# Meridian Diagrams

A meridian diagram is a sketch of the sky along the observer's meridian, showing the North Point, the North Celestial Pole (or South Celestial Pole if in the southern hemisphere), the zenith, the celestial equator, and the South Point.  Meridian diagrams are useful for working out the altitude at which an object with a known declination will cross the meridian.  Since the declination of the Sun varies in a predictable way over the course of a year, these diagrams are very useful for tracking the sunshine.  We will talk more about this in Units 4 and 5.

For the following you will need the following two transparencies - click on the link to the right to get the transparencies.  The diagram a the top of the page is a circle, representing your meridian (including the parts you can't see), with the directions to the North Celestial Pole and the celestial equator marked.  The top half of the diagram also has marked on it the declination at 10 degree intervals.

The second diagram at the bottom of the image, again, has a circle for the meridian.  However, on the second diagram the observer's horizon and zenith are marked.

By cutting the transparency in half and overlaying the halves, one can create a meridian diagram for anywhere on Earth.  Here are two examples of what you might see.  The first is for 30 degrees and the one below it is for 60 degrees.

Click here to call up a new page that will have the transparency on it of a graphics (gif) file.  You can print the image to a transparency if your computer is connected to a printer.  Once the page has loaded on your browser, click on "File" and then "Print."  When you are finished, click the "Back" button on your browser to return to this page.

The outer scale is on the "altitude dial".  The inner scale is on the "declination dial" and thus shows where objects of different declinations will cross the meridian.

Here is a simplified, streamlined, meridian diagram that retains the most essential information.  We have included a three dimensional and two dimensional diagram for clarification.  Both mean the same thing.

An observer in this diagram would be standing in the middle of the "field" (the dark area) where all the lines intersect.

The illustration on the right shows the basic meridian diagram.  In order to figure out where celestial objects will cross the meridian, one must first locate the North Celestial Pole -- on the meridian at an altitude equal to your latitude -- and the celestial equator -- 90o away from the North Celestial Pole.  Then, since declination is measured from the celestial equator, if we know the declination of an object we can find its altitude when it crosses the meridian.  This is illustrated below