Astronomy is not just about the night sky, but about the motions of all planets, including our own. These motions produce the noticeable changes in the length of day and night throughout the seasons. In this section, I will try to explain how the tilt in the Earth's axis produces the effect of seasons.

Why are the poles cooler than the equator?

The polar regions of earth are not cooler because they are farther away from the equator, and they are not cooler because they are farther from the sun. The poles are only about 3000 km farther from the sun than is the equator at noon. This is about 0.002% of the earth-sun distance -- hardly significant.
The polar regions are cooler for the same reason as the morning and evening are cooler. From the polar regions, the sun is low in the sky, so that the sunlight hits the earth at a low angle, as it does for us in the morning and evening. This low angle means that the sunlight is more spread out, and thus warms the surface less.

Why are there seasons?

The Earth's orbit is eliptical - so it is sometimes closer to the sun than at other times. There is a misconception that this causes the seasons. If that were the case, then the whole planet would experience summer at the same time, and winter at the same time. But when it is summer in the northern hemisphere, it is winter in the southern hemisphere. The Earth is actually closest to the sun (perihelion) during the northern hemisphere winter.

In fact, the reason for seasons is the tilt of the earth.

The earth's axis is tilted by about 23.5° from vertical. This tilt remains constant as the earth orbits around the sun. As a result, in part of the earth's orbit the northern hemisphere is tipped more towards the sun, and in part of the orbit it is tipped away from the sun. For the same reason as in the previous section, when the northern hemisphere is tilted away from the sun, the angle of sunlight is lower, and there is less surface heating. This makes it winter.

seasons.PNG

 

The sun in the sky

The tilt of the earth means that the position of the sun in the sky changes with the seasons. The following table illustrates the change in the position of the sun at sunrise, sunset, and mid-day at various times of the year for a midnorthern location (44° North).
The date when the sun is lowest in the sky and furthest south at sunrise and sunset is called the Winter Solstice, and usually falls around Dec. 21st. This is also the shortest day of the year. The term "solstice" means stationary or stopped -- because the sun has stopped migrating south, and starts heading north again.
The summer solstices around June 21st is the longest day of the year (for the northern hemisphere), and the sun is as far north as it will get at sunrise and sunset.
March 21 and Sept. 21 are the equinoxes, when the sun is half way between the summer and winter solstices.

 

date

time of

 sunrise

direction of sunrise

maximum altitude

time of sunset

direction at sunset

1-Jan

7:51

E33° S

22

16:51

W 33° S

1-Feb

7:35

E24° S

28

17:28

W 24° S

1-Mar

6:53

E10° S

38

18:07

W 10° S

21-Mar

6:18

East

45

18:32

West

1-Apr

5:58

E 7° N

49

18:45

W 7° N

1-May

5:09

E 22° N

60

19:21

W 22° N

1-Jun

4:39

E 32° N

67

19:53

W 32° N

21-Jun

4:36

E 34° N

68.5

20:03

W 34° N

1-Jul

4:40

E 33° N

68

20:03

W 33° N

1-Aug

5:07

E 25° N

63

19:40

W 25° N

1-Sep

5:42

E 11° N

53

18:53

W 11° N

21-Sep

6:04

East

45

18:16

West

1-Oct

6:16

E 5° S

42

17:58

W 5° S

1-Nov

6:54

E 20° S

31

17:08

W 20° S

1-Dec

7:32

E 31° S

23

16:42

W 31° S

21-Dec

7:48

E 34° S

21.5

16:44

W 34° S

 

Ancient Astronomers


Through observation, ancient peoples learned to identify the location of sunrise and sunset, and the solstices and equinoxes were of particular importance. Structures and monuments were often built to align with sunrise or sunset on specific dates.
Stonehenge, for example, is built so that the rays at sunrise on the summer solstice shine between two stones, and directly onto the altar.
Some ancient burial mounds are built so that the central chamber is always dark, except for at sunrise on the solstice when the sun shines through a small window down a long passageway directly into the chamber.

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