Milankovitch Cycles: Earth's Wobble Makes Ice Ages
About 20,000 years ago, a sheet of ice up to 3 kilometers thick sat on top of where Chicago, New York, and London are today. Then, slowly, the ice melted. Long before that, thick ice had come and gone many times. What keeps switching the great ice ages on and off? A Serbian scientist named Milutin Milankovitch spent years doing math (without a single computer!) and found the answer in the sky. Earth's own movements around the Sun change in slow, repeating patterns. Those patterns are now named after him: the Milankovitch cycles.
What You'll Learn
In this lesson you will be able to: - Name the three ways Earth's movement slowly changes over thousands of years - Explain how these changes let more or less sunlight reach the far north - Connect the cycles to why ice ages begin and end - Understand that these changes are super slow, not something you feel day to day
Wobble Number One: A Stretchy Orbit
Earth travels around the Sun in a path called an orbit. You might think it is a perfect circle, but it is not. Sometimes the orbit is more stretched out like an oval, and sometimes it is rounder. This slow change is called eccentricity, and one full cycle takes about 100,000 years. When the orbit is more oval, Earth spends part of the year farther from the Sun and gets a little less warmth. That can help ice grow.
Wobble Two and Three: Tilt and Spin
Earth is tilted, like a spinning top leaning to one side. Right now the tilt is about 23.5 degrees. But over about 41,000 years the tilt slowly nods between 22.1 and 24.5 degrees. This is called obliquity. A bigger tilt means hotter summers and colder winters. Earth also wobbles like a slowing top, so it points at different stars over time. This is called precession, and it takes about 26,000 years. Together, tilt and wobble decide how much summer sunshine hits the far north, and that is the key to melting or growing ice.
Ice ages are not really about super cold winters. Snow always falls in winter. What matters is whether the summer is warm enough to melt it. When northern summers stay cool for thousands of years, snow piles up and up, and an ice age begins!
How Slow Is Slow?
These cycles are unbelievably slow. A single obliquity cycle, 41,000 years, is longer than all of written human history. You will never feel a Milankovitch cycle in your lifetime. But if you could speed up time and watch Earth for a million years, you would see ice sheets grow and shrink again and again, like a slow heartbeat, all set by these three gentle wobbles.
Match each Earth cycle to what it changes and how long it takes.
Terms
Definitions
Drag terms onto their definitions, or click a term then click a definition to match.
According to Milankovitch, what really decides if an ice age begins?
Why won't you ever feel a Milankovitch cycle during your life?
Flashcards — click each card to reveal the answer
Be Earth's Wobble
Grab a globe or a ball with a pencil pushed through as the axis. Have a lamp be the Sun. First, tilt the axis a little, then more, to show obliquity, and notice how the top of the ball catches different amounts of light. Then slowly circle the ball around the lamp in an oval path to show eccentricity. Finally, make the tilted axis trace a slow circle in the air for precession. Afterward, draw all three motions on one page and write one sentence explaining how a cooler northern summer could start an ice age. Your deliverable is the labeled drawing plus your sentence.
Want to keep learning?
Sign up for free to access the full curriculum — all subjects, all ages.
Start Learning Free