It is in urban areas today where seventy-five percent of the world’s population lives. most of the land is used for housing, transportation, commercial interests, and manufacturing. one-quarter of each city’s land is still being used for farming. most residents either grow, harvest, or sell crops.

One of the brightest comets of the 20th century was comet hyakutake, which passed close to the sun in early 1996. the orbital period of this comet is estimated to be about 30,000 years. find the semi-major axis of this comet’s orbit. compare it to the average sun-pluto distance and to the distance to alpha centauri, the nearest star to the sun, which is 4.3 light-years distant.