As the Earth revolves our Sun, the location of its axis in relation to the Sun varies. As a result, the observed height of our sun over the horizon changes. The Sun is seen to trace a higher route above the horizon in the summer and a lower path in the winter at any given place on Earth. This is because the axis about which it orbits is tilted with respect to the plane of the Earth's orbit around the Sun. The apparent altitude of the Sun is a key factor in determining when plants will flower and when crops will be ready for harvest. Scientists use information on solar elevation to predict when and where there will be enough sunlight to grow crops profitably. They also use this information to determine if an area should be planted in wheat or corn, for example. Elevation data are gathered by government agencies and commercial companies. For example, the National Oceanic and Atmospheric Administration (NOAA) gathers data on solar elevation every hour of each day throughout the year. These data can be found at http://eosweb.larc.nasa.gov/ssevents/. Data on solar elevation are also collected by many satellite services. For example, the NASA-sponsored Solar Dynamics Observatory (SDO) sends back images of the Sun every few hours that show how high it is in the sky at that moment.