About this experiment
Move a noon observation site between latitudes and through an approximate annual calendar. Compare the angle of sunlight, solar-noon irradiance, day length and daily mean irradiance. The same Sun can provide a lower noon flux but a longer day at high latitude. The graph compares the selected date with an equinox reference above the atmosphere.
Learning goals
- Relate latitude and solar declination to noon solar altitude
- Compare noon irradiance with energy received over a full day
- Identify polar day and polar night in the ideal geometric model
- Explain why the tropics are warmer than the poles in terms of the angle of sunlight
- Explain the seasons as a result of the tilt of Earth's axis, not its distance from the Sun
How to use it
- 1Choose the equinox preset and compare the equator with 40 degrees north and 40 degrees south.
- 2At 40 degrees north, compare the June and December solstice presets.
- 3Move to 80 degrees north in June and December. Compare day length and noon altitude.
- 4Play a year and compare the seasonal changes at the same latitude. Remember that the calendar and orbit are approximations.
- 5Compare the daily mean irradiance at the North Pole and at the equator on the June solstice. Which is larger, and why?
The model
Assumptions
- Solar irradiance perpendicular to the rays is fixed at 1361 W/m². Earth follows a circular orbit, so distance variation is omitted.
- Declination follows a sinusoidal 365-day teaching calendar. This is not a precision sunrise or calendar calculator.
- Atmospheric absorption, clouds, atmospheric refraction, surface albedo and terrain are excluded.
- Latitude is limited to 80 degrees north or south. Angles and day length describe the geometric Sun center, not the refracted visible solar disk.
Try this in class
- Q1Why can a polar summer have a long day even when the noon Sun stays relatively low?
- Q2Why are northern and southern latitudes symmetric on the equinox curve?
- Q3Is a high noon irradiance sufficient to infer the daily energy received?
- Q4Why is it summer in Australia when it is winter in Canada?
- Q5Why is the Arctic still cold in midsummer even though it receives a lot of sunlight per day?
Frequently asked questions
Are these the solar watts reaching the ground?
No. They are geometric values above the atmosphere on a horizontal surface. Real surface irradiance also depends on the atmosphere, weather and local terrain.
Why can the noon altitude be negative?
During polar night the Sun can remain below the horizon even at solar noon. The noon irradiance is then clamped to zero, and the geometric day length is zero.
Are the seasons caused by Earth being closer to the Sun?
No. Earth is actually closest to the Sun in early January, during the northern winter. The seasons come from the 23.4° tilt of the axis, which changes the angle of the sunlight and the length of the day through the year.