Satellites, weather or not, have become so much a part of life that their proliferation gives new meaning to the romantic injunction of Lorenzo to young Jessica: "Look how the floor of heaven is thick inlaid with patines of bright gold." But there are several ways in which these golden ornaments can be arranged.
Geostationary satellites, as we saw yesterday, are positioned over the equator, with their speed in orbit synchronised exactly with the rate of rotation of the planet. Such craft appear to be fixed in space when viewed from Earth, and have the distinct advantage for the weather forecaster that they gaze down constantly on the same segment of the globe; successive pictures can therefore be combined to form a "movie" of the evolving weather situation. This trick can be achieved only if a satellite is launched so that it ends up 23,000 miles above the equator.
But this type of orbit has its limitations, too. For example, northern countries are viewed by geostationary satellites at a very oblique angle because of the curvature of the earth, and their image, therefore, is distorted. Moreover, owing to its relatively great distance from the ground, temperature values and other data from such a satellite are less accurate than scientists would like.
Polar orbiting satellites, on the other hand, cope with both these difficulties. They travel around the globe from pole to pole, following, as it were, the lines of longitude, and are only 500 miles or so above the surface. They also photograph the whole globe, bit by bit, from directly overhead as the Earth revolves on its axis underneath them.
But scientists have been even more ingenious. It is convenient if polar-orbiting satellites are deployed so that each time the satellite comes around it crosses the equator on one side of the Earth at noon, local time, providing broad daylight for its images. This will happen if the orbital plane of the satellite is orientated so that it intersects the sun - if the satellite is in a position, once in each orbit, such that if it were big enough, it would eclipse the sun.
This is not as simple as it seems. It must be remembered that the Earth and its satellites go around the sun once every 12 months, so for the satellite to retain its correct relationship with the sun, its orbital plane must rotate, or precess, through 360 degrees in the course of a year. This can be achieved if the satellite does not pass exactly over the poles, but only nearly so - resulting in what space scientists call a "near-polar sun-synchronous orbit".