GraphDark Hint - Equation of Time

  The Equation of Time is a good example of how transits can be used, since it is shown by the changing transit time of the Sun. The solar transit time varies by up to 16 minutes during the year because of the combined effects of the eccentricity of the Earth's orbit and the tilt of its axis. This is why accurate sundials need small corrections, the equation of time, to be applied at different times of year. The equation of time is the thick wavy vertical line down the centre of the graph below.

 
   
 
To show this effect, turn on Sun transits using Preferences, Timetable objects and check the Transits box in the Sun column. Because solar transit occurs close to midday you will also need to make sure the display is suitable for this. Go to Preferences, Timetable display and check the Day mode box. While there, it is probably also a good idea to select Horizontal day lines. This will make the changing transit times a little more obvious because the hours are now spread across the width of the screen.

Choose Sun from the list of objects on the input bar. Set a date range of, say, a calendar year and draw the timetable.

Toggling the grid on will make the deviations more obvious, as will using the maximise button to fill the whole screen. Setting a very narrow day width can get a whole year onto one screen, this makes the trend through the year much clearer.

Transits may be useful for other purposes and are indicated by a narrow dash across the day line when an object is on the meridian. For the moon this a white dash, for the Sun it is black. For All Planets the planet colour is used but for other objects a different colour can be set to show up against the object colour.

 

   Richard Fleet 2004

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