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Physical Geography (GE 306)

Books and websites on specific research topics for physical geography, as well as practice quizzes

Earth/Sun Relations and Seasons Tutorial

This tutorial explains the relationship between the Earth and the Sun and how that causes the seasons, with questions embedded throughout to help you check your understanding. Answers to all the questions are on the next page.

Tutorial

To understand why we have seasons, we need to look at the relations between the Earth and the Sun.

The plane of the ecliptic is the plane of the Earth’s orbit around the Sun.

Most of the planets in the solar system also orbit on or near the plane of the ecliptic. (You can see in the drawing below that the planets are more or less lined up, instead of circling the Sun at just any old angle. No, this is not drawn to scale.)

Solar System

The Sun’s rays are pretty close to parallel to each other when they reach the Earth:

Incoming rays are parallel to each other

So, if the Earth’s axis of rotation were perpendicular to the plane of the ecliptic,

Shows the Earth perpendicular to the plane of the ecliptic

then the direct rays of the Sun would always shine on the equator (that is, the angle of incidence at the equator would be 90 degrees), and you would always see the Sun just on the horizon at the North and South Poles (that is, the angle of incidence at the Poles would be zero):

Incoming rays hitting an Earth perpendicular to the plane of the ecliptic

If that were the case, we would have no seasons.

However, the Earth’s axis is not perpendicular to the plane of the ecliptic. If you drew a line perpendicular to the plane of the ecliptic, the Earth’s axis of rotation would be tilted, or inclined, at an angle of about 23.5 degrees with respect to the perpendicular line. It is this inclination of the Earth’s axis of rotation that is the reason for the seasons.

the Earth’s axis is inclined at an angle of about 23.5 degrees with respect to the perpendicular line

The Earth maintains the same angle of inclination throughout its orbit. Another way of saying this is that the Earth’s axis is parallel to itself at all points in its orbit.

The Earth maintains the same angle of inclination throughout its orbit

Question 1

1. To which star does the Earth’s axis point? That is, which star is almost directly overhead at the North Pole?

Tutorial

The inclination of the Earth’s axis means that at different locations on its orbit around the Sun, different parts of the Earth are tilted toward the Sun, and the part that is tilted toward the Sun is the part that is having summer. If the Northern Hemisphere is tilted toward the Sun, the Northern Hemisphere receives the most direct rays of the Sun (that is, the angle of incidence is higher), and it is summer in the Northern Hemisphere. If the Southern Hemisphere is tilted toward the Sun, the Southern Hemisphere receives the most direct rays of the Sun (that is, the angle of incidence is higher), and it is summer in the Southern Hemisphere. In the intermediate situations, halfway between the two extremes, the Earth’s axis is still tilted, but not tilted with respect to the Sun’s rays, and the Sun’s rays strike directly on the equator.

Let’s look at this more closely. At one point in its orbit, the northern part of the Earth has its maximum inclination toward the Sun. Since the Earth’s axis is tilted 23.5 degrees, then on this particular day, the Sun’s rays are striking the Earth directly at a latitude approximately 23.5 degrees north of the equator (that is, the Sun’s rays are coming in at an angle of 90 degrees here; this is the subsolar point). This occurs on June 21 or 22 each year, and the day is known as the summer solstice (summer for those who live in the Northern Hemisphere, that is; it might be safer and more generally applicable to call it the June solstice).

On the June solstice the Sun’s rays strike the Earth directly at a latitude approximately 23.5 degrees north of the equator

If you were at 23.5 degrees north latitude on that day, and you looked up at noon, the Sun would be directly overhead. This latitude is the farthest north on the Earth that the Sun’s rays will be directly overhead. Farther north, at noon, the Sun will never be directly overhead, but always at least a little bit to the south. The latitude of 23.5 degrees north is known as the Tropic of Cancer, and it is the northern boundary of the tropics.

Question 2

2. On June 21, the subsolar point is at about 23.5 degrees north. The next day, will the subsolar point be north or south of the Tropic of Cancer?

