Date: February 28, 2012
Title: Astronomy Word of the Week : Declination
Podcaster: Christopher Crockett
Description: In this astronomy word of the week podcast, we’ll start to explore the tools that astronomers use to chart the night sky. If you’ve ever wondered how an astronomer knows how to point a telescope exactly where she wants it, then this podcast is for you!
Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.
Sponsor: This episode of the “365 Days of Astronomy” podcast is sponsored in-kind by Astronomers Without Borders. Astronomers Without Borders connects people worldwide through our common passion for astronomy. From small programs to the world’s largest celebration of astronomy, Global Astronomy Month each April, there is something for just about everyone. Along with sharing the same sky, sharing resources with astronomy organizations and schools in developing countries is a core mission. More than 100 countries connect under our motto: One People, One Sky. Find us at www.astronomerswithoutborders.org.
One common question I get when manning a portable telescope on the sidewalks of Flagstaff is: how do you know where to point that thing? Fortunately, the objects I typically show – like Jupiter, the Orion Nebula, or the Pleides – are pretty bright and I can get them by eye or even from memory for some of the fainter objects. But that’s not going to work for the vast number of much more distant and fainter wonders out there. It’s a really good question: if you had to chart the night sky, how would you even begin to make sense of it all? With no landmarks to speak of, mapping the byways of the celestial sphere is a formidable challenge. Without some formalized system, astronomers would have to resort to identifying where stars are with arcane instructions like “see that bright star? no, not that one, the one next to it. now go about a hand’s width to your left and find those three faint stars in a row. follow those until you see the pair of stars and then just to the left is….” such a system would clearly become quickly cumbersome for all but the brightest stars and most routine of observations. So how do astronomers know where to point their telescopes and begin to map the inky blackness of space?
There are actually a number of ways to divide the sky into standardized reference points but the most common is known as the “equatorial coordinate system” which just means that we use the Earth’s equator as a starting point. Imagine that the night sky is actually the inside of a gigantic black sphere that encompasses our planet with the Earth sitting right in its center. The stars can be thought of as points of light attached to the inside of this sphere – a concept not far removed from how the heavens were once thought to actually work. Now imagine a ridiculously long pencil with its eraser end firmly rooted on the Earth somewhere along the equator and the lead touching the celestial sphere. As you spun the Earth around, the pencil would trace out a line on the inside of the sphere, a line that would be directly over the Earth’s equator. This line on the sky is what astronomer’s refer to as the celestial equator. It’s simply an abstract extension of the Earth’s equator into space, neatly dividing the celestial sphere into two halves: a northern half and a southern one. If you could actually see this line on the sky, it would appear quite different depending on where you were on the Earth. Standing at the Earth’s equator, you would see a line rising up from the eastern horizon and going right up over your head and sinking back down in the west. Were you to travel a bit north to say somewhere in Europe or North America, the line would still be anchored to the eastern and western horizons, but it would arch up in the sky to the south of you. As you moved further north, the line would get lower and lower in the southern sky until, standing at the north pole you wouldn’t see the line at all. It would completely encircle you, sitting right on the horizon. But now, standing at the North Pole, you have another reference mark to add to the sky. If you could now add a dot to the celestial sphere directly over your head, you would mark what astronomers call the North Celestial Pole. It is an imaginary point in the sky that sits directly over the Earth’s spin axis. If you traveled to the South Pole and did the same thing you would mark the South Celetstial Pole.
So now we have three reference points in the sky: the Celestial Equator and the two Celestial Poles. To locate how far north or south a star is, we can further divide up the sky in a manner exactly analagous to lines of latitude on the Earth. Much like latitude tells you how many degrees north or south of the equator you are on our planet, these celestial latitude lines measure how many degrees north or south a star is from the celestial equator. That number is what astronomers call a star’s “declination”. By convention, stars north of the equator have a postive declination while those to the south have a negative number.
