Day
and night
Our Solar System has one central star, the Sun. This makes it very easy to define day and night:
This is fairly simple, but why then does the length of day and night vary at different times of the year and in different places on the Earth's surface? Another question to ponder is why one person's day in Europe is our night in Australia? Our rotating globe
Day and night are caused by the rotation of the Earth on its axis. There
are two basic types of day. The Sidereal Day is the time it takes for
the Earth to rotate in relation to the distant stars; its length is 23
hours 56 minutes and 4 seconds. The difference arises because the Earth moves about one degree around its orbit over the course of a day, and must rotate a little more for a given point on Earth to exactly face the Sun again. In the remainder of this section, we shall use the term day to indicate the 24-hour solar day. Copernican
System
This model was put forward by the Polish Astronomer Nicolas Copernicus. He published details of the Copernican System in 1543, many years after writing his ideas down - and then only when he was dying. To the Earth-centric church of the middle ages, such ideas were heresy, and Copernicus may not have lasted until 1543 if he'd tried to publish them earlier! The length of day and night From 22nd September to 20th March, days are longer than nights in the Southern Hemisphere and correspondingly shorter in the Northern Hemisphere. From 22nd March to 20th September, days are longer than nights in the Northern Hemisphere and correspondingly shorter in the Southern Hemisphere. This is taken to the extreme at the North and South Poles, where daylight lasts for 24 hours for the summer half the year and nights for 24 hours for the winter half year. That is, a day lasts 6 months followed by a 6 month night. On an angle Carefully watch the observer in the following animations as the Earth rotates once to see the difference in the length of a day between summer and winter.
Summer and Winter Solstice Six months later on, 22nd June, the shortest day occurs in the Southern Hemisphere and the longest day in the Northern Hemisphere. Both these days are three months from the Equinoxes.
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