Which Way is Up? - Directions in the Eyepiece
by Jeff Polston (December 5, 1997)
You look through the eyepiece of your telescope. You're amazed at the detail you are seeing on Jupiter. Belts, festoons, and knots all show up in such clarity. What's that? A tiny moon shadow is making a transit across the Jovian disk! Your observing partner is having trouble seeing it in his telescope. "Where?", he frantically asks. "In the top left corner", you reply. He still can't see it. Is it his untrained eye? Is his telescope functioning poorly? Or is it the fact that your directions only applied to your telescope?
Most of the observers I know are not all that concerned about "which way is up" in their telescopes. When they find an object they just observe it. But sometimes you do need to know exactly which direction you are looking in. Maybe a dim object is described as being "north" of a certain star. Someone says "the Great Red Spot is on the eastern edge of Jupiter", but which side is east? If all telescopes and eyepieces were equal, then a relative direction, such as left or right would work. But, as most of us know, not all telescopes are the same.
This article is meant as a simple guide to help you figure out the right directions in your field of view based on what kind of instrument you are using. I've actually printed out the diagrams and keep them with my notebook so I don't have to perform "mental gymnastics" to know which way is north, south, east, and west.
Normal View - let's get our bearings
Go outside, and face due south. Now, which way is which? Well, if you go "up" from south, that will be north, so "up" is north and "down" is south. If you've been paying attention to our "local star", then you know that it rises to your "left", which must be the east. That means it sets to your "right", which must be the west. So, up-down, left-right, is north-south, east-west respectively. I think these are called celestial directions. In any case, these also apply to the views in the eyepiece.
Now, think of the Moon or a planet hanging in the sky in front of you. Let's label the directions as referenced on the surface (sort of like terrestrial). North and south are easy (up and down). East and west can trick you if you are not careful. I actually use a mental image of the Earth to help me. I know the sun comes from across the Atlantic ocean, crosses the United States, and sets on the other side of the Pacific ocean. With the United States in the middle, the Atlantic ocean is on the right, so that must be the east side. The Pacific ocean is on the left, so that must be the west side. So, up-down, left-right on the Moon (or your favorite planet) is north-south, west-east respectively. This is a "normal" view and is illustrated by the image below.
The
directions spelled out (i.e. North, South, etc.) are the directions in the
sky. The directions illustrated by letters (i.e. N, S, etc.) are the
directions or limbs of the planet being observed. The arrow shows the
direction of rotation for the planet. Again, this is a "normal" or
naked eye view. This would also be the view through the eyepiece if
you were using a refractor or Schmidt-Cassegrain telescope with an image
erecting diagonal (sometimes called an amici prism).
Inverted View - the common "astronomical" telescope
When most people think of a telescope, they think of a common refractor. A refractor with a normal eyepiece will give you an inverted view. Basically, you are just seeing everything upside down. This is also the common view through a straight-through finder scope and a Schmidt-Cassegrain (without a diagonal). The inverted view is illustrated by the image below.
Some
people can easily figure out these directions in their head. They just
take the "normal" view and flip it. I sometimes need a little help,
which is why I created this diagram. This is also the view commonly
seen through a reflecting telescope. The image in the reflector is
sometimes not quite upside down though. It is rotated based on which
way the scope is pointing and which way you are looking through it. For
the most part, you can consider it as an inverted view.
Mirror View - diagonals and mental gymnastics
If you take a telescope that normally presents an inverted image, such as a refractor or Schmidt-Cassegrain, and add a diagonal to it to make it easier and more comfortable to use, the resulting image will be a mirror image. Just like in a real mirror, up and down are the same, but left and right have been switched. This can be very confusing if you try to do it in your head (although some people have no problem with it). Again, I like to refer to a diagram. The mirror view is illustrated by the image below.
As you
can see, north and south are still up and down respectively, but east and
west have switched places. This also applies to the directions on the
planet (and its apparent rotation direction).
Conclusion
Applying these three illustrations to your view through your particular telescope, you should be able to figure out "which way is up". I find them especially helpful when observing Jupiter. There were many times when I would glance at Jupiter and see a tiny moon right at the edge of disk. Without knowing which way is which, I wouldn't know whether or not the moon was getting ready to slip behind Jupiter or slide across its face (the moons travel in the same direction as the rotation of Jupiter). Now, with these illustrations in my notebook, I always know "which way is up".