directions in space - Moonstick Information Site

Directional Names Used in Outer Space

After all, up and down aren't very useful without a well defined reference frame. On the Earth, we have a well defined reference frame, the Earth itself. All of the rocks, trees, and houses move together, uniformly* accelerating in a direction we call "up". The opposite direction we call "down". Because of the acceleration, you will often find the more dense particles (like dirt and water) in the "down" direction and the less dense particles (like air and clouds) in the up direction; there are exceptions however (like caves). All of the rocks, trees, and houses are (while accelerating) also rotating (as the Earth rotates) about an axis whose counterclockwise end points is a direction that we will for now leave nameless, but the part of this direction that is perpendicular to up and down, we call "north". The opposite direction we call "south". To the left of "north" we call "west". "Which way is left?" you may ask. It is counterclockwise as viewed from "up". "Which way is counterclockwise?" you may ask. Perhaps you should go buy a clock. And opposite of "west" we call "east". Thus the relationship between the terrestrial directions is as follows.

As you can see, something that is quite simple can seem very complicated when you are not already familiar with it.

Beginning again, I will (in the same way) describe the directions in outer space. First, you should realize (and hopefully already do) that the Earth orbits the sun. This simply means that the Earth follows a circular path around the relatively stationary sun. The Earth orbits the sun about an axis whose counterclockwise end points in a direction that we call "ecliptic north". The opposite direction we call "ecliptic south". In addition, the Earth is turning about an axis whose counterclockwise end points is a direction that we will once again leave nameless, but the part of this direction that is perpendicular to "ecliptic north" and "ecliptic south" we will call "summer solstice". The opposite direction we call "winter solstice". Ninety degrees counterclockwise from "summer solstice" as viewed from "ecliptic north" we call "fall equinox"; and the opposite direction we call "spring equinox". Thus the relationship between the celestial directions is as follows.

These words are necessary to describe a three-dimensional model in outer space. For example, the rotational axis of the Earth points 23.4° from ecliptic north toward the summer solstice. The orbital axis of the Earth points directly toward ecliptic north (by definition). The moon orbits the Earth about an axis whose counterclockwise end (in the year 2001) points 5° from ecliptic north toward the spring equinox. In the year 2006, this axis points 5° from ecliptic north toward the winter solstice. If these points are conceptually difficult, don't worry. The important thing you learn from this page is the directional names in the celestial reference frame.