1. Begin by leading the class in a discussion about Earth’s seasons. Ask students to identify some characteristics of each of the Earth’s seasons: spring, summer, autumn, winter. (Answers will vary but should cover weather, climate, hours of daylight). Write on board or chart paper. Ask students what causes these changes over the course of a year? (The Earth’s orbit around the sun.)
2. Use a large beach ball to represent the sun and a globe to represent the Earth. Ask for a student volunteer to hold the beach ball (Sun). Ask for another student volunteer to hold the globe (Earth) perpendicular to the plane of Earth’s orbit around the sun (straight up and down, with the North Pole facing the ceiling). Ask the student holding the globe to walk slowly in a small circle around the Sun, spinning the globe and keeping the Earth’s axis straight. Ask students if this is an accurate representation of the Earth’s orbit around the Sun. (No.) Why not? (The Earth should be tilted.) Explain that if the Earth’s orbit was really like this then every place on Earth would always have 12 hours of daylight and 12 hours of darkness, and there would be no seasons.
3. Ask the student holding the globe to tilt it slightly, so that the Poles are no longer aligned with the floor and the ceiling. Ask the student again to walk slowly around the Sun, spinning the tilted globe. Explain to students that the Earth is always tilted at approximately 23°, so depending on the time of year different places get different amounts of daylight. This is what causes variances in temperature and daylight hours that make up the seasons.
1. Write the following vocabulary terms on the board and/or ask students to write them in their notebooks.
Go through the terms one at a time, asking students to contribute working definitions for each of the terms, continuing to use the beach ball and globe as visual aids if desired. (Orbit: the Earth’s path around the sun. Rotation: Earth’s spin on its axis. Axis: the imaginary line around which the Earth rotates, tilted approximately 23 degrees. Hemisphere: half of the Earth; split into Eastern/Western or Northern/Southern.)
2. Explain to students that these attributes account for the seasonal changes that we experience, such as changes in temperature and weather, and increased or decreased daylight hours. During the winter (in the Northern Hemisphere) the Sun sets earlier and it is colder. Ask students if they can think of why the Earth is colder at certain times of the year and warmer at others? Explain that in the winter the Sun’s rays hit the Northern Hemisphere at an angle but in the summer the Sun’s rays are direct.
3. Tell students that they are going to see some more examples of the effects of seasonal changes. Direct students to the Seasons Interactive, and FRAME the Web site by telling them that this Web interactive will allow them to explore these seasonal changes in four cities located in different places around the world. Divide class into pairs or small groups. Give students a FOCUS by asking them to note the differences in temperature and daylight hours in the four cities over the course of the year. Distribute the Seasons Interactive Organizer (RTF) (PDF), which allows students to note the information for March, June, September, and December. Give pairs/groups 10-15 minutes to explore the interactive and enter the information on the organizer.
4. Review answers with class, and FOLLOW UP by asking the following questions. You may display the interactive on a screen for the whole class as a visual reference, if desired
- Which city experienced the most variation in temperature and daylight hours? (Anchorage, AK) Which city experienced the least? (Quito, Ecuador)
- Why was there such a fluctuation in the data for Anchorage? (The city is close to the North Pole, and therefore experiences the most change due to the tilt and orbit.)
- Why is the data in Quito basically the same year-round? (The city is on the Equator so the amount of sunlight that it gets is not affected by the Earth’s tilt.)
- Compare the data for Chicago, IL and Melbourne, Australia. What is the same? What is different? (Their temperatures and daylight hours are comparable due to their latitude, but their seasons are reversed.)
5. Explain to students that the seasons are “reversed” in the Southern Hemisphere. Ask students why they think that is? If they need visual clues they can refer to the Web interactive or the globe and beach ball. (Because of the tilt of the Earth, the Northern Hemisphere and Southern Hemisphere point in different directions and so they are affected differently by the Sun.)
1. Explain to students that there is a reason they only had to collect information from four months out of the year from the Web interactive. These are the times of the year that the seasons change, and are marked by the spring equinox, summer solstice, autumn equinox, and winter solstice. Tell students that they are going to watch a short video clip. FRAME the clip by explaining that it focuses on these seasonal milestones. Give students a FOCUS for watching by asking them to observe the dates and characteristics of each of these milestones. Distribute the Seasonal Milestones Organizer (PDF) (RTF) for students to use to note their observations.
2. PLAY the clip. Give students a few minutes to complete the questions on the organizer. Replay the clip if necessary. FOLLOW UP by reviewing the information students pulled from the video clip about the spring equinox, summer solstice, autumn equinox and winter solstice. Ask students what they think the dates for these seasonal milestones are in the Southern Hemisphere. (Spring Equinox: September 21, Summer Solstice: December 21, Autumn Equinox: March 21, Winter Solstice: June 21.)
3. Explain to students that the dates of these milestones are not always exactly on the 21st of March, June, September, and December. Direct students to Times Solstices & Equinoxes 2000 – 2010, or display the Web site on a screen for the class. Observe some of the dates for the solstices and equinoxes throughout the decade. Students should observe that the dates are always in the vicinity of the 21st, but not always exactly on that date. Ask students: why would the dates be different from year to year? (Variances in the length and shape of the Earth’s orbit.)