Uses of the Sun

1. Uses of the Sun

2. The Seasons on Earth • The combined effect of the Earth's annual motion around the Sun and the tilt of its axis cause the seasons. • The length of time the Sun is above the horizon (daylight) changes. The path the Sun follows across the sky varies; the Sun's declination changes.

3. EARTH'S AXIS IS TILTED: Here is the most important fact about why we have seasons. The Earth’s axis is tilted. Because the Earth’s axis is tilted, we lean back from the sun in winter getting only INDIRECT energy from the sun. Shadows are long and the sun is only up for nine hours. Temperatures decrease. In the summer, the sun’s energy strikes us DIRECTLY. The sun’s energy is more concentrated and temperatures get warmer. Shadows are short around noon and the sun is up for 15 hours.

4. the North Pole will be dark, and the South Pole will be illuminated

5. As the Earth revolves around the Sun, the seasons in the northern and southern hemispheres are opposite.

7. Eratosthenes Measures the Earth Using the Sun • It was around 250 BCE when Eratosthenes was a librarian in the famous library of Alexandria He discovered in his reading that there was a yearly festival in the town of Syene (modern Aswan) on June 20. On that date, and only on that date, it was possible to look down into a well and see the sun! This remarkable phenomenon was possible because on June 20 the sun was direcly overhead and its light rays would go straight down the well and reflect back to the observer.

8. Eratosthenes Measures the Earth Using the Sun • Eratothenes also observed that on that same date in Alexandria an obelisk cast a distinct shadow. Therefore, the sun was not directly overhead in Alexandria on June 20. One can understand that if the earth is a sphere. In fact, from the angle of the sun’s rays from the vertical on June 20 and the distance from Alexandria to Syene he was able to deduce the circumference of the earth.