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.
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!
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:
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
exhibits a tilt (inclination) relative to the plane of its orbit around
the sun (plane of the ecliptic). Figure 5 also shows how the
of illumination changes through the year. There is one final
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
is always pointing in the same direction in space? The North Pole
is always pointing at the "North Star" (Polaris). This constant
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
North Pole is pointed toward the sun (A) and six months later it is
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
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
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.
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.