day). On a daily basis, temperatures peak at local solar noon and reach a minimum at local midnight. This is similar to the effect in Earth's deserts, only much more pronounced. The axial tilt and eccentricity of Earth (or Mars) are by no means fixed, but rather vary due to gravitational perturbations from other planets in the Solar System on a timescale of tens of thousands or hundreds of thousands of years. Thus, for example Earth's eccentricity of about 1% regularly fluctuates and can increase up to 6%, and at some point in the distant future the Earth will also have to deal with the calendrical implications of seasons of widely differing length and the major climate disruptions that go along with it. Aside from the eccentricity, the Earth's axial tilt can also vary from 21.5° to 24.5°, and the length of this "obliquity cycle" is 41,000 years. These and other similar cyclical changes are thought to be responsible for ice ages (see Milankovitch cycles). By contrast, the obliquity cycle for Mars is much more extreme: from 15° to 35° over a 124,000-year cycle. Some recent studies even suggest that over tens of millions of years, the swing may be as much as 0° to 60°. Earth's large Moon apparently plays an important role in keeping Earth's axial tilt within reasonable bounds; Mars has no such stabilizing influence and its axial tilt can vary more chaotically. The color of the sky The normal hue of the sky during the daytime can vary from a pinkish-red to a yellow-brown “butterscotch” color; however, in the vicinity of the setting or rising sun it is blue. This is the exact opposite of the situation on Earth. On Mars, Rayleigh scattering is usually a very small effect. It is believed that the color of the sky is caused by the presence of 1% by volume of magnetite in the dust particles. Twilight lasts a long time after the Sun has set and before it rises, because of all the dust in Mars' atmosphere. At times, the Martian sky takes on a violet color, due to scattering of light by very small water ice particles in clouds. Generating accurate true-color images of Mars's surface is surprisingly complicated. There is much variation in the color of the sky as reproduced in published images; many of those images, however, are using filters to maximize the scientific value