Do You Know the Way to Aldebaran?

How on Earth does Astropotamus figure out where to point the Time Machine? It’s a little bit science, a little bit wanderlust, and a little bit luck. Sometimes, it’s just plain random.

If you look at a globe, you’ll see lines marking longitude and latitude. Longitude tells you how far east or west of a central point you are while latitude tells you how far north or south of the equator (which is an imaginary line around the middle of the Earth) you are. That central point by the way, is at the Royal Observatory in Greenwich, England. Using lat/lon coordinates, you can find any point on the surface of the Earth. That’s fine if you’re standing in the heavens looking down at the Earth, but what about if you’re on the Earth looking up at the heavens?

There is a similar coordinate system for the sky called the Equatorial Coordinate System. This uses two measurements called right ascension and declination, (usually written as RA/dec) just like the global uses lat and lon. RA is how far right you are of some central point, and declination is how far north or south you are of some line called the celestial equator. Sounds very similar to lat/lon, doesn’t it?

Similar to longitude, RA is measured from a central point, known as the first point of Aries (the constellation). It really doesn’t matter where this is in space, just like it really doesn’t matter where the Royal Observatory is on Earth, just so long as everyone who uses these coordinates knows where to start, they can figure out how far away from that point something is. RA is measured in hours, rather than degrees. This is called sidereal time. Since the stars move 360 in 24 hours, there are 15 degrees of sky in each hour of sidereal time, or 15 arc minutes in each minute of sidereal time, or 15 arc seconds of each second of sidereal time. We don’t really need to know this exactly, we just need to know that star charts and other astronomical tools tell us that something is at RA X hours, Y minutes, and Z seconds. We’ll come back to this later.

Declination is similar to terrestrial latitude. The celestial equator is simply the Earth’s equator projected out into space. So if you imagine an infinitely long and wide, but very thin razor slicing through the Earth at the equator, the surface of this razor would be the same as the celestial equator. Every star is at some (mostly unchanging) location above or below this celestial equator. Stars at X degrees of declination appear overhead when you are at X degrees latitude. If you are at 40 degrees north of the Earth’s equator (40 degrees north latitude), then you might be in Philadelphia. If you’re in Philadelphia and look at a star that has 40 degrees declination (good luck finding a dark spot in Philly!), then the star would be just about directly overhead.

So if we know one object in the sky, and we know it’s RA and declination, then we can turn the Time Machine to point at that object and adjust some dials on the Time Machine so that they show the RA and declination of that object. If we want to point at something else, now all we have to do is move the Time Machine so that the RA and declination on the dials match the thing we want to find. It’s the same thing as saying

If you start at the Royal Observatory in Greenwhich, and walk due south until you find the equator, you know you are at zero degrees north, zero degrees west. From here, you can go north 40 degrees and west 75 degrees and you’ll be just on the New Jersey side of the Schuylkill River from Philadelphia.

So now we know how to find something else if we’ve found something to start with. Luckily, in the northern hemisphere, we have a star that’s just about at the north celestial pole. It’s called Polaris, and it’s in the constellation Canis Minor. Why is this star important? Well, if we point the Time Machine at it (actually, just a wee bit off, but we’re ignoring that) then we’ve set up the Time Machine so that it rotates in right ascension along with the sky. That means that if we hook up an electric motor to the base of the Time Machine, and that motor turns once every 24 hours, then once you’ve put something in the viewfinder of the Time Machine, it will stay there all night long. Or until the sun comes up and you can’t see stars anymore. Or, I uppose, until the Time Machine bangs into its mount support and crashes down around you. Either way, you’re done for the night.

This makes it fun to talk about, and it sounds really easy, but remember, the farther away something is, not only the farther back in time it is, but the more a small change in RA or declination will move the object away from its place in the Time Machine viewfinder. So it’s always good to have a backup system. It’s called star hopping, and I’ll tell you about that next time.

French Navisphere
French Navisphere from User Chevassu on fr.wikipedia