Which Way Is Up?

Mike Swanson

As most folks know, in space there is no true up or down (and since there is no air, sound doesn't carry, so Darth Vader's tie-fighter doesn't make cool "whoosh" sounds).  But, since most folks are down-to-earth with their feet firmly planted on the ground, astronomers have always felt compelled to establish "directions" in their field.  Here are some of the ups and downs in astronomy.

Planets, Moons and the Right-hand Rule
Long ago, someone discovered that magnetized iron, if allowed to turn freely, would always point in the same general direction.  Besides being a real help to the mapmakers of the time (who made really terrible maps, even with compasses), this was the beginning of establishing which way was "up" for the planet Earth, and by extension, the rest of the planets and even the moons of those planets.  Apparently, all the important places you ever wanted to get to were north of wherever the users of these first compasses lived, because north became "up".

Many hundreds of years later, scientists like Copernicus and Galileo convinced folks that the Earth and the other planets traveled around the Sun.  Adventurers like Columbus and Magellan proved that the Earth was not flat.  This, coupled with experiments by other astronomy-minded scientist proved that the reason for day and night was the rotation of the Earth on its axis.  As telescopes improved and we were able to witness the rotation of the planets Jupiter and Mars, a rule was needed to establish north - enter the Right-hand Rule.

Looking at the rotating object, whether a planet or moon, wrap your right hand around the object with your fingers pointed in the direction of the rotation.  Stick your thumb up like you are hitchhiking and your thumb is pointing north for that object.

The Night Sky and the North Celestial Pole
"North" when referring to objects on the celestial sphere (in the sky) is the direction that points straight at the North Celestial Pole (NCP).  You will recall that Polaris (the North Star) is currently situated within one degree of the NCP, just for our convenience.  For example, here are the directions of north for Capella and Dubhe:

Another way to think of it is that north for a celestial object is the line of right ascension of that object, going in the direction that takes you to the NCP. 

Directions in the celestial sphere must be based on the NCP since terrestrial (Earth-based) directions applied to the sky would change as the Earth rotates.  So, if an astrophoto is printed "north is up", you would hold the photo up against the sky to cover the location of the object and orient the top of the picture so that it is pointed towards Polaris.

The Solar System
Up for the Solar System is pretty easy - if you are outside the Solar System watching all the planets revolving around the Sun, simply apply the right-hand rule.  Point the fingers of your right hand in the direction of travel of the planets and your thumb points north.  Turns out it is the same general direction pointed towards by the North Pole of planet Earth.  So, in other words, if you were very high above the Earth's North Pole, the planets would be traveling counterclockwise from your point of view.

The Galactic Up
Astronomers seem to have gone out of their way to confuse us when they decided which way is up for the Milky Way Galaxy.  Like all spiral galaxies, the Milky Way rotates.  So, you would have thought they would just follow the right-hand rule and keep things simple.  Nope, instead it is just the opposite - the left hand rule applies.  This seems to have been done since that direction in space most closely matches the north of our Solar System.  Hard to say what they have done with directions in other galaxies, especially those that don't rotate properly like irregular dwarf galaxies...  Anyway, since it is unlikely any of us will be traveling outside the Solar System anytime soon, we'll leave this for a future discussion.

The View through a Telescope
And then when you think you've got it all figured out, you have a look through your telescope and nothing seems to make sense.  The problem is various combinations of lenses and mirrors flip, invert and otherwise disorient the view.  The following summarizes:

So, depending upon the type of telescope you are using and what equipment you have mounted, you are most likely to see a mirrored left-to-right or 180-degree inverted view.  Note that the usual straight-through finderscope is the equivalent of a refractor without a diagonal and thus the views are as shown in the sample to the far right.  For this reason, some star atlases are printed with an inverted view, to match the view in such finderscopes.  But, nonetheless, determining directions in the eyepiece can be daunting. 

Here's a trick to take the guesswork out of what your mirrors and lens are producing in the eyepiece.  First, you should understand if your view is like the middle or the right-hand sample above.  Point your scope outdoors during the day to determine which applies.  Then, at night, center the field you are interested in and turn off tracking (if your scope has a motorized drive).  West is the direction all the objects are now traveling.  The "y" in the images above is always pointed west, so north is "above" the word astronomy.   Do not be surprised if the direction of travel is diagonal across the field of view - scopes mounted alt-azimuth or with the eyepiece pointed at an angle away from the direction of the movement of the scope in declination will do just that.

Copyright 2002-2006,
Ryukyu Astronomy Club

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