Which Way Is
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
Planets, Moons and the Right-hand Rule
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".
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.
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
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:
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.
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
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
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:
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.
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.