Hour, a unit for measuring time, defined as either 1/24 of a day or 3600 seconds (60 minutes). Used alone, the word hour refers to clock time, a composite of mean solar time and atomic time. There are other types of hours, such as sidereal hours (9.83 seconds shorter than clock-time hours), but these are primarily of theoretical interest. Scientists, who are most concerned with precise time intervals, tend to use decimal fractions of the various types of days and multiples of seconds instead of hours. (Seconds and days are based on measurable physical phenomena; the hour is derived from seconds and days.)
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Condensation, the formation of a liquid from a vapor or gas. It occurs in the formation of clouds and when cooled, water vapor turns into dew or rain.
Every November, the Leonid meteor shower fills the sky with "shooting stars." This year, the shower is expected to peak twice. Learn how Leonid and other meteor showers work.