Time has been a source of angst and fascination to man since the days of the sundial and the hour glass, but not until we started to travel over great distances has standardised time been an issue.  In the days of the sundial we relied on Local Apparent Time (LAT) which based its calibration on the sun being directly overhead meaning it was now noon at that specific geographical location.  Problem was that the sun can run up to 16 minutes fast or as much as 14 minutes slow depending on the season not to mention that the sun would reach noon at different times from village to village.  Once accurate mechanical clocks were developed this would mean that this form of calibration would not be consistent.

When ships started to travel great distances they needed not only an accurate timepiece (chronometer), they also needed a reference to a time at a single geographical location at noon.  This was important as they could compare the time of noon at their current location with the time of noon at this fixed geographical point, thus giving them their longitude.  This fixed geographical point was established at the observatory in Greenwich, England (Greenwich Mean Time).

In the 1800's, North America was being settled and local time was a matter of each town using some form of local solar time (like LAT) that was maintained by a single central timepiece located some where prominent in the town (i.e. church steeple, or a jeweller's window).  Everything was going fine until the railways started to criss-cross the continent at great speeds.  How could you catch a train that was scheduled to arrive at noon in Upthecreek, Alberta if the train was running on a clock that was set for noon in Downthecreek, Ontario?  Even travel within a single Province meant that arrival times could vary by an hour or so.  The solution came in 1883 with the advent of Standard Time which divided the continents into several time zones. This meant that each area within that timezone (from North to South)would be at approximately the same time.

In astronomy, time becomes very important as celestial events and positions will vary with the time and the season.  The universe has its own clock that runs independent of our crude mechanical timepieces and if we are to observe, and communicate our observations another method of timekeeping is required.

Sidereal time is where we get our celestial co-ordinates from that allows us to generate star charts and such.  Since the rotation of the Earth and our rotation around the Sun exists, our view of the constellations change from season to season relative to our position in the solar system.  Sidereal time is the right ascension of stars on your local meridian (midpoint or highest point of that star's travel across the horizon)at any moment.  Because of the above mentioned rotations, Sidereal time runs 4 minutes a day faster (0.986° angle of observed difference) than our standard measurements of time.  Astronomers can use this to tell which constellations are on the meridian at a given point in time, and thus determine which star chart to use.  For example if  you have a clock that is running fast at a rate of 4 minutes a day, and that clock reads13:00, then the constellation Virgo(RA= 13h00m) is at the meridian.  This is all academic today as most amateurs look up at the sky and choose their charts, or use astronomy software that will predict times and locations of various celestial objects.

What most astronomers will find useful in their everyday observing is Co-ordinated Universal Time (UTC).  Without getting into the differences between Universal Time which would be insignificant to the amateur, UTC means that this is the position of a given celestial object as it is passing over the meridian of Greenwich.   Thus, for our purposes, UTC is the same as saying GMT.  What UTC allows us to do is to record our observations in a single time frame so that any astronomer any where in the world knows at what time you observed a celestial object.  For example, on my web page I can give best viewing times for a certain object in UTC, and know that an astronomer looking at the page at the other end of the country can convert that time into their local time.  Below you will find a table that will allow you to convert UTC into your own time zone.

Thus, 11:00 UTC would be the same as 5:00pm CST.  You must also take into account the date as 01:00 UTC on May 2 is the same as 7:00pm CST on May 1.  It is a new day in Greenwich, England while it is still the day before here.  Also, UTC is expressed in the 24 hour time system so  00:00 is midnight, 03:00 is 3am, 12:00 is noon, 15:00 is 3pm, 18:00 is 6pm, and 21:00 is 9pm.  Takes a bit of getting used to, but with practice you will be able to convert UTC to your local time and exchange observations in a universal format.