Directions? Don't need 'em

Jack Kramer

It's commonly known that when traveling in unfamiliar places, men won't ask for directions. And for us amateur astronomers that's fine - there really are none of the usual directions in space. The familiar conventions of north, south, east, and west are Earth-centered concepts that become meaningless where there's no up or down.

Some habits of thought are hard to break though. I recall old sci-fi shows where a rocket ship is on its way to the far corners of the universe. As it heads out, we see it passing the orbits of Mars, Jupiter, Saturn, etc. But why does it have to head off across the plane of the Solar System if it's not going to a "local" planet? It could just as well shoot off at some other angle with respect to the plane of the Solar System. This is one example of man's need to relate directions in space to something familiar. To be sure, some sort of orientation is necessary to point to things once you're in space. Satellite platforms like the Hubble Space Telescope have to know where to aim, and since it's an extension of its human makers, it needs familiar directions. That can be provided by certain bright stars or the position of the Earth and sun.

While traditional directions are pretty much irrelevant, we observers are still confronted with the application of Earth-oriented directions to other objects in space. It's easy enough to know that Ursa Major is a north circumpolar constellation and that we have to look toward our northern sky to see it. But when you're looking at, let's say, Jupiter through your telescope, in essence you're no longer in an Earthly domain. How do you know whether a cloud belt is in the Jovian north or south hemisphere? (By the way, the residents of Jupiter don't give a hoot.) Add to the confusion the fact that our telescopes do funny things to directions, like turning everything upside down and left to right (when there's an even number of reflections), or just flipping them left to right (odd number of reflections). Those new to observing sometimes find this unduly vexing. Some disorientation can also occur when we rotate a star diagonal in the focuser of our telescope and we make a bodily shift while the object in view doesn't shift.

As long as we're stuck with Earthly directions, there are a few hints that can help sort them out on another Solar System body. For example, Jupiter's Great Red Spot lies in the south equatorial belt. But if the GRS doesn't happen to be in view, then where is "up" on Jupiter? Uh ... let me think.

Then which pole of Mars or Saturn is currently tipped toward Earth? I always have to look that one up. And farther out, when we're observing a galaxy oriented at an oblique angle, are we looking at it from the "top" or "bottom"? (And do you really care?)

The moon is another case in point. If someone tells us a particular feature is near the northeast limb, where do we look for it? My memory jogger for the moon is the rugged, heavily cratered area around Tycho, known as the "Southern Highlands". The opposite hemisphere is flatter, with more maria. Here we see the crater Plato, so that tells us we're now looking at the moon's northern hemisphere. But which way is lunar east?


To show how arbitrary compass points can be, consider that directions on the moon have actually been changed by us Earthlings. (Again, the residents of Luna don't give a hoot.) Prior to 1961, it was common on lunar atlases to designate east to the left when north is up, as a person in Earth's northern hemisphere would reckon it when looking at the Moon in the sky. With the advent of the Space Age, it became necessary to adopt the orientation used in all mapping. So nowadays east is on the right when the atlas has north up. The orientation of my old 1959 issue of the Lick Observatory photographic atlas of the moon was made obsolete by the stroke of a pen! Time for mental gymnastics. Just remember that the day/night terminator moves from the moon's east toward its west during the course of a lunar month ... like the sun traveling across Earth's sky.

If all this talk about directions in space is making your head spin, imagine you're actually out there. We already use that vernacular. For example, recently an LCAS member was asked what he was looking at during an observing session; his answer was "I'm in Pisces".

I say don't fret about directions. Instead, simply enjoy observing freed from the shackles of an Earth-bound orientation. Space can be very liberating.

Published in the January 2009 issue of the NightTimes