10 define axis, circle of illumination and sub-solar point. Ideas

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Relations and Seasons

Tutorial 22:

Earth-Sun Relations and Seasons

I Earth-Sun Relations:

Figure 1 below shows that the orbit
of the Earth about the sun is not circular.  The path is elongated
or ellipitcal.  This means that the distance from the Earth to the
sun varies through the year.  Two special events are depicted in
the
diagram.  Aphelion (July 4) is when the Earth is as far away from
the sun as it ever gets.  Perihelion (Jan. 3) is when the Earth is
as close to the sun as it ever gets.  Note that these events do
not
correspond to the coldest and hottest months for us in the Northern
Hemisphere.
The purpose of this is to show that distance from the sun has nothing
to
do with seasons.

• One orbit around the sun is
called
a Revolution.
• One revolution takes 365 days
or 1
year
to complete (on each birthday, you have completed one more lap around
the
sun!).
• Aphelion distance is 9.45 x 107
miles.
• Perihelion distance is 9.15 x 107
miles.

Figure 2 looks rather
complicated.
It does, however, reveal some very important facts about the Earth and
its orbit abound the sun.  First note the purpleish
rectangle.
This represents the plane of the Earth’s orbit about the sun or the Plane
of the Ecliptic
.  We now want to measure the orientation of
the Earth with respect to the plane of its orbit, the plane of the
ecliptic.
Now note the orange rectangle which represents the plane of the
equator.
We can clearly see that the two planes do not coincide.  That is
to
say, the Earth is tilted with respect to the plane of the
ecliptic.
Figure 2 also shows the Earth’s axis of rotation.  If the Earth
were
not tilted with respect to the plane of the ecliptic, then there would
be a right angle (90°) between the axis and the plane of the
ecliptic.
Note that the axis is shy of 90° by 23°30′.  This
deviation,
or tilt, is called
Inclination.  We will find
that this inclination is vital for seasons on Earth.  Make sure to
memorize the amont of inclination as we will see this number pop up
time
and again!

• The spinning of the Earth about
its
axis
is called Rotation.
• One rotation takes about 24
hours
or 1
day.

Figure 3 reveals two more important
parts of the seasons story.  First note that 50% of the Earth is
in
daylight and 50% is in darkness.  This is always the case for the
whole Earth, but equal parts of each hemisphere may not be in daylight
and darkness.  The dividing line between day and night is called
the
Circle of Illumination.  The orientation of
the circle of illumination changes with the seasons.  Note in
Figure
3 that the circle of illumination does not pass through the
poles.
Look carefully and you will see that more of the Northern Hemisphere is
in daylight than in darkness which means that the day is much longer
than
the night!  What is important here is that the changing
orientation
of the circle of illumination alters the lengths of daylight and
nighttime
hours.

The second major concept shown in
Figure
3 is the Subsolar Point.  The subsolar
point
is the latitude on the Earth’s surface where the sun’s rays strike at a
90° angle which is the highest possi
ble solar angle.  Figure 3
shows a special event when the subsolar point is as far north as it
ever
gets, the Tropic of Cancer.  The subsolar
point
is where the sun’s rays are most direct and, therefore, most
concentrated.
The concentration of the solar energy heats the surface.
Important
rules emerge from this fact:

• When the subsolar point is as far
north
as it can go, it is the Northern Hemisphere’s Summer.
• When the subsolar point is as far
south
as it can go, it is the Northern Hemisphere’s Winter.
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Figure 4 is a view of
the Earth from space showing the circle of illumination.  Again,
you can see that half of the planet is all ways in darkness and half is
in daylight.   The amounts of the northern and southern
hemispheres in daylight and darkness, however, may NOT be equal.

Figure 5 below shows the position of
the Earth relative to the sun at four times of the year.  You can
see that the orbit is elliptical, as described earlier, and that the
Earth
exhibits a tilt (inclination) relative to the plane of its orbit around
the sun (plane of the ecliptic).  Figure 5 also shows how the
circle
of illumination changes through the year.  There is one final
element
that this figure shows that has a direct affect on seasons.  Note
the orientation of the Earth’s axis.  Do you see that the North
Pole
is always pointing in the same direction in space?  The North Pole
is always pointing at the “North Star” (Polaris).  This constant
orientation
of the Earth’s axis in space is called Parallelism
Look at the axis at position A and then at position C.  Do you see
that the axis is parallel in these two positions?  Also, note that
the axis is again parallel at positions B and D.  The inclination
of the Earth coupled with parallelism means that at one time of year
the
North Pole is pointed toward the sun (A) and six months later it is
pointed
away (C).  This shift from A to C and back again causes the circle
of illumination and the subsolar point to move and for the planet to
experience
seasons.  When studying the seasons, make sure to note the tilt of
the Earth, the position of the subsolar point, the orientiation of the
circle of illumination, and the relative lenths of daylight and
nighttime
hours.

II Seasons:

Let’s begin talking about seasons at
March 21 (position D in Figure 5 above and in Figure 6 below).  At
this point in time, the axis is neither pointed toward nor away from
the
sun.  This causes the subsolar point to fall on the equator.
The circle of illumination also passes through both poles making
daylight
and nighttime hours equal (see below).  When daylight and
nighttime
hours are equal, the event is called an Equinox
We, in the Northern Hemisphere, call March 21 the Vernal
Equinox
.

