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University of Texas, Rio Grande Valley *
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Course
1421.90L
Subject
Astronomy
Date
Apr 29, 2024
Type
Pages
8
Uploaded by noneya12456 on coursehero.com
Physical Science 1421
Seasons
Equipment Needed
Quantity
Equipment Needed
Quantity
Seasons Simulator (NAAP) - UNL
Astronomy
1
Seasons Excel File
1
The purpose of this laboratory activity is to study how the position of the Earth in space, the Earth’s
tilt, direct light, and indirect light combine to produce a phenomenon that we call Seasons.
Part 1: Background
Position of Earth in Space
The shape of the Earth is an ellipsoid; however for most practical educational purposes we
approximate its shape to a sphere. It can be divided into two hemispheres, the Northern Hemisphere and
Southern Hemisphere. On the top of the Northern Hemisphere is the North Pole and the bottom of the
Southern Hemisphere is the South Pole. The Earth’s axis is an imaginary line running from the North to
the South poles. The Earth spins on its own axis all the time, turning around completely once every 24
hours. The rotation of the Earth about its axis is what provides us with the alternation between day and
night. What do you think makes the seasons happen?
Direct and Indirect Light
The Earth is actually tilted 23.5
o
from the plane of the ecliptic (the plane the planets orbit about the
Sun). It should look like this:
Figure 1 the Earth
Figure 2 Light as it hits the surface.
In the left (Figure 2), the light is striking the paper at an angle. That is, it is shining on the surface
with indirect light. In the right, the light is striking the paper perpendicularly. That is, it is shining as a
direct light. Direct light produces more heat (gives more rays per area and therefore more energy per
area) than indirect light (where light striking a surface at an angle spreads out).
Length of Day and Night
During the year the length of a day changes, due to the tilt of the Earth. The Sun rises earlier in the
summer than in the winter and sets later in the summer than in the winter. The table below gives the time
of sunrise and sunset for three latitudes.
0
o
latitude
30
o
N latitude
60
o
N latitude
Date
Sunrise
Sunset
Sunrise
Sunset
Sunrise
Sunset
Jan. 1
6:00
18:08
6:56
17:12
9:02
15:06
Feb. 1
6:10
18:18
6:51
17:37
8:15
16:13
Mar. 1
6:09
18:16
6:28
17:58
6:55
17:30
Apr. 1
6:00
18:07
5:49
18:19
5:24
18:45
May 1
5:53
18:00
5:17
18:37
3:56
19:59
June 1
5:54
18:01
5:00
18:56
2:48
21:08
July 1
6:00
18:08
5:02
19:05
2:40
21:26
Aug. 1
6:02
18:10
5:18
18:55
3:36
20:34
Sep. 1
5:56
18:03
5:44
18:22
4:50
19:10
Oct. 1
5:46
17:53
5:53
17:45
6:03
17:36
Nov. 1
5:40
17:48
6:13
17:14
7:24
16:04
Dec. 1
5:45
17:54
6:38
17:00
8:35
15:03
Table 1
Sunrise and sunset times at different latitudes throughout the year
The Seasons
As mentioned in the previous section, the Earth’s rotation axis is tilted 23.5
o
throughout the year
pointing the same direction in space. The Earth moves around the Sun once every year. It follows an
elliptical orbit. This means that it goes almost in a circle around the Sun, but gets a little further away at
some times. The path it takes is like an oval.
p. 2
Figure 3 Orbit of Earth around the Sun
As you can see, because the Earth is tilted, some parts of it get direct light while other parts get
indirect light. Therefore, the amount of heat that the Earth’s surface receives is not evenly distributed.
Figure 4 shows that when the Northern hemisphere is tilting towards the Sun, sunlight falls most
directly on it. This is
Summer
in the Northern Hemisphere. As the Earth moves around to the other side
of the Sun the Northern Hemisphere is tilted away from the Sun therefore light falls indirectly on it. It is
Winter
in this hemisphere.
Notice that the exact opposite is happening in the Southern hemisphere. When the Northern
Hemisphere has Summer, the Southern Hemisphere has Winter. Now, how do we determine which has
Spring and which has Fall? This is simple:
Spring
follows Winter and
Fall
follows Summer!
Figure 4 Position of the Earth relative to the Sun. The seasons are written for the northern
hemisphere.
Summary
Most places on Earth experience four seasons every year. These are
Spring
,
Summer
,
Fall
(or
Autumn) and
Winter
. Here we are going to find out why we have different seasons.
The seasons are caused by a combination of two things:
A.
Earth’s axis is tilted as it moves around the Sun.
p. 3
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