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.)
The Smithsonian Institution began with an unexpected donation and has now grown to a massive collection of American artifacts. The Smithsonian is world renowned for its museum and for scientific endeavors. Take this quiz and learn more about the famous Smithsonian Institution.
When a star dies, it becomes a black hole. Once something passes through the it's gone for good, never to be seen or heard from again. If we lived next to a black hole, would it suck us in too?