Sketching Technique: Establishing Cardinal Directions

The four cardinal directions or cardinal points are the compass directions of north, south, east, and west.

Establishing these directions through the eyepiece is not always intuitive. The type of telescope you use, where it is pointed, and what angle the eyepiece extends from the scope all effect the position of these directions in the eyepiece.

The quickest way to determine directions is to place a prominent star in the center of the view, turn off the tracking motor if you have one, and allow that star to drift out. The edge of the field where it exits is your WEST point. Now all you have to do is determine north or south and the other two directions will fall in place. To do this, look through the eyepiece and nudge or skew your scope toward Polaris. The direction that stars ENTER the eyepiece will be NORTH, and this can be marked 90 degrees from WEST. (If you are in the southern hemisphere, nudge generally toward the south celestial pole, and stars will enter from the SOUTH.)

If you are using a telescope with an even number of mirror reflections, such as a Newtonian (2 reflections) or a refractor with no mirror diagonal (0 reflections), NORTH will appear 90 degrees counterclockwise from WEST. If you have an odd number of reflections, such as an SCT with a mirror diagonal (3 reflections) or a refractor with a mirror diagonal (1 reflection), then NORTH will appear 90 degrees clockwise from WEST.

Once you know which side of west the north point appears for your scope, you only have to determine west and you will know where the rest of the points fall.

It is always a good idea to mark at least two perpendicular cardinal points on your sketches so that viewers can relate your observation to their own or to other references.