Why are Days Longer in Summer?

As per the heliocentric model of our solar system, the Earth, just like other planets, revolves around the Sun. However, it is important to note that the Earth, while revolving around the Sun, also rotates around itself. For the sake of understanding this phenomenon with ease, it is assumed that the Earth spins along an imaginary, vertical axis.

The vertical axis extends from North Pole to South Pole. With this basic knowledge of the Earth's alignment/position with respect to the Sun, it becomes easier to understand the phenomenon of longer days in summer season.

We know that the Earth spins or rotates around itself and also revolves around the Sun. But how does this type of Earth's movement affects the length of days in summer? The tilted axis of Earth is the cause behind the planet receiving light for varying amount of time in a day, in different seasons.

It is the time of the year when the Earth's semi-axis - either northern or southern - appears to be inclined towards the Sun.

The northern solstice, also known as June solstice, occurs on the 20th, 21st or 22nd of June; most of the time, the June solstice occurs on the 20th or 21st of June; it rarely comes on the 22nd. It was in 1971 that the June Solstice occurred on 22nd of June. The next June solstice on the 22nd will take place in 2203 AD.

The southern solstice can occur on the 21st or 22nd of December; therefore, southern solstice is also known as December solstice.

For studying the phenomenon of longer summer days, the Earth can be divided in two semi-circular halves, the northern hemisphere and southern hemisphere, along an imaginary line called the equator. During the summer of a particular hemisphere (northern or southern), the Earth appears to be tilted towards the Sun.

The angle at which the Earth tilts towards (or away from) the Sun is 23° 26'. It is the highest degree to which the Earth can undergo inclination. As a result, the Sun appears to have reached the highest point in the sky in that hemisphere; the expression, "summer sun standing still" is used in reference with this state.

It means the Sun reaches the northernmost point in the northern hemisphere. Therefore, days get longer in summer and on 21st June, the northern hemisphere receives light for the maximum duration in a day's time.

However, it is necessary to understand that although the Sun appears to traverse the sky, it is actually the Earth which revolves around the Sun.

The Earth is tilted towards the Sun, which means the northern hemisphere receives light for a longer period. The situation in winter is opposite to that in summer. In the winter season, the semi-axis of northern hemisphere tilts away from the Sun; it means that the north pole tilts away from the Sun.

As a result, the Earth receives light for the lowest amount of time in a day; therefore, the length of days in winter season becomes shorter and shorter.

However, when there is winter in the northern hemisphere, the southern hemisphere witnesses summer season due to inclination of the southern semi-axis towards the Sun. Therefore, countries which lie in the southern hemisphere, experience summer in December.

Equinox is the time of the year when days and nights are of the same length. The equinox occurs twice in a year's time i.e. on 21st or 22nd of March and 22nd or 23rd of September.

However, the March equinox can occur as early as 19 March. In 2013, the March equinox came on 20th March. At the time of equinox, axis of the Earth and rays of Sun are aligned perpendicularly with each other.

You can see iin the illustration that during an equinox, the Earth is tilted neither towards the Sun nor away from it. This also means that both the northern and southern hemispheres receive an equal amount of light.

The positions of Earth and Sun in the solar system and the orbit in which the Earth revolves (around the Sun) and rotates (around itself), are responsible for the occurrence of different seasons and the solstice and equinox. The knowledge of these basic concepts associated with different seasons, should make it clear as to why days are longer in summer.