The sun is the most important factor in our lives and to us is the most important star in the universe. It is our source of heat and light. Without the sun, life on Earth could not have developed and could not be maintained.
The sun is a star, spherical in shape, and although its diameter is 109 times that of the Earth, it is by no means large in comparison with some other stars. In fact, astronomers classify it as a yellow dwarf star—even though its diameter is 865,000 miles. The main difference between the sun and any other afar, as far as Earth people are concerned, is that we on Earth are so close to it. The relative distances of the other stars can be appreciated by the fact that although light traveling at 186 thousand miles a second takes just over eight minutes to reach us from the Sun, it takes over four years to reach Earth from the nearest other star.
The Earth revolves around the sun at a mean distance of 93 million miles and its orbit is so nearly circular that this distance does not vary by more than one-and-one-half inches during the course of the year. It is of interest to note that the Earth is nearest to the sun in January and furthest away early in July showing that the variation in distance is
not
responsible for the seasons. The seasons are caused by the fact that the Earth’s axis (which maintains an almost constant direction in space) is inclined to the ecliptic at 66,1/2°—or as more often stated, the Earth’s equator is inclined to the ecliptic at an angle of 23, 1/2° (Figure 1). This means that during the course of the year the sun can be as far north of the equator as 23,1/2°, giving summer in the Northern hemisphere and winter in the Southern hemisphere. Also, it can be south of the Equator as 23,1/2°, giving winter in the Northern hemisphere and summer in the Southern hemisphere.
HOW THE EARTH’S ROTATION AROUND THE SUN
APPEARS IN DIFFERENT PLACES ON EARTH
|
SUN’S
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NORTH OF
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|
SOUTH OF
|
|
DECLINATION
|
THE EQUATOR
|
OVER EQUATOR
|
EQUATOR
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|
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March 21-
|
March 21 and
|
September 2
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|
Time of year
|
September 2
|
September 2
|
March 21
|
|
|
Summer in
|
|
Winter in
|
|
Seasons
|
northern
|
Equinoxes
|
northern
|
|
|
hemisphere
|
|
|
hemisphere
|
|
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Winter in
|
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Summer in
|
|
|
southern
|
Equinoxes
|
southern
|
|
|
hemisphere
|
|
hemisphere
|
|
Observer ‘s
|
|
Days & nights
|
|
latitude:
|
Perpetual
|
of equal
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Perpetual
|
|
North Pole
|
daylight_
|
length
|
darkness
|
|
A high
|
|
Days & nights
|
|
northern
|
Long days,
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of equal
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Short days,
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|
latitude
|
short nights
|
length
|
long nights
|
|
|
Days & nights
|
Days & nights
|
Days & nights
|
|
The equator
|
of equal
|
of equal
|
of equal
|
|
|
length
|
length
|
length
|
|
A high
|
|
Days & nights
|
|
southern
|
Short days,
|
of equal
|
Long days,
|
|
latitude
|
long nights
|
length
|
short nights
|
|
|
|
Days & nights
|
|
South Pole
|
Perpetual
|
of equal
|
Perpetual
|
|
|
darkness
|
length
|
daylight
|
At the Equator, throughout the year, whatever the season and wherever the Sun, days and nights are of equal length.
At the Equinoxes (March 21 and September 2), all places have equal days and nights.
