Imagine you’re installing a new array of solar panels for an energy-conscious company in California. You want the largest amount of sunlight possible to reach the panels. The panels are fixed, so you need to figure out which direction to point them in order to gather the most light. The company manager wants you to point them west towards the sun—but they’d have to remain facing that direction every day. Which direction should you actually point them in?
What direction should solar panels face, and why?
- Magnetic compass
- Empty tissue box
- Pen and paper
- 4 outdoor/indoor thermometers
- Sunny day
- Sand (or other form of weight)
- Plastic wrap
- Fill your empty tissue box with sand.
- Tape each of the four thermometers to the tissue box, one to each side, with all the bottoms facing the same direction.
- Tape a layer of plastic wrap over each thermometer using a square of equal size for each of them. What do you think the Plastic Wrap is for?
- Find a spot outside that you know will get sunlight all day.
- Try to wake up before the sun rises so that you can place your tissue box in this spot. Use your compass to find north, and rotate one side of your tissue box to face that direction. Label this side with an ‘N,’ and make sure to all the other sides with their corresponding directions on the compass. Do you think the sun rises directly to the east and sets directly in the west? Why or why not?
- After the sun begins to rise, wait half an hour and look at the temperature for each thermometer. Record the time for each thermometer in a chart like this:
- Do this after every hour over the course of the day, until late in the afternoon (or until sunset, if you can wait that long!).
- Collect your tissue box and make a graph of temperatures using the data you collected. What’s the difference between the highest and lowest temperature thermometers? Is this surprising to you? Are the temperatures the same at dawn and sunset?
What data you get will depend on your latitude and what time of year it is, but if you’re in the United States you should see a higher overall temperature on the thermometer that faced south than the thermometer that faced north. Someone in the southern hemisphere would see a higher temperature on the thermometer facing north.
Your building manager was wrong. The Sun feels warmer in the afternoon just because everything is hotter in the afternoon—after all, everything has had ample time to warm up all day! It turns out that no matter how far north or south the sun is, it’s always somewhere in the southern part of the sky all day, and it doesn’t rise or set directly in the east or west, respectively (except at special times of year in special places on the Earth). So how come?
The earth is tilted relative to its orbit around the Sun. When the northern hemisphere is enjoying its summer, the earth tilts that hemisphere towards the sun. This is why the sun appears higher in the sky during the summer months. It’s winter in the southern hemisphere during the northern hemisphere’s summer, because during this time, the southern hemisphere is tilted away from the sun.
If you live north of the Tropic of Cancer, the sun will always be in the southern half of the sky. Because the Earth tilts in different directions over the course of a year, the sun’s apparent position seems to wander from places directly above the Tropic of Cancer to places directly above the Tropic of Capricorn and back—but because the sun is almost always in the south of the sky in the northern hemisphere, it will preferentially illuminate south-facing objects, such as buildings or solar panels. A good rule of thumb is to tilt your solar panels south towards the sun (and if you want to get really picky, tilt them at an angle determined both by how far north you are and the balance of energy you want during the summer and winter months).
So what’s the deal with wrapping our thermometers in plastic? The plastic wrap traps heat, allowing the sunlight hitting the entire face of the tissue box to heat that face’s thermometer. It behaves like a greenhouse: light from the sun passes through the transparent material and heats up anything underneath it. That heat can’t pass back through the material and escape. This allows us to see a bigger change in temperature, which makes our data much easier to interpret.