As someone who has spent thousands of hours observing the night sky, I like to think that I know it pretty well and can navigate it with some ease. That’s certainly true on a large scale: returning from one constellation to another or searching for bright stars.
But when I’m at the eye of my telescope, struggling to find a faint, distant galaxy, I get lost pretty easily. My situation is like knowing your neighborhood very well, but trying to find a specific blade of grass somewhere. The sky is big, and the objects in it seem small. How can astronomers find them?
The answer is similar to how we navigate on Earth: we use a set of coordinates that are very similar to latitude and longitude, except in the sky.
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It makes sense to do this. It is the earth a sphere approx and rotates on its axis. This naturally determines the two points on the planet where the spin axis crosses the surface: the North Pole and the South Pole. At the halfway point, which we call the equator, we can also define the globe-ginding line.
We see this movement reflected in the sky; As we look out from the rotating Earth, we see the heavens rotating around us once a day. This again defines two points: the north and south celestial poles, equivalent to the points connected to our planet. Another way to think of these points in the sky is, say, if you were standing exactly at the Earth’s North Pole, north heavenly the pole would be directly above, at your peak. As it happens, The medium-bright star Alpha Ursae Minoris happens to be near that spotso he takes the nickname Polaris in honor of the position.
Midway between the two poles is the celestial equator, marking the boundary between the northern and southern sky. Mirroring the Earth’s coordinate system, we define the north celestial pole as 90 degrees, the south celestial pole as –90 degrees, and midway between them the celestial equator as zero degrees, just as we define their equivalents on the Earth’s surface.
Length is more difficult. Our rotating planet easily defines north and south, but there is no obvious marker where to start measuring east and west. What we choose must be arbitrary! The zero-degree line of longitude—called the prime meridian—was chosen in 1884 at a conference dedicated to determining how to create a unified, unified Earth coordinate system. Those present voted that the prime meridian would pass through the site of the Royal Observatory in Greenwich, London—astronomers used the positions of the stars to tell time because the daily motion of the stars mirrors the Earth’s rotation, making it a celestial clock we can measure.
Unlike Earth’s longitude, however, we do it A point in the sky that is relatively fixed and not arbitrary. It is the intersection of the ecliptic of the celestial equator, the sun’s path across the sky as it moves relative to the fixed stars (because our perspective on the sun changes as the Earth revolves around it). These two slightly inclined circles intersect at two points called nodes. Every year, on or around March 21st, the sun is at one of these nodes: the March equinox. While most people think of this equinox as a time of year, astronomers think of it as a place in the sky where the sun happens to be at that time. In any case, this gives us a decent reference, so astronomers use it as a zero point for measuring longitude.
To distinguish their coordinate systems from latitude and longitude, astronomers call them right ascension (RA) and declination (dec). These are used for historical reasons, which is the usual explanation for ancient sounding astronomical slang. History is also behind the unusual choice of using latitude-like degrees north and south for declination but not right ascension.Because of its origins in longitudinal timekeeping, we measure RA in hourly units, again reflecting the Earth’s rotation, with numbers increasing eastward.
So how do we use this admittedly clunky system? A star located exactly on the celestial equator meeting the ecliptic would have an RA of zero hours and a declination of zero degrees. A star slightly to the east would have a higher RA, and one on the opposite side of the sky would have an RA of 12 hours. North declination is measured in positive degrees, and south declination in negative degrees.
They are pretty big units, though. There can be thousands of prominent galaxies within a single square degree in the sky, so we divide the units into smaller minutes and seconds, sometimes called arcminutes and arcseconds, to avoid confusion with time units. Here are the things really Befuddling: One degree of declination is divided into 60 arc minutes (usually called arc minutes), and each arc minute is divided into 60 arc seconds (so there are 3,600 arc seconds in a degree). But for RA, we split each one time in 60 minutes and every minute in 60 seconds!
This leads to the irritating circumstance that one arc minute of celestial declination is not equal to one minute of right ascension! There are 24 hours around the sky in RA, but 360 degrees of declination, a difference that represents a factor of 15. So one hour in RA is equal to 15 degrees, and one minute in RA is equal to 15 minutes of arc. . Phew
At least (big sigh) they level at the equator. But to make things almost unbelievably unbearable, RA’s physical units zoom out to each celestial pole. Because a star near the north celestial pole makes a small circle in the sky as the Earth rotates, one near the equator makes a much larger one. To account for this, astronomers understand that RA units change length depending on where they are in the sky and take this into account in their calculations (for those math lovers out there, the change in length depends on the cosine of the declination). Yes, this is a terrible pain. And make no mistake, it’s not like many astronomers love it! But we are stuck with it because we perceive the sky as a sphere. Other units in the coordinate system might work, but I think they’re even harder to use, so we’d go back to the old system. It’s based on the rotating Earth, and we live here, so we’re literally here.
And that’s how we find objects in the sky. For example, a galaxy might have a coordinate of 16 hours, 34 minutes, and 3.25 seconds in RA and –32 degrees, 10 minutes, and 49 seconds in December, and astronomers can use that to find it easily enough in the telescope—at least for now.
The subtle one that runs on the earth’s axis it means that the coordinate system drifts slowly across the sky, so the coordinates of objects change a little all the time! We account for it by declaring an observation period—for example, the year 2000—and use coordinates based on that year, taking into account the drift since then. This is essential for very precise observations, as with most space telescopes, where you can miss the target if you don’t account for the vortex. But to the casual observer, this accuracy is too much.
Well—if you’ll excuse the expression—thank heavens! This is hard enough. And we haven’t even touched on other less geocentric coordinate systems centered on the sun or the heart of the galaxy. Astronomers use all of them depending on the type of object they are looking for. It’s a mess.
But it works. We can measure the sky and transmit the positions of these observations to other astronomers around the world so they can see them too. It just takes a little coordination.
