E. Earth and Space Science: Space Exploration · Space Exploration: The Sun, the Solar System and the UniverseInvestigating the Seasons and the Sun's Path
E2.6 A1.2 A1.5About 75 minutes
Learning goals- I can explain the causes of the seasons using Earth's tilt, the angle of sunlight and the length of day.
- I can calculate the height of the noon Sun at different times of year for my location.
- I can plan and carry out a fair-test investigation about the angle of sunlight.
- I can explain why the seasons are not caused by Earth's distance from the Sun.
- I can explain the midnight sun and polar night in Canada's North.
Words to know
axis
The imaginary line through Earth's North and South Poles that Earth spins around. It is tilted about 23.4°.
solstice
The two days each year (around June 21 and December 21) when the noon Sun is highest or lowest in the sky and day length is longest or shortest.
equinox
The two days each year (around March 20 and September 22) when day and night are about equal everywhere.
altitude (of the Sun)
The angle of the Sun above the horizon, from 0° (on the horizon) to 90° (straight overhead).
angle of incidence
The angle at which light strikes a surface.
Arctic Circle
The line of latitude at about 66.5° N; north of it, there is at least one day a year when the Sun does not set and one when it does not rise.
perihelion
The point in Earth's orbit when it is closest to the Sun (early January).
Remember from Grade 6
In Grade 6 you learned that Earth's axis is tilted about 23.4° and always leans the same way in space. Around June, the Northern Hemisphere tilts toward the Sun (summer); around December, it tilts away (winter). Now you will measure and calculate the effects of the tilt and test them with investigations.
A surprising fact to start
Earth is closest to the Sun in early January (about 147 million km) and farthest in early July (about 152 million km). So if distance caused the seasons, Canada would have summer in January! Clearly, something else is going on.
Cause 1: the angle of sunlight
When the Northern Hemisphere is tilted toward the Sun, the Sun climbs higher in our sky. Sunlight strikes the ground more directly, so the same beam of energy falls on a smaller area and heats it more. In winter, the low Sun spreads the same beam over a larger area, and its light also passes through more atmosphere.
The math: a beam hitting the ground at an angle of 30° spreads over twice the area it would cover at 90°, so each square metre gets half the energy.
Cause 2: the length of day
When the Northern Hemisphere tilts toward the Sun, places in Canada spend more hours of each rotation in sunlight. In Toronto, the longest day (around June 21) has about 15½ hours of daylight, and the shortest (around December 21) about 9 hours. More hours of sunlight means more energy absorbed each day.
Both causes come from the tilt. Without a tilt, there would be almost no seasons.
Calculating the noon Sun's height
You can predict how high the Sun will be at solar noon (when it is highest, due south) with this formula for the Northern Hemisphere:
noon altitude = 90° − your latitude + adjustment
- adjustment = +23.4° at the June solstice
- adjustment = 0° at the equinoxes
- adjustment = −23.4° at the December solstice
For Toronto (latitude 43.7° N): June: 90 − 43.7 + 23.4 = 69.7°; equinox: 46.3°; December: 90 − 43.7 − 23.4 = 22.9°. The noon Sun is about three times as high in summer as in winter.
The midnight sun and polar night
North of the Arctic Circle (66.5° N), the tilt has dramatic effects. Around the June solstice, the Sun stays above the horizon all day and night: the midnight sun. Around the December solstice, it does not rise at all: polar night. In Alert, Nunavut, the northernmost permanently inhabited place in the world, the Sun stays below the horizon for months in winter and stays up for months in summer. Communities just south of the Arctic Circle, like Iqaluit, still get very long summer days and very short winter days.
Worked exampleExample 1: Noon altitude in your city
Find the noon altitude of the Sun in Thunder Bay (latitude 48.4° N) on the June and December solstices.
- June: 90° − 48.4° + 23.4° = 65.0°.
- December: 90° − 48.4° − 23.4° = 18.2°.
- Compare: the December Sun is much lower, so the light is more slanted and there are fewer daylight hours.
About 65° in June and about 18° in December.
Worked exampleExample 2: Planning a fair test
Plan an investigation to test: 'Does the angle of light affect how much a surface warms up?'
- Independent variable (what you change): the angle of the light (90°, 60°, 30°).
- Dependent variable (what you measure): temperature rise of the surface in 10 minutes.
- Controlled variables (keep the same): same lamp and distance, same surface (e.g. black paper on cardboard), same thermometer position, same starting temperature, same time.
- Prediction: if light hits more directly, then the surface will warm more, because the energy is concentrated on a smaller area.
- Repeat each angle 3 times and average the results.
A fair test changes only the angle, measures the temperature rise, and keeps everything else the same.
Hands-on activityInvestigation: angle of light and the spot it makes
You need: a flashlight, graph paper, a protractor, a ruler, a book or box to rest the flashlight on, and tape.
- Question: How does the angle of a light beam change the area it lights up?
- Hypothesis: write your prediction with a reason.
- Tape the graph paper to a table. Darken the room.
- Hold the flashlight straight down (90°), 20 cm above the paper (measure along the beam). Trace the bright spot and count the squares inside it.
- Repeat with the beam tilted to 60° and 30° from the table (use the protractor). Keep the distance along the beam at 20 cm.
- Record the results in a table and draw a bar graph: angle vs. area.
- Conclude: connect your result to why winter sunlight in Canada heats the ground less.
