Due to variations in the Earth's orbit, the amount of insolation varies; there are periods of around 21,000, 40,000, 100,000, and 400,000 years. Variations in the amount of solar energy drive changes in the climate of the Earth, and are recognised as a key factor in the timing of start and termination of glaciations. Looking at strata, isotope analysis can be performed. One of the most precise methods consist in using Oxygen-18. The amount of oxygen 18 mainly depends on the amount of ice that can be found and on the temperature of the planet. 