“By the way, Cathleen, is there any rhyme or reason to that three-object object‘s funky name? I’ve still got it on Old Reliable here.”
“It’s nothing like funky, Sy, it’s perfectly reasonable and in fact it’s far more informative than a name like ‘Barnard’s Star.’ The ‘PSR‘ part says that the active object, the reason anyone even looked in that system’s direction, is a pulsar.”
“And the numbers?”
“Its location in two parts. Imagine a 24-hour clockface in the Solar Plane. The zero hour points to where the Sun is at the Spring equinox. One o’clock is fifteen degrees east of that, two o’clock is another fifteen degrees eastward and so on until 24 o’clock is back pointing at the Springtime Sun. Got that?”
“Mm, … yeah. It’d be like longitudes around the Earth, except the Earth goes around in a day and this clock looks like it measures a year.”
“Careful there, it has nothing to do with time. It’s just a measure of angle around the celestial equator. It’s called right ascension.”
“How about intermediate angles, like between two and three o’clock?”
“Sixty arc-minutes between hours, sixty arc-seconds between arc-minutes, just like with time. If you need to you can even go to tenths or hundredths of an arc-second, which divide the circle into … 8,640,000 segments.”
“OK, so if that’s like longitudes, I suppose there’s something like latitudes to go with it?”
“Mm-hm, it’s called declination. It runs perpendicular to ascension, from plus-90° up top down to 0° at the clockface to minus-90° at the bottom. Vivian, show Sy Figure 3 from your paper.”“Wait, right ascension in hours-minute-seconds but declination in degrees?”
“Mm-hm. Blame history. People have been studying the stars and writing down their locations for a long time. Some conventions were convenient back in the day and we’re not going to give them up. So anyway, an object’s J designation with 4-digit numbers tells you which of 13 million directions to look to find it. Roughly.”
“That’s what the ‘J‘ is about. If the Earth’s rotation were absolutely steady and if the Sun weren’t careening about a moving galaxy, future astronomers could just look at an object’s angular designation and know exactly where to look to find it again. But it’s not and it does and they won’t. The Earth’s axis of rotation wobbles in at least three different ways, the Sun’s orbit around the galaxy is anything but regular and so on. Specialists in astrometry, who measure things to fractions of an arc-second, keep track of time in more ways than you can imagine so we can calculate future positions. The J-names at least refer back to a specific point in time. Mostly. You want your mind bent, look up epoch some day.”
“Plane and ship navigators care, too, right?”
“Not so much. Earth’s major wobble, for instance, shifts our polar positions only about 40 parts per million per year. A course you plotted last week from here to Easter Island will get you there next month no problem.”
Old Reliable judders in my hand. Old Reliable isn’t supposed to have a vibration function, either. Ask her about interstellar navigation. “Um, how about interstellar navigation?”
“Oh, that’d be a challenge. Once you get away from the Solar System you can’t use the Big Dipper to find the North Star, any of that stuff, because the constellations look different from a different angle. Get a couple dozen lightyears out, you’ve got a whole different sky.”
“So what do you use instead?”
“I suppose you could use pulsars. Each one pings at a unique repetition interval and duty cycle so you could recognize it from any angle. The set of known pulsars would be like landmarks in the galaxy. If you sent out survey ships, like the old-time navigators who mapped the New World, they could add new pulsars to the database. When you go someplace, you just triangulate against the pulsars you see and you know where you are.”
If they happen to point towards you! You only ever see 20% of them. Starquakes and glitches and relativistic distortions mess up the timings. Poor Xian-sheng goes nuts each time we drop out of warp.
~~ Rich Olcott
4 thoughts on “Naming the place and placing the name”
Re: “… it’s called declension”
Check your article’s fourth tag, Rich. 🙂
From a Google on “declension definition”:
(in the grammar of Latin, Greek, and other languages) the variation of the form of a noun, pronoun, or adjective, by which its grammatical case, number, and gender are identified.
a condition of decline or moral deterioration.
“the declension of the new generation”
Thanks for the catch, Richard. Fixed it.
Re: “… runs perpendicular to ascension”
In the context of celestial coordinates, it’s always “right ascension”, never just “ascension”:
As for why it’s _right_ ascension, note what is underlined below.
An old term, right ascension (Latin: ascensio recta) refers to the ascension, or the point on the celestial equator that rises with any celestial object [u]as seen from Earth’s equator, where the celestial equator intersects the horizon at a right angle[/u]. It contrasts with oblique ascension, the point on the celestial equator that rises with any celestial object as seen from most latitudes on Earth, where the celestial equator intersects the horizon at an oblique angle.