VIII. Using Central Ideas Based on Evidence to Develop an Explanatory Model for the Earth’s Seasons

A.  Explaining the Earth’s seasons with a heliocentric model

Question 5.37 Why is it hot in the summer and cold in the winter?

An additional aspect of this heliocentric model is to envision the Earth as tilted on its axis. The tilt of the Earth’s axis of rotation has profound implications for variations in climate as the Earth revolves in its orbit around the Sun as shown in Fig. 5.69.

Note that the tilt is envisioned as always being in the same direction throughout the Earth’s annual trip around the Sun:

In the northern hemisphere, as shown in Fig. 5.69 for example, the Earth is envisioned as moving in a counter-clockwise direction around the Sun with the tilt is always in the same direction, toward the north star, Polaris. In spring, the tilt is toward Polaris but opposite to the counter-clockwise direction of motion around the Sun. In summer, the tilt is still toward Polaris and now toward the Sun but perpendicular to the counter-clockwise direction of motion. In autumn, the tilt is still toward Polaris but also in the counter-clockwise direction of motion around the Sun. In winter the tilt is still toward Polaris but away from the Sun, perpendicular to the counter-clockwise direction of motion.

To model the revolution of the tilted rotating Earth around the Sun :

• Which wall is in the north direction? east? south? west?
• Have someone hold the globe. Toward which wall should this person always tilt the axis of the globe?
• Which season is being represented by where this person is standing with respect to the lamp, the tilted globe, and the designated wall?
  • Tilt backwards along the path around the lamp but toward the designated wall for spring
  • Tilt both toward the designated wall and the lamp for summer
  • Tilt forward along the path around the lamp and toward the designated wall for autumn
  • Tilt away from the lamp but toward the designated wall for winter
•  Have the person holding the globe keep the globe rotating on its axis and tilted toward the designated wall while walking around the lamp
• Where should the person be standing to represent:
  • the spring equinox?
  • the summer solstice?
  • the autumn equinox?
  • the winter solstice?

as the person walks around the lamp, while keeping the rotating globe’s axis tilted toward the designated wall?

Now everyone model the tilted rotating Earth revolving around the Sun:

• Tilt in the direction of the designated wall.
• Practice modeling the Earth’s daily rotation while tilted in this direction.
• Which season are you modeling?

• • Where are you with respect to the lamp and the constellations on the walls?
  • Are you:
  • tilted toward the designated wall but tilted backwards along the path around the lamp as you move forwards ?    (spring)
  • tilted toward both the designated wall and the lamp?  (summer)
  • tilted toward the designated wall and forward along the path around the lamp ?  (autumn)
  • tilted toward the designated wall but away from the lamp but? (winter)
• Now put the rotating and revolving motions together. As you revolve around the lamp, keep your body tilted toward the designated wall while rotating on that axis.
• What is the connection between this model of a tilted rotating Earth revolving around the Sun and the evidence of changes in the Sun’s maximum angular altitude α and the tilt’s effect on seasonal changes in the climate?
• Draw two diagrams, one representing a tilted Earth revolving around the Sun in an almost circular orbit and the other representing a lamp and students modeling the reasons for the Earth’s seasons.

The data reported in section VI.B.3 indicate that during summer the Sun is above the horizon for more hours and seems to travel in a higher arc across the sky so that its rays shine more directly down through the atmosphere. Therefore, more energy from the Sun reaches the surface for a longer time period during the summer compared to the amount of energy from the Sun reaching the surface during winter.

In summary:

• The Sun is higher in the sky when the Earth is in a position in its orbit where its axis is tilted toward the Sun during the summer solstice.
• The Sun is lower in the sky when the Earth is in a position in its orbit on the other side of the Sun, where the Earth’s axis is tilted away from the Sun during the winter solstice.
• During spring and autumn equinoxes, the Earth’s tilt is parallel to the direction of travel and the tilt does not affect the number of hours that the Sun is above or below the horizon.
• Note that the Earth’s axis of rotation always tilts in the same direction, in the northern hemisphere, for example, always toward Polaris, the North Star. What changes is where the Earth is in its orbit around the Sun.

Many people associate being warm with being close to a heat source and being cold with being far away from a heat source. Thus it may seem reasonable to associate summer with being closer to the Sun and winter with being farther away. This is not the case here, however. The Sun is about 100,000,000 miles away. The diameter of the Earth is only about 10,000 miles. The fractional difference in being on one side (tilted toward) or the other side (tilted away) from the Sun is only 10,000/100,000,000  = 1/10,000. This is not enough to make a difference.

The northern and southern hemispheres are effectively the SAME distance from the Sun; yet the southern hemisphere experiences winter while the northern hemisphere is experiencing summer; the southern hemisphere experiences summer while the northern hemisphere is experiencing winter. The difference is caused by where the Earth is in its orbit around the Sun and how this position affects the direction of the tilt of Earth’s axis of rotation with respect to the Sun, not by a very minor difference in distance from the Sun.

Fig 5.69 assumes that the Earth’s orbit around the Sun is circular. The orbit is, however, slightly elliptical. This means that the northern hemisphere is actually slightly closer to the Sun in January than in July, the opposite of what one might expect.

If you have a globe, light source, and a light sensor or a smartphone with a built-in light sensor or a light sensor app, you can explore the effect on the brightness  of light from a source (flux) of a location’s latitude on a globe as well as the effect on brightness of the orientation of the globe’s tilt to the source. You also can explore the effect of differences in distance of the globe from the source to model the effect of the slight elliptical shape of the Earth’s orbit around the Sun so that the northern hemisphere is closer to the Sun in January (perihelion) than in July (aphelion). See J. Durelle, J. Jones, S. Merriman, and A. Balan, “A smartphone-based introductory astronomy experiment: Seasons investigation,” The Physics Teacher, 55, 2, 122-123 (2017).

• Complete entries in Table V.13. Then write a summary of what you have learned about developing an explanatory model for the Earth’s seasons.