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Earth and Space 6-8 / Week 04 / Monday
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Week 04 · Seasons

Monday

Not the distance
// A tilted axis that always points the same way
⏱ about 20 min

Monday: Not the Distance

Rocket arrives at the Sky Deck in a coat. "It is colder than last month. Earth must be farther from the Sun."

Raven pulls the Sky Log from her pocket. "What do you notice about the Sun these days?"

"It sets farther along the fence than in week 1," Rocket says. "And it is lower at noon. The shadows are longer."

"Lower," Raven repeats. "Not farther. Lower."

Nova hovers beside the railing. "Here is a clue," she says. "Earth is closest to the Sun in January."

Rocket stops. "January? That is winter here." He frowns. "Then distance cannot be the reason."

Nova hums. "Good. Now find what is."

Rocket, Raven and Nova at the Sky Deck on a porch step in cool weather. Rocket wears a coat and points at a long shadow across the yard. Raven holds the Sky Log open beside a flashlight and a sheet of grid paper. Nova the companion drone hovers beside the railing with a soft glow.

A common mistake

Many people believe the seasons come from Earth's changing distance from the Sun. OpenStax Astronomy says the facts do not bear this out.

Earth's orbit is an ellipse, but its distance from the Sun varies by only about 3 percent. That is not enough to change the heating much.

Worse for the idea, Earth is actually closest to the Sun in January, in the middle of Northern Hemisphere winter.

And if distance were the cause, why would the two hemispheres have opposite seasons at the same time?

The real cause: a tilt

The seasons are caused by the tilt of Earth's axis, about 23.5 degrees. NASA gives the tilt as 23.4 degrees.

The axis keeps pointing the same direction in space all year as Earth travels around the Sun.

So in June the Northern Hemisphere leans into the Sun. In December the Southern Hemisphere leans in and the Northern leans away.

In March and September Earth leans sideways, neither toward nor away. Both hemispheres get about equal sunshine.

MonthWhich hemisphere leans toward the SunSeason in the northSeason in the south
JuneNorthernsummerwinter
Septemberneitherautumn beginsspring begins
DecemberSouthernwintersummer
Marchneitherspring beginsautumn begins
CAUSE OF THE SEASONS
  • Read the question.
  • Tap your answer.
When is Earth closest to the Sun?
What causes the seasons?
When the Northern Hemisphere has summer, what season is it in the Southern Hemisphere?
StatementTrue or false?
Earth's distance from the Sun changes by only about 3 percent.?
Summer happens when Earth is closest to the Sun.?
Earth's axis points the same direction all year.?
In December the Northern Hemisphere leans toward the Sun.?
WHY THIS EXERCISEThe standard says the seasons result from the tilt, not from distance.
About how many degrees is Earth's axis tilted? Type the number OpenStax gives.
WHY THIS EXERCISEThis one number explains summer, winter and why the hemispheres are opposite.
Try it
Hold a ball with a pencil through it as the axis. Tilt the pencil and walk around a lamp, keeping the pencil pointed at the same wall.
Notice the top of the ball on each side of the lamp. On one side it leans in, on the other it leans away.
For a grown-up
This week your student shines a flashlight straight down and at a slant onto grid paper and counts the squares it lights.
The flashlight is never shone into anyone's eyes. The Sun is never looked at; its height is judged from shadow lengths.
The facts this week come from OpenStax Astronomy at openstax.org, NASA at science.nasa.gov and the Naval Observatory at aa.usno.navy.mil.
Draw the Sun in the middle and Earth at four points around it, with its axis tilted the same way in each. Label June and December.

Good start. Tomorrow you learn the two ways the tilt makes summer warmer: steeper sunlight and longer days.