Revolution of the Earth around the Sun
The revolution (or translation) of the Earth around the Sun is the movement that the Earth is around its star the sun. This move follows a kind of “stretched circle”: an ellipse. A complete tour of the circuit lasts 365 days 6 hours 9 minutes (approximately). This movement determines the times of day and night – which vary during the year – and the seasons over most of the Earth’s surface.
The change in the length of day and night inequality
Due to the inclination of the axis of rotation of the Earth and of the revolution, the Earth seems to tilt its northern hemisphere toward the Sun of March to September . By cons, from September to March, it is the southern hemisphere that seems tilted toward the Sun (see sketch of the above section).
In the sketch below against the right, where the revolution of the Earth is supposed to be seen “from above,” we see the change in the length of day and night to different points on the earth’s surface.
- The north poleis in the full day for half the year (March to September) and in total darkness during the other half (September to March).
- Point A is located at latitudesaverage (this is the case of Paris ). Equinoxes in March and September, the day equals the night. Between March and June, the length of the day increases. At the June solstice, day length is maximum (it lasts longer 16 hours in Paris). From late June to mid-September day length decreases while remaining higher than that of the night. At the September equinox, day length equals that of the night (12 hours).From late September to mid-December day length decreases by being less than that of the night. In December solstice day length is minimum (over 8 hours in Paris).
- Point B is located in the northern polar regions experiencing an intermediate position. It has a period of days equal to the duration of the night in the March and September equinoxes, so 12 hours of daylight and 12 hours of night.As of March, day length increases, until it reaches days without night: day lasts 24 hours. Those days are spread around the June solstice and the number of days increases as one approaches the pole. Then again we encounter days with nights (after June and before September) day length decreases, but remains higher than that of the night until the September equinox. From the end of September, the day length becomes lower than that of the night. Comes a time (before December), where there are more day. These days without a day are distributed around the December solstice and the number increases as one approaches the pole.
For places in the southern hemisphere, the opposite phenomenon is observed (day length is maximum at the December solstice and lowest in June solstice).
The revolution of the Earth around the Sun, combined with the inclination of the axis of rotation of the Earth, explains the existence of climatic zones on Earth.
North of the Arctic Circle, a large part of the year there is no light , so no warming . Also during the period when the illumination is, the solar radiation reaching the area, have lost some of their energy entering the atmosphere (some is reflected back into space) and they are pretty low on the horizon (the energy is distributed over a larger area so less heat). The climate is cold all year round. It is called cold zone or polar zone.
By cons, in tropical regions (between the two tropics thus including the equator ), the duration of the illumination little enough varies during the year (between 10 and 14 hours), so these regions are heated constantly. More sunlight are close to vertical and even go through vertically on a few days per year, so their energy is distributed over a very small area that is very heated. These regions are hot throughout the year (seasons are then characterized by the presence or absence of rain ).
In temperate regions (such as those of Europe Western) heat received depends on the inequality of the duration of the illumination during the year and the change of the position of the axis of rotation of the Earth relative to the sun. This double influence determines a hot season (the summer ) and a cold season (the winter ). The seasons are related to changes in temperature (hence the temperate name).