Time is measured by the revolution of the Earth and its path as it orbits the Sun.
Ancient astronomers built rock circles, towers and other structures to follow the Sun's seasonal path and the march of the constellations in the night sky.
Stonehenge in England is thought to be an astronomical structure that tells the seasons as the Sun lines up with huge rocks set in a circle. Click on image or here for a larger view of Stonehenge.
Click on image for a detail view.
Medieval inventors built mechanical clocks to keep track of time. Electronic digital clocks that measure time by the vibrations of quartz crystals are accurate to a few seconds a year. As our instruments improve scientists are able to more accurately measure time. We can now calculate time down to billionths of a second!
Greenwich Mean Time (GMT) or Universal Time, Coordinated (UTC) is a standard for setting time zones. It is based on an imaginary line running through Greenwich, England, with other lines marking each hour East and West around the globe. Eniwetok, in the Marshall Islands in the Pacific, is halfway around the Earth so its time can be calculated by subtracting 12 from GMT. When the sun is high overhead at noon in Greenwich it is midnight in Eniwetok. New York City is 5 hours East of Greenwich so it is "behind" by 5 hours or GMT - 5. Wellington, New Zealand, is 12 hours "ahead" so the new day starts in that timezone, continues around the world and ends in Eniwetok's timezone.
The times below are given in "24 hour" format so that there is no confusion with a.m. (morning) or p.m. (afternoon). For numbers larger than 12 just subtract 12:00 from the time to find out what "12 hour" time is: 14:00 is 2:00 p.m. (14 - 12 = 2). Look for your time zone below by comparing the Local Time to the list of time zones. The times are based on your computer system's clock. See links below to the US Naval Observatory to see if your clock is set to the standard. Remember, Daylight Savings Time will adjust the calculation "ahead" by one hour.
The US Naval Observatory is charged with keeping standard time. They also calculate astronomical events like moon phases, eclipses, tides and orbits of the planets. Explore the USNO website in a new window by clicking on the links below. Return to Kid's Cosmos by closing the window.
Astronomers use a similar coordinate system to map stars. A series of lines make up the equatorial coordinate system. East-West positions, called Right Ascension, measured in hours, minutes and seconds follow the longitude lines on Earth. North-South positions are indicated by Declination, the number of degrees from the Equator (0 degrees) to the North Pole (90 degrees) or South Pole (-90 degrees). Research the Celestial Sphere for more information on this topic.
What Time Is It?
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