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 possible 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.

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.  Read on and try to answer a question about this diagram posed below.

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.

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.