Directions in Outer Space
Many of us have a crude map in our head of our country or our state or the continents. If you live in the middle of the USA you probably know that Miami is to your southeast and NYC is to your northeast, without having to know or care what their longitude and latitude coordinates are. What if you could do the same thing for outer space, using these same general directional tools along with some distance values to let you intuitively visualize a simple 3-d map of things such as the interstellar neighborhood, the local group of galaxies, or even neighboring superclusters? Just as you might have an intuitive understanding of where Africa is in relation to other continents, you could also develop an intuitive sense of where Alpha Centauri is in relation to other stars and the sun.
The precise direction of objects in space using the equatorial coordinate system are given by 2 values, the declination (like latitude) and the right ascension (like longitude). There is also the concept of a ‘celestial north’ and a ‘celestial south’ that are directions based on the declination values.
celestial north (declination +90) celestial south (declination -90)
However we could go further and define 2 more outer space directions based on the right ascension value. These are based on an observer outside Earth, for example, on a space station, facing Earth from direction Decl:0; RA:0.
celestial east (RA=6:00) celestial west (RA=18:00)
Now we have celestial north, south, east, and west. But we still need to define 2 more directions because space is 3-d not 2-d. Celestial noon is the direction of the observer’s front, and celestial midnight is the direction to the observer’s back.
celestial noon (RA=noon) celestial midnight (RA=midnight)
Now we could slice the celestial sphere 3 ways into celestial hemispheres, just for clarity.
northern hemisphere = declination+ southern hemisphere = declination- eastern hemisphere = RA 0:00 to 12:00 western hemisphere = RA 12:00 to 0:00 midnight hemisphere = RA 18:00 to 6:00 noon hemisphere = RA 6:00 to 18:00
We can also combine directions to get 8 RA (longitude) directions of 3 hours each.
midnight = RA 22:30 to 1:30 east-midnight = RA 1:30 to 4:30 east = RA 4:30 to 7:30 east-noon = RA 7:30 to 10:30 noon = RA 10:30 to 13:30 west-noon = RA 13:30 to 16:30 west = RA 16:50 to 19:30 west-midnight = RA 19:30 to 22:30
We could combine them with declinations as well to add north/south. e.g.
north = decl: >+60 north-east = decl: +30 to +60; RA 4:30 to 7:30 east = decl: +30 to -30; RA 4:30 to 7:30 south-east = decl: <-30 to >-60; RA 4:30 to 7:30 south = decl: <-60
Now we can start to visualize a little interstellar map in 3-d. In this map, north, south, east, and west are exactly where you think it is. Noon is in front of you, and midnight is behind you. You are an observer facing Earth from Decl:0; RA:0.
Barnard’s Star [RA 17:58; DECL +4] is 57 petameters (6.9ly) to our west Sirius [RA 6:45; DECL -17] is 91 petameters (8.6ly) to our east Wolf 359 [RA 10:59; DECL +7] is 74 petameters (7.86ly) to our noon direction Epsilon Eridani [RA 3:33; DECL -9] is 99 petameters (10.3ly) to our east-midnight direction Procyon [RA 7:39; DECL +5] is 108 petameters (11.4ly) to our east-noon direction Alpha Centauri [RA 14:40; DECL -60] is 41 petameters (4.37ly) to our south-west-noon direction Polaris [RA 2:31; DECL +89] is 3500 petameters (115ly) to our north (not in our neighborhood) The galactic center [RA 17:46; DECL -29] is 236 exameters to our south-west Andromeda [RA 0:43; DECL +41] is 24 zettameters to our north midnight direction