Objective – Students will understand how Earth’s tilt on its axis changes the length of daylight and creates the
seasons.
What made Earth a Tilt-a-World?
What made Earth a Tilt-A-World? I'm Bob Hirshon and this is Science Update.
Every 24 hours, the Earth spins around once on its axis—an axis that's tilted 23.5o with respect to Earth's orbit.
Vijay Thurimella of Denver, Colorado, wants to know how Earth got that way.
We consulted Clark Wilson, a Geophysicist at the University of Texas at Austin. He says Earth's tilt came about
early in the formation of the solar system, four and a half billion years ago. Back then, a lot of dust and rocks were
floating around and crashing into each other. That debris eventually stuck together to form the planets.
Wilson:
That process is a little messy, and in the case of the Earth, probably led to some big impacts that eventually tilted
the axis to what it is, 23.5o now.
One of those big impacts, for example, ejected a lot of debris that eventually coalesced to form the moon.
But Wilson says Earth doesn't get knocked around much anymore.
Wilson:
Most of those objects are gone, and we're left with sort of little pieces, asteroids and so on, that would not
significantly change the Earth’s orbit or rotational axis direction now.
For the American Association for the Advancement of Science, I'm Bob Hirshon.
Making Sense of the Research
First of all, let's be sure we're clear on what the Earth's axis is. It's an imaginary stick going through the center of the
Earth. Picture the Earth spinning like a top, straight up and down. Now picture a stick going right through the
center of the Earth. If the Earth weren't tilted, it would rotate like that as it revolved around the Sun. We wouldn't
have seasons—only areas that were colder (near the poles) and warmer (near the Equator).
But the Earth is tilted, and that's why the seasons happen. When the Northern Hemisphere is pointed toward the
Sun, it gets more hours of sunlight. Temperatures rise, and you get summer in New York. At the same time, it's
darker and cooler “down under” in Australia. Six months later, the reverse is true. It's the Southern Hemisphere
that experiences summer. The 23.5o degree tilt also explains why changes in daylight during the seasons are very
dramatic near the poles (which are flooded with sunlight all day long in summer and get no light in mid-winter) but
barely perceptible near the equator (where the Sun shines more or less equally throughout the year).
The axis exists as a result of the rough-and-tumble environment of the early Solar System. Objects collided and
clumped together, which increased their gravitational pull, which in turn drew more objects in, which made the
object even more gravitationally powerful. Today, the Solar System looks like a Sun and eight fairly neat planets.
There is not much stray junk flying around.
Occasionally these forming objects were hit by rocky objects big enough to knock it off-kilter. That's what probably
happened to the Earth, after it was already large enough to start rotating. Actually, Wilson says it probably took
several impacts to whack the Earth into the position it's in today.
Objective – Students will understand how Earth’s tilt on its axis changes the length of daylight and creates the
seasons.
Back in the “old days,” the Earth used to rotate a lot faster—once every 6 to 10 hours. The Moon's gravity has
played a big role in slowing us down to 24 hours. It's a good thing too, because a 6-hour day would certainly lead to
an awfully hectic work week.
Answer the following questions using the information above.
1. Define axis.
a. An imaginary line going from the North Pole to the South Pole. The Earth spins on this
line.
b. The path that a planet travels as it goes around the Sun or a moon takes as it goes around
a planet.
c. The Earth spinning. It takes the Earth 24 hours (one day) to spin once on its axis.
d. It’s not straight up and down, instead it leans at a 23.5o angle.
2. If the Earth were not tilted, we would still have seasons.
a. True
b. False
3. What happens when the Northern Hemisphere is tilted towards the Sun?
a. Daylight hours are shorter and temperatures are warmer
b. Daylight hours are shorter and temperatures are colder.
c. Daylight hours are longer and temperatures are colder.
d. Daylight hours are longer and temperatures are warmer.
4. Daylight hours at the equator are the same throughout the entire year.
a. True
b. False
5. During the early Solar System… (There are two or more answers to this question.)
a. objects collided and clumped together.
b. gravitational forces decreased.
c. there still is a lot of stray junk flying around.
d. object slowly became larger and the gravitational pull increased.
6. Earth has a tilt because of several impacts whacking the Earth into the position it’s in today.
a. True
b. False
7. Early in the Earth’s history, the days were _________ hours long.
a. 2 to 6
b. 6 to 10
c. 12 to 18
d. 18 to 24
Objective – Students will understand how Earth’s tilt on its axis changes the length of daylight and creates the
seasons.
What made Earth a Tilt-a-World? – Key
1. Define axis.
a. An imaginary line going from the North Pole to the South Pole. The
Earth spins on this line.
2. If the Earth were not tilted, we would still have seasons.
b. False
3. What happens when the Northern Hemisphere is tilted towards the Sun?
d. Daylight hours are longer and temperatures are warmer.
4. Daylight hours at the equator are the same throughout the entire year.
a. True
5. During the early Solar System… (There are two or more answers to this
question.)
a. objects collided and clumped together; d. object slowly became larger
and the gravitational pull increased.
6. Earth has a tilt because of several impacts whacking the Earth into the position it’s
in today.
a. True
7. Early in the Earth’s history, the days were _________ hours long.
b. 6 to 10
Objective – Students will understand how Earth’s tilt on its axis changes the length of daylight and creates the
seasons.
What made Earth a Tilt-a-World? – Scoring Guide
1. a
2. b (2 choices)
3. d
4. a (2 choices)
5. a, d
6. a (2 choices)
7. b
Scoring Guide
6-7 – 3
5 – 2.5
4–2
3 – 1.5
2–1
1 – .5
0–0