Planets do not produce light, they only reflect sunlight, which is the star of the Solar System.
Theories claim that the planets were also formed from very hot portions of mass and that they are all cooling down. Some, including the Earth, have cooled enough to present a solid surface.
A celestial body is considered a planet when, besides having no light of its own, it revolves around a star.
The planets have approximately spherical shape. Its main movements are that of rotation and the of translation. Each planet has a rotation axis with respect to the sun, the most inclined of them is the dwarf planet Pluto, because its rotation axis with respect to the sun is 120º, look at the picture.
In the rotational motion, the planets rotate around their own axis, an imaginary line passing through their center. The terrestrial observer has difficulty perceiving the rotational motion of the earth. For this it should be noted that the sun from dawn to dusk seems to move from the east to the west. The same happens at night with the moon, the stars and other stars we see in the sky.
The earth's rotational motion lasts approximately 24 hours - which is one day. The earth, being spherical, is not illuminated at once. As the earth rotates around its axis, the rays of sunlight fall on one part of the planet and the other is in shadow.
O day and night cycle occur thanks to the rotation. While the planet is spinning on its own axis, it is day in the regions that are illuminated by the sun (light period) and simultaneously it is night in the unlighted regions (dark period).
The movement of translation is performed by the planets around the sun, and the time it takes to make a full turn is called orbital period. For Earth, this period takes about 365 days and about 6 hours to complete. The Earth, in its translational motion, forms a slightly elongated (very close to circular) ellipse. Already the planet Neptune traces its elliptical orbit of very elongated form.
Because of the translational movement and the tilting position of the earth's axis, each hemisphere is alternately more exposed to sunlight during a period of the year. This results in the four seasons of the year: summer, fall, winter and spring. From December to March, the Southern Hemisphere - located south of the Equator Line - is most exposed to the Sun. That's when the sun's rays shine perpendicularly on at least some points of the Southern Hemisphere. It's summer in this hemisphere.
After six months, from June to September, the Earth has already traveled half its orbit. The Northern Hemisphere - located north of the Equator Line - is more exposed to the Sun, so the sun's rays strike perpendicularly to at least some points of the Northern Hemisphere. It is summer in the Northern Hemisphere.
While it is summer in the Northern Hemisphere with the longest days and the shortest nights, it is winter in the Southern Hemisphere, where the days get shorter and the nights longer. And vice versa.
In two periods of the year (from March to June and from September to December) there are positions of the Earth in its orbit, in which the two hemispheres are illuminated equally. It is when the spring and autumn weather occur alternately in both hemispheres.
The seasons are reversed between the southern and northern hemispheres. So it is possible, at the same time of the year, for example, for people to enjoy summer on a beach in the Southern Hemisphere, while others wrap up in a winter blizzard in the Northern Hemisphere.
In regions near the equator, in both hemispheres and in the other, there is a constant incidence of sunshine, and it is hot all year round. There is only the rainy season and the dry season.
Because of the "curvature of the earth" and the inclination of the earth's axis of rotation relative to its plane of orbit, the poles receive fairly slanted sunbeams. For a long period of the year, the sun's rays do not reach the poles; that's why these are very cold regions.
For residents of these regions, there are only two weather seasons:
- One they call winter, that is, the long period when the sun's rays don't reach the pole;
- another so-called summer, when sunset doesn't happen for months.