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Ice ages are among the largest changes Earth's climate has seen, with ice sheets spreading across continents and then retreating. Milutin Milankovic (often written Milankovitch) spent his career trying to explain their timing with nothing more than the geometry of Earth's orbit and a great deal of calculation. This post covers his life, the three cycles, how his theory was finally tested, and a worked example you can do with Class 11 physics.
Who was Milutin Milankovic?
NASA's Earth Observatory biography of Milankovitch says he was born in 1879 in the village of Dalj, then in the Austro-Hungarian Empire and now in Croatia. He earned a doctorate in technical sciences from the Vienna Institute of Technology in 1904 and worked briefly as chief engineer for a construction company. In 1909 he took a post in applied mathematics at the University of Belgrade, which he held for the rest of his life. NASA's science article on the cycles gives the year of his death as 1958.
What are the three Milankovic cycles?
NASA describes three slow changes in Earth-Sun geometry:
Eccentricity, the shape of the orbit, which varies from nearly circular to slightly oval in a cycle of about 100,000 years, mainly because of the pull of Jupiter and Saturn.
Obliquity, the tilt of Earth's axis, which has varied between 22.1 and 24.5 degrees over the last million years, in a cycle of about 41,000 years. More tilt means stronger seasons.
Precession, the slow wobble of the axis, which combines with the turning of the orbit itself into an overall cycle of about 23,000 years on average.
These change where and when sunlight arrives more than how much arrives in total. NASA notes that the change in global yearly sunlight from eccentricity is very small.
How did he turn this into a theory of ice ages?
The Earth Observatory explains that Milankovic built a mathematical model of sunlight at different latitudes and the matching surface temperatures for 600,000 years before 1800. On the suggestion of the climatologist Vladimir Koppen, he focused on summer sunlight at 65 degrees north. His reasoning: ice sheets grow when summers are too cool to melt the winter snow.
How was his theory finally tested?
For about 50 years, the Earth Observatory says, the theory was largely ignored. NASA's article adds that serious attention came about ten years after his death. The test came from the ocean floor. In Variations in the Earth's Orbit: Pacemaker of the Ice Ages (1976), Hays, Imbrie and Shackleton studied 450,000 years of climate recorded in Southern Hemisphere sea-floor sediments. They found climate variation concentrated at periods of 23,000, 42,000 and about 100,000 years, matching the orbital cycles. They concluded that changes in Earth's orbital geometry are the fundamental cause of the succession of Quaternary ice ages. They also noted that the 100,000-year link probably needs the climate to respond in a nonlinear way.
The theory is in your syllabus too. NCERT's chapter World Climate and Climate Change, in Class 11 Fundamentals of Physical Geography, lists "Millankovitch oscillations" among the astronomical causes of climate change.
Worked example: how much does Earth's distance change the sunlight?
Sunlight follows the inverse-square law: intensity is proportional to 1/r². NASA's Earth fact sheet gives perihelion as 147.095 million km and aphelion as 152.100 million km, with an eccentricity of 0.0167.
Step 1: ratio of intensities. I(perihelion) ÷ I(aphelion) = (152.100 ÷ 147.095)² = (1.0340)² ≈ 1.069. So Earth receives about 6.9% more sunlight in early January than in early July. NASA's article gives about 6.8%, the difference coming from rounding.
Step 2: the same result from eccentricity. For an ellipse, r(perihelion) = a(1 − e) and r(aphelion) = a(1 + e). The ratio is [(1 + e) ÷ (1 − e)]² = (1.0167 ÷ 0.9833)² ≈ 1.069. Same answer.
Step 3: run it backwards. NASA says that at the orbit's most oval, about 23% more sunlight arrives at perihelion than at aphelion. So (1 + e) ÷ (1 − e) = √1.23 ≈ 1.109, which gives e = 0.109 ÷ 2.109 ≈ 0.052.
Notice the trap: January is perihelion, yet it is winter in India. Seasons come mainly from tilt, not distance. Our post on Kepler's laws covers elliptical orbits in more depth.
What mistakes do students make with these cycles?
Blaming seasons on distance. Tilt sets the seasons. Distance changes them slightly.
Mixing up the periods. Roughly 100,000 years for eccentricity, 41,000 for tilt, 23,000 for precession.
Thinking the cycles act alone. NASA notes that ice reflects sunlight and causes further cooling, a feedback that amplifies small orbital changes.
What can a student learn from Milankovic?
Milankovic took a vague question, why ice ages come and go, and turned it into calculations he could carry out by hand over many years. He chose one measurable quantity, summer sunlight at 65 degrees north, and followed it through. He did not live to see the 1976 test, but he had made his theory precise enough to be tested.
If you want to revisit orbits, ellipses and the inverse-square law before your next test, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing.
Try it now: calculate how much more sunlight Earth would get at perihelion if the eccentricity were 0.03.