Tutorial

What else is happening at this time? Look at the circle of illumination, and see where it is. 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 not tilted. Instead, the Sun’s rays pass over the North Pole and in fact go “over the top” to a latitude 23.5 degrees south of the North Pole. Since the North Pole is 90 degrees north, the circle of illumination goes to 66.5 degrees north (90-23.5=66.5). This is the Arctic Circle, and on this day, June 21 or 22, every location north of the Arctic Circle is illuminated for the entire 24 hours (this is the Land of the Midnight Sun: the Sun does not rise or set here on the solstice, but just circles around above the horizon).

On the other end of the globe, the circle of illumination never reaches the South Pole, and in fact it doesn’t get to within 23.5 degrees of the South Pole. So every place more than 66.5 degrees south of the equator misses out on the Sun’s rays entirely that day; they get 24 hours of darkness (twilight, really, since the Sun is not that far below the horizon). The latitude 66.5 degrees south is known as the Antarctic Circle.

Questions 3-5

3. If day length is 24 hours at the North Pole during the June solstice, and 0 hours at the South Pole during the June solstice, what is the day length at the equator on that day?

 

Bonus questions:
4. If the Sun is directly overhead at the Tropic of Cancer (that is, the angle of incidence is 90 degrees), what is the angle of incidence at noon at the equator?
5. If the Sun is directly overhead at the Tropic of Cancer (that is, the angle of incidence is 90 degrees), what is the angle of incidence at noon at the North Pole?

Tutorial

Let’s take a look at the opposite end of the year, six months later, on December 21 or 22. On this day, the Earth has gone halfway around its orbit, and is opposite the point where it was on the June solstice. The situation is exactly reversed. The Southern Hemisphere of the Earth is tilted toward the Sun, and the Sun’s rays are perpendicular to the Earth’s surface at 23.5 degrees south. This is the subsolar point: the Sun is directly overhead at noon at this latitude. The farthest south that the Sun’s rays shine directly is latitude 23.5 degrees south, the Tropic of Capricorn. This is the southern boundary of the tropics. On this day, the area north of the Arctic Circle gets no sunlight, and the area south of the Antarctic Circle gets 24 hours of light.

This is called the winter solstice (or December solstice).

On the December solstice the Sun’s rays strike the Earth directly at a latitude approximately 23.5 degrees south of the equator

It is summer for the Southern Hemisphere (and the longest day of the year for the Southern Hemisphere), and winter for the Northern Hemisphere (and the shortest day of the year for the Northern Hemisphere).

Questions 6-8

6. At the December solstice, the Sun is shining directly on the Tropic of Capricorn. The day after the solstice, will the subsolar point be north or south of the Tropic of Capricorn?
7. Six months after the December solstice (on, say, June 21), where will the subsolar point be?
8. Three months after the December solstice (on, say, March 20), where will the subsolar point be?

Tutorial

Now for the intermediate situation. The subsolar point is at the Tropic of Cancer in June, and at the Tropic of Capricorn in December. Midway between the solstices are two dates when the Sun shines directly on the equator, and we have a situation like our hypothetical one, where the Earth’s axis is perpendicular to the ecliptic. (The axis is still tilted, still pointing at the North Star, but it is tilted sideways with respect to the Sun, rather than toward or away from the Sun).The circle of illumination passes through the Poles, the Sun’s rays strike the equator at an angle of 90 degrees, and every part of the Earth has 12 hours of daylight and 12 hours of night. This happens twice in the year. These two days are called the equinoxes. (Equinox means “equal night”, since day and night are equal at 12 hours each.)

On the equinox the Sun’s rays strike the Earth at the equator

March 20 or 21 is the vernal (or spring) equinox for the Northern Hemisphere, and the subsolar point is heading north, on its way to the Tropic of Cancer. September 22 or 23 is the autumnal (or fall) equinox for the Northern Hemisphere, and the subsolar point is heading south.

Equinoxes and solstices during the year

Throughout the year, day length is 12 hours at equator. At the Poles, day length varies from none (at the winter solstice for that Hemisphere) to 24 hours (at the summer solstice for that Hemisphere). At points in between, day length will be somewhere in between. The closer a location is to the equator, the more even the day length is, not varying much from 12 hours all year. At high latitudes, the day length is very long in the summer and very short in the winter.

