Abstract
The chief variations in upper-atmosphere density are described, and it is shown how each of them affects the orbits of close Earth satellites. The main features discussed are: 1. the irregular variations in density dependent on solar activity, which may cause great changes in a few days, as well as a 27-day recurrence tendency; 2. the day-to-night and semi-annual variations in density, which cause nearly-periodic changes in orbits: for the day-to-night variation the period is usually between 2 months and 2 years, while the semi-annual effect leads to maxima of density in April and October, and minima in January and July; and 3. the large changes in density during a sunspot cycle, which give rise to a 10-or 11 -year recurrence tendency.
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