To precisely measure a star’s declination, astronomers divide the sky between the equator and each pole into ninety degrees, just like lines of latitude. A star with a declination of zero degrees sits exactly on the celestial equator. A star with a declination of ninety degrees sits right over the Earth’s north pole and one with 45 degrees sits halfway between the two. To be more precise, each degree is divided into sixty increments called arcminutes and each arcminute is further divided into sixty arcseconds. This lets us be incredibly precise about measuring the positions of objects in the sky. To give you some idea of the size of these numbers, your fist held at arm’s length covers about ten degrees on the sky. The full moon subtends roughly half a degree. An arcminute is the same as width of your index finger seen from about 70 meters away; an arcsecond is that same finger at a distance of over four kilometers.
Knowing a star’s declination not only tells you where it is in the sky, but can tell you something about what’s visible from different vantage points on the Earth. Stars with declinations close to your latitude will always appear to pass high in the sky at some point during the year. For example, the brilliant red star in Orion’s shoulder – a supergiant named Betelgeuse – has a declination of +7 degrees, 24 arcminutes, 25 arcseconds. Observers at a latitude of near 7 degrees north will see Betelguese pass high overhead in the early evening of late winter. Those further to the north or south will see Orion’s shoulder sitting closer to the horizon. This bit of knowledge can also be helpful for navigators at sea. Since a star’s declination is the same as the latitude that it passes over simply looking up can tell you how far north or south you are. Armed with a detailed star atlas, identifying what stars are passing overhead and then looking up those stars’ declinations will let you determine your latitude.
While this may seem like a pretty simple and straightforward way of locating stars on the sky, it’s actually fraught with some subtle complications. One problem is that where the earth’s equator intersects the celestial sphere is not always the same! Our planet does more than simply spin on its axis and orbit around the Sun. Like a spinning top that has started to slow down, our planet also wobbles. That is, where the north pole of our Earth points changes over time, slowly tracing out an enormous circle in the sky once every 26,000 years. Astronomers refer to this as “precession”. This has a number of effects not the least of which is our supposedly fixed reference system – which is tied to the Earth – is always drifting across the sky. The change is small and completely unnoticeable to the casual observer over many human lifetimes. But at the precision astronomers need to find distant celestial beacons, it makes a huge difference. So astronomers have come up with a fix and that is to tie the coordinate system to a specific year – referred to as the ‘epoch’ of the coordinates – and then adjust the coordinates depending on how long it has been since that epoch. The most popular epoch in use for current celestial coordinates is the year 2000. That is, when looking up a star’s coordinates, you will generally find what the star’s coordinates were 12 years ago. One then needs to make a small mathematical adjustment to account for the amount the Earth has precessed since then. Fortunately, computers are fantastic at doing that for us.
Having a coordinate system is a great first step, but it’s not much help if you can’t locate all these coordinates on the sky with ease. Fortunately, telescopes come equipped to help with that job. Amateur telescopes – and some older professional research observatories – come equipped with what are called ‘setting circles’. This is just a circle around each axis of the telescope marking off degrees in declination, for example. When properly calibrated and aligned, all one needs to do is move the telescope until a smaller pointer is aligned with the declination you’re seeking and you’ll be pretty close to where you want your telescope to point. Modern research telescopes come equipped with advanced computer algorithms which take care of all this messiness behind the scenes. Sitting at a computer console in a warm control room, the astronomer enters the coordinates of there star, galaxy, or nebula that she wants to observe and then presses a button and lets the computer figure it all out. These algorithms take into account the exact location of the observatory and the current time to figure out where those coordinates are in the sky and then translates that into signals which instruct the drive system how far north/east/south/west to move the telescope taking into account subtlties like the precession of the earth’s axis, atmospheric refraction, and even the slight flexure in the telescope tube as it swings around in the dome to precisely point a couple hundred ton machine to an accuracy akin to the thickness of a human hair seen from 20 meters away.
Now, declination is only one half of the puzzle. To locate a star in the sky you actually need two numbers. Declination tells you how far north or south that star will be. To figure out east and west, well, that sounds like a good topic for a later word of the week podcast….
End of podcast:
365 Days of Astronomy
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