1. Tidal forces are periodic variations in gravitational attraction exerted by celestial bodies. 2. Where \ scriptstyle N is the number of periodic variations . 3. In addition there are periodic variations , as Earth moves within the Solar system. 4. Terraces record natural, periodic variations driven by cycles such as the Milankovitch cycle. 5. In this case the periodic variation is a variation in the insertion loss of the network against increasing frequency. 6. Instead, it is perceived as a periodic variation in the amplitude of the first term in the expression above. 7. Unusual periodic variations in the pulsation amplitude were also noticed, similar to the Blazhko effect in RR Lyrae variables. 8. When ultrasonic waves are propagated in a liquid, the density varies from layer to layer due to periodic variation of pressure. 9. A pumped wave excited in the active medium produces a periodic variation of the volumes of the dispersed particles ( gas bubbles ). 10. The small periodic variations in the magnitude of Beta Librae suggest the presence of a companion star which is not directly observable from Earth.
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