Earth's Tilted Axis and Seasonal Patterns

Seasonal Patterns and Observed Changes

• Seasons are regular, repeated yearly patterns of changes in temperature, daylight length, and the sun's path.

• In the Northern Hemisphere, December, January, and February are winter months, while June, July, and August are summer months; the Southern Hemisphere experiences the opposite pattern.

• Temperature variation depends on latitude: areas near the equator, like Hawaii, have less variation, whereas higher latitudes, such as Alaska or New York, experience more significant temperature shifts.

Solar Energy and Earth's Geometry

• Sunlight carries energy that heats Earth. The concentration of this energy depends on the angle at which light strikes the surface.

• Light hitting a surface perpendicularly at a $90^{\circ}$ angle is more concentrated. Light striking at a larger angle spreads over a larger area (an oval shape), transferring less energy per spot.

• Because Earth is a sphere, sunlight hits the equator nearly perpendicularly, while it hits latitudes north and south of the equator at larger, non-perpendicular angles, leading to colder climates at the poles.

Earth's Axial Tilt

• Earth's axis is tilted approximately $23.5^{\circ}$ relative to its orbital plane and points toward the same location in space throughout its revolution.

• A system of connected parts, including Earth's round shape, its orbit, and its axial tilt, causes different latitudes to receive varying amounts of concentrated sunlight throughout the year.

Solstices and Tropics

• A solstice is when the North or South Pole is closest to the sun during noon.

• June Solstice: The North Pole is tilted toward the sun. Sunlight hits the Tropic of Cancer ($23.5^{\circ}N$) at a $90^{\circ}$ angle. This marks the first day of summer in the Northern Hemisphere and winter in the Southern Hemisphere.

• December Solstice: The South Pole is tilted toward the sun. Sunlight hits the Tropic of Capricorn ($23.5^{\circ}S$) at a $90^{\circ}$ angle. This marks the first day of winter in the Northern Hemisphere and summer in the Southern Hemisphere.

Equinoxes

• An equinox occurs when the North and South Poles are an equal distance from the sun, and the sun is directly overhead at the equator at noon.

• During an equinox, the equator ($0$ latitude) receives sunlight at a perpendicular angle.

• September Equinox: Marks the first day of autumn in the Northern Hemisphere and spring in the Southern Hemisphere.

• March Equinox: Marks the first day of spring in the Northern Hemisphere and autumn in the Southern Hemisphere.

Variation in the Sun's Path

• The sun's apparent path in the sky changes seasonally. In June in Chicago, Illinois, the sun arcs high and is visible for approximately $15\,hours$. In December, the path is lower, and the sun is visible for just over $9\,hours$.

• In the Northern Hemisphere, the sun rises and sets farther south in the winter and farther north in the summer.

• Solargraphs, made using pinhole cameras and light-sensitive photographic paper, can record these sun tracks over months. Artist Tarja Trygg collected over $350$ solargraphs to demonstrate this, showing lower tracks in high latitudes like Varkaus, Finland ($62^{\circ}N$) compared to lower latitudes like Baltimore, Maryland ($39^{\circ}N$).

Questions & Discussion

• How do the seasons change where you live? Experiences vary by latitude; New York and Alaska have hot summers and cold winters, but Alaska's summers remain cooler than New York's.

• What causes the seasons to be so different? All differences in temperature and daylight are caused by changes in the position of Earth's axis relative to the sun.

• How did ancient civilizations track the seasons? Ancient people built structures like Stonehenge to monitor the sun. Stonehenge rocks frame the sun at sunrise on the June solstice and sunset on the December solstice.

• What is a solargraph and how is it taken? A solargraph is a long-term exposure of the sun's path using a pinhole camera. The camera usually consists of a can with a pinhole aperture ($8\,cm$ from the bottom) and light-sensitive paper inside. The camera should face south in the Northern Hemisphere or north in the Southern Hemisphere.

Solargraphs are a form of long-term exposure photography that captures the sun's path over several days or months using a pinhole camera. They take a long time to make because the exposures last from weeks to months, with each bright line representing the sun's path for one day. The differences in solargraphs stem from variations in location, exposure time, and camera direction:

• Location Impact: The latitude affects the sun's path; for example, Solargraph A from Varkaus, Finland (62°N) shows lower arcs due to high latitude and was taken during fall/winter over three months, recording about 90 sun tracks. (Lower arcs occur at higher latitudes.)

• Solargraph B from Baltimore, Maryland (39°N) displays higher arcs because it was taken in summer, illustrating a higher sun path due to its lower latitude.

• Solargraph C from Singapore (1°N) captures sun tracks so high that they exceed the camera’s frame, demonstrating how close to the equator allows the sun to travel high.

• Solargraph D was taken in Quito, Ecuador (at the equator) but differs in appearance due to the camera's direction, showing nearly vertical tracks as the sun moves directly overhead.

Creating a Pinhole Camera:

1. Cut the top off a clean, tall aluminum can.

2. Create a lid from black cardstock and duct tape it to prevent light entry, but ensure it remains removable for the exposure process.

3. Poke a small hole (the aperture) about 8 cm from the bottom with a sewing pin and cover with duct tape as a shutter.

4. Place 5×7-inch black-and-white photographic paper inside the can, ensuring the light-sensitive side faces in.

5. Secure the camera outside with the pinhole directed south (in the Northern Hemisphere) or north (in the Southern Hemisphere) in a clear view area and remove the shutter tape to start exposure.

6. Leave for a minimum of two weeks to capture sun tracks.

7. After exposure, cover the pinhole, remove the photographic paper in a dimly lit environment, and scan it. Invert the colors to complete the solargraph image.

Conclusion

Making a solargraph requires patience and creativity, but the process and result can yield unique artistic expressions of sunlight's journey across the sky.