The Sky From One Month To The Next


Before we proceed, let me define a couple of convenient terms which I shall be using from now on.  The zenith is that point in the sky which is directly above the observer’s head.  The  meridian is an imaginary line which starts at the northern horizon, passes through Polaris and the zenith, and ends at the southern horizon.  Thus, if you face toward the North Star and look at the place that is directly below it on the horizon, you are looking at the starting point of the meridian.  Conversely, if you then turn around 180 degrees so that you are facing due south, the point on the horizon you are facing is where the zenith ends. 

Also, when observing you should use Local Mean Time.  Local Mean Time is actually the true time of day of your location as determined by the sun.  Time zones are artificial inventions used to keep large masses of people on the same time table; therefore, the time you use in your every day life is not the true time for your location!  When you are under Daylight Savings Time you’re even worse off!  Fortunately,  finding out your Local Mean Time is a matter of doing some simple arithmetic.  As you will see, there is a little more arithmetic to do the first time you figure Local Mean Time than there will be when you figure it later.  After the first time there is a short cut that you can use. 

 First, you need to know the longitude of your location to the nearest degree.  This information may be obtained from an atlas or your local library.  My favorite weather forecasting website gives the latitude and longitude of any location for which you request a forecast.   Find out what the Universal Time is from CNN International (when they announce the time, they always give it in Universal Time), from a short wave radio receiver tuned to station WWV, or from this web site via the Internet:  If any of these sources is unavailable to you, call your local airport. 

Divide your longitude by 15.  This result is the number of hours that you have to add or subtract from Universal Time to obtain your Local Mean Time.  Should your longitude be to the west of Greenwich, England you will subtract.  If you are to the east you will add.   Example:  suppose the longitude of your location is 82 degrees west.  Dividing this number by 15 yields 5.47 or approximately 5¾ hours.  Say the Universal Time is 1 AM.  1 AM minus 5¾ hours is 7:15 PM.  Thus, your Local Mean Time is a quarter past 7 in the evening.

Now to get the information you need for the shortcut that you will use hence forth!  Once you have determined your Local Mean Time, do the following.  Get the “official” time from your clock.  How much later or earlier is your Local Mean Time than your “official” time?   You can use the difference between the two to figure Local Mean Time from now on!  Example: If your “official” time is 7 PM and your Local Mean Time is 7:30 PM, it means that your Local Mean Time is one half  hour ahead of your official time!  Therefore, from now on, you just add one half hour to your “official time” and you have your local time.  IMPORTANT NOTE: If  you did your first time calculation when Daylight Savings Time WAS NOT in effect, and you are now under Daylight Savings Time, subtract an hour from your calculated Local Mean Time.  If  your first time calculation was made when Daylight Savings time WAS in effect and you have switched back to regular “official” time, add an hour to your calculated Local Mean Time.


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