Solstices, Equinoxes and Variation in Day Length

At a Glance

• Summer solstice About 21 June, Sun over the Tropic of Cancer
• Winter solstice About 22 December, Sun over the Tropic of Capricorn
• Equinoxes About 21 March and 23 September
• At equinox Equal day and night everywhere
• Polar regions Midnight sun and polar night
• Exam link UPSC Prelims, Geography: Earth, Latitudes, Longitudes and Motions

Where Does It Fit?

This is the fifth topic of the category on the Earth, latitudes, longitudes and motions. The previous topic covered revolution and the seasons, and this one covers the key dates of the year and the length of day. The next topic covers eclipses and lunar phases. Previous topic: Revolution, Inclination of the Axis and Seasons. Next topic: Eclipses, Lunar Phases and Supermoons.

What Are the Key Positions of the Earth in a Year?

In short: four dates mark where the Sun is overhead and how long the day lasts.

As the Earth revolves with its axis tilted, the position of the vertical (overhead) rays of the Sun moves between the Tropic of Cancer and the Tropic of Capricorn during the year. Four dates mark this journey. The two solstices are the extremes, and the two equinoxes are the midpoints.

The word solstice means a standstill of the Sun, since its apparent northward or southward movement pauses and turns back.

What Happens at the Summer and Winter Solstices?

On 21 June the northern hemisphere is tilted towards the Sun, and the vertical rays fall on the Tropic of Cancer. The days are longest and nights shortest in the northern hemisphere, and the Arctic Circle region has continuous daylight. This day is the summer solstice in the north and the winter solstice in the south. On 22 December the vertical rays fall on the Tropic of Capricorn, the southern hemisphere has its longest days, and the northern hemisphere has its shortest days. The conditions are exactly reversed.

The Sun is overhead at midday on the Tropic of Cancer on 21 June, which is why the Tropic of Cancer is the northern limit of the torrid zone, as in Latitudes and Important Parallels.

What Happens at the Equinoxes?

On about 21 March and 23 September the vertical rays of the Sun fall on the equator. The circle of illumination passes through both poles, so every place on the Earth has about 12 hours of day and 12 hours of night, from which the term equinox, meaning equal night, comes. In the northern hemisphere 21 March marks the start of spring and 23 September the start of autumn, and in the southern hemisphere the reverse holds. The seasonal relation is explained in Revolution, Inclination of the Axis and Seasons.

How Does the Length of Day Vary with Latitude?

At the equator, day and night are about 12 hours each through the year. As one moves away from the equator, the difference between summer and winter day length increases. Beyond the Arctic and Antarctic Circles, the Sun stays above the horizon for more than 24 hours at midsummer and below it for more than 24 hours at midwinter.

What Are the Midnight Sun and the Polar Night?

Within the polar circles the Sun does not set at midsummer and does not rise at midwinter. The phenomenon of continuous daylight is called the midnight sun, and it is seen in northern Norway, Sweden, Finland and Alaska in the north. The opposite, when the Sun does not rise above the horizon for a long time, is called the polar night. At the poles the Sun is above the horizon for about six months and below it for about six months. Twilight and the angle of the Sun’s rays near the horizon keep these regions cold even in summer. As an analytical point, long summer days at high latitudes do not bring tropical heat because the Sun stays low in the sky, so the rays are slanting and spread over a larger area.

Why Is This Topic Important?

• Dates Frequent Prelims facts
• Overhead Sun Links to Tropics and heat zones
• Day length Explains polar phenomena

What Came Next?

The next topic describes eclipses, lunar phases and supermoons. Next topic: Eclipses, Lunar Phases and Supermoons.

Key People and Terms

• Solstice Sun overhead at a Tropic
• Equinox Sun overhead at the equator
• Midnight sun Continuous polar daylight
• Polar night Continuous polar darkness
• Vernal equinox Spring equinox, about 21 March
• Autumnal equinox About 23 September

Exam Corner

Mains Angle

Questions ask how day length changes. These points add depth.

• Dates Overhead Sun moves between the Tropics
• Equator Nearly constant day length
• Poles Extremes of light