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Three months later we arrive at June 21
(position A in Figure 5, and in Figure 7).  Here the inclination
of
the Earth points the Northern Hemisphere toward the sun.  This
causes
the subsolar point to be as far north as it ever goes (23°30′ N),
the
Tropic of Cancer.  The circle of
illumination
doesn’t pass through both poles making daylight and nighttime hours
differ
to the extreme.  Note that more of the Northern hemisphere is in
daylight
than in darkness.  This represents the Northern Hemisphere’s
longest
day of the year or the
Summer Solstice.  June
21 is also the shortest day in the Southern Hemisphere or their Winter
Solstice.  Since seasons are hemisphere specific, the June 21
event
is called the June Solstice.  Note that
strange
things happen on the June Solstice.  Figure 7 shows that Repulse
Bay
will not get rotated into darkness on this day.  Anywhere on
Repulse
Bay’s latitude will experience 24 hours of daylight.  This
latitude
is 23°30′ from the North Pole or at a latitude of 66°30′
N.
This is called the Arctic Circle.  The Antarctic
Circle
, at 66°30′ S experiences 24 hours of darkness on the
June solstice.

By September 22 (position B in Figure 5
and Figure 8) the Earth is no longer pointed toward or away from the
sun,
and the subsolar point has returned to the Equator.  The circle of
illumination again passes through both poles making daylight and
nighttime
hours equal.  This is the second equinox know as the
Autumnal
Equinox
in the Northern Hemisphere.

On December 21, the north pole is
pointed
away from the sun (C in Figure 5 and Figure 9).  This causes the
subsolar
point to be as far south as it ever goes, 23°30′ S (the
Tropic
of Capricorn
).  The circle of illumination is offset once
again
this time making the day short and the night long in the Northern
Hemisphere.
This is the Northern Hemisphere’s Winter Solstice
Do you see that the rule regarding the location of the subsolar point
holds.
The subsolar point is as far south as it ever gets making the period
the
winter for the Northern Hemisphere.  At the same time, this marks
the beginning of the summer in the Southern Hemisphere.  This
event
is technically called the December Solstice
Note once again where strange things happen.  Figure 9 shows that
the Arctic Circle experiences 24 hours of darkness while the Antarctic
Circle has 24 hours of daylight.

• When a circle experiences 24
hours
of darkness,
its noon-time solar angle will be 0° which means the sun does not
rise
above the horizon.
• At the North pole, the sun
rises
above
the horizon on March 22 and does not set until Sept. 22.  Once the
sun sets, it will not rise above the horizon until March 22.
• There are six months of
daylight
and six
months of darkness at the poles.
• The circles experience 24 hours
of
daylight
and 24 hours of darkness on their hemisphere’s summer and winter
solstices
respectively.
depiction of the seasons
from the Prentice Hall Geosciences Animations Library.

Now that you have read this tutorial,
take another look at Figure 4.  What
time of year does Figure 4 depict and how do you know?

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Earth-Sun Relations and Seasons

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• Sumary: I Earth-Sun Relations:

• Matching Result: Half of the Earth is illuminated, but the circle of illumination does not pass through the North and South Poles, as would be the case if the Earth’s axis were …

• Intro: Earth-Sun Relations and Seasons Tutorial 22: Earth-Sun Relations and Seasons I Earth-Sun Relations: Figure 1 below shows that the orbit of the Earth about the sun is not circular.  The path is elongated or ellipitcal.  This means that the distance from the Earth to the sun varies through the year. …

1.5: Seasons – Geosciences LibreTexts

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• Sumary: What causes seasons? The distance between the Earth and the Sun does not account for the seasons. Look at page xii in Goodes World Atlas. First, you will note that the earth’s orbit is indeed not quite circular. The…

• Matching Result: The circle of illumination is the circle that separates the day hemisphere from the night hemisphere. The subsolar point is the single point …

• Intro: 1.5: Seasons Last updated Save as PDF Page ID2233 What causes seasons? The distance between the Earth and the Sun does not account for the seasons. Look at page xii in Goodes World Atlas. First, you will note that the earth’s orbit is indeed not quite circular. The distance between…

Frequently Asked Questions About define axis, circle of illumination and sub-solar point.

If you have questions that need to be answered about the topic define axis, circle of illumination and sub-solar point., then this section may help you solve it.

What in geography is the subsolar point?

The subsolar point refers to the latitude where the sun appears directly overhead at noon, with its rays striking the planet exactly perpendicular to its surface.

What connection exists between the Earth’s rotational axis and the circle of illumination at the equinox positions?

Figure 6h-6: The fact that the circle of illumination passes through the poles and the axis of the Earth is not tilted toward or away from the Sun during the equinoxes does not imply that the Earth’s 23.5 degree tilt has disappeared.

What is a quiz on subsolar points?

The exact location on Earth where the sun’s rays heat the surface at a 90-degree angle is known as a subsolar point.

What is the summer solstice’s subsolar point?

The subsolar points at the solstices define the Tropics, with the Tropic of Cancer being at 23.5 degrees North and the Tropic of Capricorn being at 23.5 degrees South on the June solstice, respectively.

The circle of illumination is what?

The term “Earth’s axis” refers to an imaginary line passing through the center of the planet from top to bottom. On the spring and autumnal equinoxes, the circle of illumination divides all latitudes in half, separating light from darkness and day from night.

How is the subsolar point located?

At 23.5 degrees south, the subsolar point, where the Earth’s Southern Hemisphere is tilted toward the Sun and the Sun’s rays are perpendicular to the Earth’s surface, the Sun is overhead at noon.

Are the illumination circle and axis identical?

The circle of illumination separates day from night, and the Earth takes 24 hours to complete one rotation around its axis. However, the circle of illumination does not coincide with the axis of the Earth because of the inclination of the axis by 2312?.

The subsolar point: Why is it significant?

Important rules are derived from this fact: when the subsolar point is as far north as it can go, it is the Northern Hemisphere’s Summer. The subsolar point is where the sun’s rays are most direct and, therefore, most concentrated.

Where is the only location where the subsolar point is found in the United States?

The event is known as Lahaina Noon when the point passes through Hawaii, the only state in the union where this can occur.