miércoles, 4 de noviembre de 2015

Days, nights and seasons

Long time ago, Ancient Greeks thought that the Earth was motionless and in the center of  the Universe. Sun appears to move through the sky from East to West. So, days and nights were explained because the Sun revolved around the Earth. Years later, Copernicus was convinced that the Earth was a planet too; so, he proposed that our planet has two motions: rotation, spining around it axis, and orbital motion, around the Sun.
As result of rotation, the Sun lights up half of the Earth and the other half is in shadow. It’s day time when Granada is in the lit by the Sun; it is night-time here when our part of the planet is facing away from the Sun. The Earth’s axis, the imaginary line through the centre of the planet, is tilted compared to the way the Earth orbits the Sun.  

As result of orbital motion, the Earth goes once around the Sun every 365 days. That is one Earth  year. And every year has four seasons: winter, spring, autumn (fall) and summer. Seasons are mostly due to the tilt of the axis. In summer, the Northen Hemisphere is tilted to the Sun, so it spends more time facing the Sun and days are longer than nights and the weather is warm. In winter, the Northen Hemisphere is tilted away  from the Sun, so, nights are longer than days and the weather is cold.

In my opinion, it’s clear the mecanism of seasons in our planet. But the Earth is not the only planet in the Solar System. Here you have a picture whit all the planet (including the dwarf one, Pluto). Can you tell what planets do have seasons? Why? Please, write your answer as a comment (Spanish version).