24 hours of daylight.

It matters because it marks one end of the Sun's yearly movement between the Tropics. This movement causes the seasons and changes how much insolation (sunlight energy reaching the Earth) each latitude gets.

Explanation

Cause: the tilted axis that always points the same way

• Solstice means the day when the Sun's noon path is furthest north or furthest south of the equator [2].
• The Earth's axis is tilted.
• It makes 66.5° with the plane of the Earth's orbit.

• This is the same as 23.5° from the perpendicular to the ecliptic (the plane of the Earth's orbit) [3].

• Parallelism of the axis: the axis keeps pointing the same way in space, towards the Pole Star, all year.

• What follows:
• The Earth goes round the Sun, but the axis does not change direction.
• Around 21 June, the North Pole leans towards the Sun.

• So the noon Sun is overhead at its furthest point north, the Tropic of Cancer.

• Distance from the Sun is not the cause. The Earth is closest to the Sun (perihelion) in early January, which is northern winter [3]. The tilt causes the seasons [2][3].

What happens on 21 June

• Overhead Sun: at the Tropic of Cancer (23.5°N).
• Day length in the Northern Hemisphere:
• The North Pole leans towards the Sun.
• So more of each northern circle of latitude lies on the lit side of the circle of illumination (the line that divides day from night).
• The further north you go, the larger the lit part and the longer the day.
• At the Arctic Circle (66.5°N) the whole circle is lit, so it has 24 hours of daylight.

• At the equator, the day stays close to 12 hours.

• Polar regions:

• North of the Arctic Circle: 24 hours of daylight. In polar summer the Sun never sets [3].

• South of the Antarctic Circle: 24 hours of night.

• Southern Hemisphere: 21 June is its winter, with its shortest day.

Why the northern summer gets more insolation

• Two effects of the same tilt work together:
• Higher Sun: the summer hemisphere gets the Sun's rays more vertically than the winter hemisphere [3].

• Longer days: the ground is heated for more hours.

• Why vertical rays heat more:

• Smaller area: a vertical beam falls on a small patch, so each square metre gets more energy. A slanting beam spreads the same energy over more ground.

• Shorter path through the air: less energy is lost to absorption (gases and dust taking up energy) and scattering (air molecules sending light off in all directions).

• High-latitude summers: in places like Siberia and Canada the Sun stays low, but the very long days still bring a lot of insolation in a day.

Its place in the yearly cycle

Event	Date (approx.)	Noon Sun overhead at
Vernal (spring) equinox	21 March	Equator
Summer solstice	21 June	Tropic of Cancer (23.5°N)
Autumnal equinox	23 September (NASA: ~22 September [3])	Equator
Winter solstice	22 December	Tropic of Capricorn (23.5°S)

• After 21 June, the overhead Sun starts moving south again.

Where it occurs

• Tropic of Cancer in India: on 21 June the noon Sun is overhead along this line. It crosses 8 states, from west to east:

• Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram.

• Indian cities, day length on 21 June: days get longer from south to north.

• Chandigarh (~30.7°N): about 14 hours of daylight.
• Thiruvananthapuram (~8.5°N): only about 12.5 hours.

• Order: Thiruvananthapuram < Chennai < Nagpur < Delhi < Chandigarh < Srinagar.

• India south of the Tropic of Cancer (8°4'N to 23.5°N):

• These places have already had the overhead Sun once, between 21 March and 21 June. They get it a second time between 21 June and 23 September.

• On 21 June the noon Sun at Thiruvananthapuram is slightly to the north of overhead.

• North-west India: strong heating in May–June, around the solstice, helps draw in the south-west monsoon. Clear-sky Rajasthan (Thar Desert) gets the most solar energy.

• World:
• North of the Arctic Circle (66.5°N): 24-hour daylight.
• South of the Antarctic Circle (66.5°S): 24-hour night.
• Siberia and Canada: long summer days give high daily insolation.

Don't confuse with

• Winter solstice (22 December): the Sun is overhead at the Tropic of Capricorn (23.5°S). The Northern Hemisphere has its shortest day, and north of the Arctic Circle it is night for 24 hours.
• Equinox (21 March and 23 September): the Sun is overhead at the equator, not a Tropic. Day and night are equal everywhere [2][3], and the Sun is on the horizon at both poles.
• Perihelion (early January): this is when the Earth is closest to the Sun [3]. It happens in northern winter, so it is not linked to the summer solstice. Tilt, not distance, causes the seasons.
• Summer solstice of the Southern Hemisphere: this falls around 22 December, when the Sun is over the Tropic of Capricorn. For places like Australia, 21 June is the winter solstice.

Prelims Hooks

• 21 June: noon Sun overhead at the Tropic of Cancer (23.5°N). It is the longest day in the Northern Hemisphere, with 24-hour daylight north of the Arctic Circle (66.5°N) and 24-hour night south of the Antarctic Circle.
• The Earth's axis makes 66.5° with the orbital plane, which is 23.5° from the perpendicular [3]. Watch for options that swap these two figures.
• The perihelion comes in early January, during northern winter [3]. So the June solstice is not caused by the Earth being near the Sun.
• On 21 June, day length increases from south to north: Thiruvananthapuram < Chennai < Nagpur < Delhi < Chandigarh < Srinagar.
• The Tropic of Cancer passes through 8 states (Gujarat, Rajasthan, MP, Chhattisgarh, Jharkhand, WB, Tripura, Mizoram). It does not pass through Odisha, Bihar or Uttar Pradesh.
• Places in India south of the Tropic of Cancer get the overhead noon Sun twice a year. Places north of it never do.

Mains Points

• One cause, two effects (GS-I):
• The 23.5° tilt, with an axis that always points the same way, brings the overhead Sun to the Tropic of Cancer on 21 June.
• This gives the Northern Hemisphere both higher Sun angles and longer days, so it gets its peak insolation.

• The resulting difference in insolation between latitudes drives the seasons, the heat balance and the general circulation of the atmosphere.

• Link to the Indian monsoon:

• As the overhead Sun moves north towards the Tropic of Cancer, north-west India heats up strongly in May–June.

• This pre-monsoon heating, strongest under the clear skies of Rajasthan, helps draw in the south-west monsoon.

• India's tropical advantage:

• The noon Sun is high all year, and south of 23.5°N it is overhead twice a year.
• Together with long days around the solstice, this gives high insolation and long growing seasons.

Related concepts

• Factors affecting insolation
• Inclination of the Earth's axis
• Angle of incidence of sun's rays
• Aspect

Read more

• Why insolation varies: tilt, angle of rays, day length and aspect