Optional extension (A1.5 safety): with a grown-up, put a thermometer under black paper in a sunny window at different angles for 10 minutes each. Do not use a lamp that gets hot near paper.
What to look for: the spot gets larger and dimmer as the angle gets lower. At 30° it should cover roughly twice the squares it covered at 90° (for perfectly parallel rays, theory says exactly 2 times; a flashlight beam spreads a little, so your result may differ). A good conclusion says the same light energy is spread over more area, so each square gets less: like low winter sunlight.
Hands-on activityLong-term investigation: the shadow stick
You need: a straight stick or pencil about 30 cm long, a flat sunny spot (or a flat board you can bring back to the same place), a ruler or tape measure, a protractor and paper.
- Stand the stick straight up (use a level or check from two sides).
- Look up solar noon for your town on a sunrise-sunset website (during daylight saving time in southern Ontario it is usually between about 12:45 and 1:45 p.m., depending on where you live and the date). Starting a little before it, mark the tip of the shadow every few minutes; the shortest shadow is at solar noon. Measure it.
- Make a scale drawing: draw the stick's height upward and the shadow along the ground, then draw a line from the shadow tip to the top of the stick. Measure the angle at the shadow tip with a protractor: that is the Sun's altitude.
- Repeat once a month for a few months. Compare with the formula.
Safety: never look at the Sun; look only at the shadow.
What to look for: from September to December the noon shadow gets longer and the Sun's altitude gets lower. Your measured angle should be within a few degrees of the formula's prediction.
Just for funQ: Why doesn't the North Pole ever get bored of daytime in June?
A: Because the Sun never leaves! It's the world's longest sleepover, and nobody gets any sleep.
Your turn
1Give the two main reasons summer is warmer than winter in Canada.
- The Sun is higher, so sunlight is more direct (concentrated on a smaller area). 2. Days are longer, so the ground absorbs energy for more hours.
2Why is 'Earth is closer to the Sun in summer' wrong?
Earth is actually closest in early January, during our winter. The difference in distance is small; the tilt causes the seasons.
3Calculate the noon altitude of the Sun in Toronto (43.7° N) at an equinox.
90° − 43.7° + 0° = 46.3°.
4What happens in Alert, Nunavut, in December?
Polar night: the Sun stays below the horizon all day.
5In the flashlight investigation, what are the independent and dependent variables?
Independent: the angle of the beam. Dependent: the area of the lit spot (or its brightness).
6When is the Southern Hemisphere having summer?
Around December–February, when it is tilted toward the Sun.
7Why should you look only at the shadow, not the Sun, in the shadow-stick investigation?
Looking at the Sun can permanently damage your eyes; the shadow gives the same information safely.
More practice: warm-up
1About how much is Earth's axis tilted?
2When is the June solstice?
3What is the Sun's altitude when it is straight overhead?
4Is the noon Sun higher in summer or in winter?
5What line of latitude marks the start of the midnight sun zone?
The Arctic Circle (about 66.5° N).
More practice: on your own
1Find the noon altitude in Ottawa (45.4° N) on December 21.
90° − 45.4° − 23.4° = 21.2°.
2A beam lights 20 squares at 90° and 40 squares at 30°. Explain what this means for energy per square.
The same light energy is spread over twice the area, so each square gets half the energy at 30°.
3Why are shadows longer in winter at noon?
The Sun is lower in the sky, so light comes in at a shallower angle and objects cast longer shadows.
4If Earth's axis were not tilted, what would happen to the seasons?
There would be almost no seasons: the noon Sun height and day length would stay about the same all year at each place.
5Why do scientists repeat each trial three times and average?
To reduce the effect of random errors and make the results more reliable.
Challenge
1At what latitude is the noon Sun directly overhead (90°) on the June solstice? Use the formula.
90 = 90 − latitude + 23.4 → latitude = 23.4° N (the Tropic of Cancer).
2Using the formula, show why the Sun does not rise at all on the December solstice at 70° N.
Noon altitude = 90 − 70 − 23.4 = −3.4°. Even at noon, the Sun is below the horizon, so it does not rise (polar night).
3A beam of sunlight hits the ground at an altitude of 23°. Compared with a beam at 90°, about how many times larger is the area it covers? (Hint: area factor = 1 ÷ sin(altitude); sin 23° ≈ 0.39.)
1 ÷ 0.39 ≈ 2.6 times larger, so each square metre gets only about 0.39 (39%) of the energy it would get from an overhead Sun. This is why Toronto's December sunlight is so weak.
Quiz
1. What is the main cause of the seasons?
2. When is Earth closest to the Sun?
3. Noon altitude in Toronto (43.7° N) on June 21 is about:
4. Why does low-angle sunlight heat the ground less?
5. In a fair test about light angle, which should be kept the same?
Big idea: E2.6 asks students to conduct investigations to explain the causes of astronomical phenomena that can be observed from Earth. Seasons were introduced in Grade 6; in Grade 9 the focus is on quantifying the effect (noon altitude, area spreading) and on planning fair tests (A1.2), with attention to safety (A1.5).
Common difficulties:
- The distance misconception. Point out that Australia has summer while Canada has winter, at the same distance from the Sun.
- Variables vocabulary. Independent = what you change; dependent = what you measure; controlled = what you keep the same.
- Using the formula with the wrong sign in December.
Safety: remind your teen never to look at the Sun; and supervise the use of any lamp.
How to help: look up your town's latitude together and calculate the noon Sun's height for today's season. Then go outside at solar noon and look at shadow lengths.