Seasons

I mentioned in the earlier post that there are three reasons why we have seasons:

  1. the 23.5 degree tilt of the earth, or obliquity.
  2. The earth’s axis of rotation is fixed and doesn’t change with the rotation of the earth. In other words, the north pole points in the same direction in space always.
  3. Revolution – the Earth revolves around the Sun.

As the earth is tilted, half of the year the northern hemisphere is closer to the sun and the southern hemisphere in the other half making it warmer.

When these hemispheres are tilted away from the sun, they are cooler. This difference in intensity of the sun causes seasons.

Earth does not move at a constant speed in its elliptical orbit. Therefore the seasons are not of equal length. The consolation in the Northern Hemisphere is that spring and summer last longer than autumn and winter.

 

earth movement-04
Seasons if marked on the celestial sphere
Seasons as the earth moves around the sun

Spring

From March Equinox to June Solstice

The Earth continues on its path, and our north pole starts moving towards the Sun. The Sun moves upwards in our skies and the days continue getting longer. Again, we reach a midpoint when day and night are both 12 hours long. This is called the ‘vernal (or spring) equinox’ and happens around 21 March.

Summer

From June Solstice to September Equinox

When the north pole is tilted maximum (23.5 deg) towards the sun, this marks the beginning of summer. During the summer, the sun’s rays hit the Earth at a steep angle because the sun is higher in the sky. The light does not spread out very much, thus increasing the amount of energy hitting any given spot. The long daylight hours during the summer months allow the Earth plenty of time to reach warm temperatures.

Because the sun is higher in the sky our shadows are shorter than they are in the other three seasons

 

Autumn

From September Equinox to December Solstice

The north pole begins to move away from the Sun. The Sun rises lower in the sky so the days start getting shorter. When the Sun is at its mid-point in the sky, we reach the ‘autumn equinox’, around 22 September. Day and night are both 12 hours long and its the beginning of autumn.

Winter

From December Solstice to March Equinox

During the winter, the sun’s rays hit the Earth at a shallow angle because the sun is lower in the sky. These rays are more spread out, which minimizes the amount of energy that hits any given spot. The long nights and short days prevent the Earth from warming up. This is why we have cold winters.

The Earth’s movement around the sun causes the seasons, but it does not affect the temperatures during the seasons. The different temperatures on the earth are caused by the lengths of our days and nights.
In summer, the days are longer and the nights shorter making it hotter. In winters, the days are shorter and the nights longer, keeping the temperatures low.

Note: These seasons are marked in the illustrations for the northern hemisphere.
They will be the opposite for the southern hemisphere.

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