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Universal Time - Windows to the Universe

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Time zones in North America. The eastern part of the United States is offset by 5 hours from Universal Time. The central part of the USA and most of Mexico is 6 hours different from UT, while the West Coast is 8 hours different.
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Universal Time

When it is noon where you live, it is midnight on the opposite side of the world. Usually when we think of time, we mean "the time of day where I live". If we say something happened at 9 AM, we mean it was morning a while after sunrise (in most places in most seasons). If we plan to meet someone at 6 PM, it might be for dinner in the evening more-or-less around sunset (once again, depending on the season and the latitude of our location). If someone tells you they did something around 11 PM, you assume it was late at night a little before bedtime.

However, there are times when something happens that isn't related to the time of day in a certain place. This is often true for astronomical events like eclipses or meteor showers. Astronomers use a special time scale in these cases called Universal Time (UT). People around the world can convert a time expressed as Universal Time to their own local time zone. Sometimes that time will be in the morning, or maybe late at night. Sometimes it will even be on the next day!

Universal Time is based on the time of day at the Royal Greenwich Observatory in Greenwich, England. If you learned that the peak activity of a meteor shower was going to happen at noon UT, you would know that the Sun would be high in the sky if you were in England and that you wouldn't be able to spot most meteors. If you were in the Americas, the local time would be sometime in the morning... so the meteor shower might be visible from your location if the Sun hadn't risen yet (for example, in California). In China, it would be evening... a pretty good time for viewing meteors. People in Siberia and New Zealand would be in prime position to spot meteors, since it would be midnight there.

Last modified February 3, 2010 by Randy Russell.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF