Day
and night
Our Solar System has one central star, the Sun. This makes it very
easy to define day and night:
- Day occurs when the part of the Earth you are on faces the Sun;
- Night therefore occurs when your part of the Earth faces away from
the Sun.
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 is caused by the rotation of the Earth
on its polar axis once every 24 hours, hence the combined length of
day and night is always 24 hours.
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 Solar Day is the time it takes for the Earth to return to the same
orientation with respect to the Sun; its length averages exactly 24
hours.
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
Long ago, it was generally believed that the Earth was the centre of
all things, with the Sun and other planets orbiting around it. Models
based on this idea were developed by ancient Greek philosophers such
as Aristotle and Ptolemy. However, as we know today, the Earth and other
planets revolve around (or orbit) the Sun and the Earth turns (rotates)
on its own axis.
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
Although the Earth rotates once every 24 hours on its polar axis through
the North and South Poles, day and night do not share the 24 hours equally
- except on two dates, around 21st September and 21st
March, known as the Equinoxes. On these two dates, day and night are
each exactly 12 hours long all over the Earth, except at the North and
South Poles where night and day exchange places on these dates.
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
The variation in the length of day over a year is due to the angle,
or tilt, at which the Earth rotates in relation to its orbit around
the Sun. If the Earth's rotation was perpendicular to its orbit, then
day and night would be 12 hours each for the whole year. Because the
Earth is tilted at an angle of 23.5° to this direction, we have
more daylight in summer than we do in winter and the opposite during
winter.
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
The longest day of the year, the summer solstice, occurs on 22nd
December in the Southern Hemisphere. In the Northern Hemisphere, this
day corresponds to the shortest day of the year or 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.
At the North and South Poles, the summer solstice marks high noon and
the winter solstice midnight.
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