One more point about the Earth’s orbit: By now you know that the reason we have seasons is the tilt of the Earth’s axis. But sometimes people wonder if seasons have anything to do with the distance of the Earth from the Sun. We know that can’t be the case, because the seasons are opposite in the Northern and Southern Hemispheres. However, if we look at the Earth’s orbit closely, we find that it is not exactly circular. Rather, it is an ellipse, which is an oval shape, or a very slightly flattened circle. The Sun is at one focus of the ellipse. This means that the Earth’s distance from the Sun does vary. The point at which the Earth is closest to the Sun is called perihelion (from the Greek peri, close or near, and helios, meaning Sun). Perihelion takes place on January 3, which, of course, is during winter for the Northern Hemisphere and during summer for the Southern Hemisphere. The point at which the Earth is farthest away from the Sun is called aphelion (Greek ap, away from, and helios, Sun).

the Earth is closest to the Sun on January 3

Question 9

9. Aphelion takes place on July 4, which is summer for which hemisphere?

Tutorial

So actually, the Earth and Sun are closest during the Southern Hemisphere summer. Does this mean that the summers are hotter in the Southern Hemisphere? Well, the Southern Hemisphere does get a little more insolation during summer, but the effects of this are moderated because the Southern Hemisphere is a water hemisphere (lots of ocean: take a look at a globe), and the water absorbs the extra heat. So the differences in summer heating between the two Hemispheres are not really very noticeable.

Summary Questions

At the June Solstice - The date is June 21 or 22

  1. The subsolar point is at what latitude?

  2. What is this latitude called?

  3. What is the length of daylight at the equator?

  4. What is the length of daylight at the North Pole?

  5. What is the length of daylight at the South Pole?

  6. On this day, every location south of what Circle is in darkness for 24 hours?

  7. Every location north of what Circle gets 24 hours of daylight?

  8. This day is the longest day of the year for which Hemisphere?

  9. At the equator, what is the Sun’s altitude at noon (the angle of the Sun above the horizon at noon)?

  10. After the June solstice, will the subsolar point will move northward or southward?

 

At the December Solstice - The date is December 21 or 22

  1. The subsolar point is at what latitude?

  2. What is this latitude called?

  3. What is the length of daylight at the equator?

  4. What is the length of daylight at the North Pole?

  5. What is the length of daylight at the South Pole?

  6. On this day, every location north of what Circle is in darkness for 24 hours?

  7. Every location south of what Circle gets 24 hours of daylight?

  8. It is the longest day of the year for which Hemisphere?

  9. At the equator, what is the Sun’s altitude at noon (the angle of the Sun above the horizon at noon)?

  10. After the December solstice, will the subsolar point will move northward or southward?

 

At the March Equinox - The date is March 20 or 21

  1. The subsolar point is at what latitude?

  2. What is this latitude called?

  3. What is the length of daylight at the equator?

  4. How many hours of daylight does every place on Earth get?

  5. At the equator, what is the Sun’s altitude at noon (the angle of the Sun above the horizon at noon)?

  6. After the March equinox, will the subsolar point will move northward or southward?

  7. In which Hemisphere is this the spring (vernal) equinox?

  8. In which Hemisphere is this the fall (autumnal) equinox?

  9. Will the days now be getting longer or shorter for the Northern Hemisphere?

  10. Will the days now be getting longer or shorter for the Southern Hemisphere?

 

At the September Equinox - The date is September 22 or 23

  1. The subsolar point is at what latitude?

  2. What is this latitude called?

  3. What is the length of daylight at the equator?

  4. How many hours of daylight does every place on Earth get?

  5. At the equator, what is the Sun’s altitude at noon (the angle of the Sun above the horizon at noon)?

  6. After the September equinox, will the subsolar point will move northward or southward?

  7. In which Hemisphere is this the spring (vernal) equinox?

  8. In which Hemisphere is this the fall (autumnal) equinox?

  9. Will the days now be getting longer or shorter for the Northern Hemisphere?

  10. Will the days now be getting longer or shorter for the Southern Hemisphere?