Having spent thousands of hours observing the night sky, I like to think I’m familiar with it and have some degree of ease navigating it. That’s certainly true on a large scale, such as hopping from one constellation to another or searching for bright stars.
But when you’re standing in front of your telescope’s eyepiece, trying to spot a faint galaxy in the distance, it’s easy to get lost. My situation is like knowing your neighborhood well, but trying to find a specific blade of grass somewhere within it. Although the sky is large, objects within it appear small. How can astronomers find them?
The answer is similar to how we move around the planet. We use a set of coordinates very similar to latitude and longitude, except for the sky.
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It makes sense to do this. The Earth is approximately spherical and rotates on its axis. This naturally defines the two points on the planet where the axis of rotation intersects the surface: the north and south poles. You can also define a line that encircles the globe between them. This is called the equator.
You can see this movement reflected in the sky. Looking out from the rotating Earth, we witness the heavens rotating around us once a day. This also defines two points, the celestial north and south poles, which correspond to the Earth’s boundary points. Another way to think about these points in the sky is that if you were standing exactly at the Earth’s North Pole, for example, north would be north. heavenly The pole is directly above, at the zenith. As it happens, a moderately bright star, Alpha Ursa Majoris, is currently near the location, giving it the nickname Polaris.
Midway between the two poles is the celestial equator, marking the boundary between the northern and southern skies. The celestial north pole is 90 degrees, the celestial south pole is -90 degrees, and the celestial south pole is 90 degrees, just as we define their equivalents on the surface of the earth by reflecting the Earth’s coordinate system. Define the equator as 0 degrees.
Longitude is even trickier. Although our rotating planet makes it easy to define north and south, there are no clear landmarks to indicate where to start measuring east and west. Whatever we choose, it must be arbitrary. The zero degree longitude line, called the prime meridian, was chosen in 1884 at a conference to decide how to create a single unified coordinate system for the Earth. Those present voted for the prime meridian to pass through the site of the Royal Observatory in Greenwich, London. Observatories are where astronomers used the positions of stars for timekeeping, as the daily motion of stars mirrors the Earth’s rotation and is a celestial clock that we can measure.
But unlike the longitudes on Earth, we do Empty points are relatively fixed and not arbitrary. It is the intersection of the celestial equator and the ecliptic, and the orbit that the sun takes across the sky as it moves relative to the stars (as our perspective changes as the earth rotates around the sun). These two slightly tilted circles intersect at two points called nodes. Every year, around March 21st, the sun is at one of those nodes, the March equinox. While most people think of the vernal equinox as a time of the year, astronomers think of it as the location in the sky where the sun happens to be at that time. In any case, this is a good reference, and astronomers use it as a zero point for measuring longitude.
To distinguish coordinate systems from latitude and longitude, astronomers instead refer to them as right ascension (RA) and declination (dec). They are used for historical reasons, and this is the usual explanation for old-sounding astronomical terms. There is also a history behind the strange choice of using northern and southern latitudes for right ascension, but not for right ascension. Because of its origins in longitude timekeeping, we measure RA in units of time from 0 to 24 (which also reflects the Earth’s rotation). The numbers increase as you go east.
So how do we use this admittedly ungainly system? A star located precisely on the celestial equator, where it intersects the ecliptic, will have a right ascension of 0 hours and a right ascension of 0 degrees. A star a little further east will have a larger right ascension, and a star on the opposite side of the sky will have a right ascension of 12 hours. The declination to the north is measured in positive degrees, and the declination to the south is measured in negative degrees.
However, these are fairly large units. Because there can be thousands of notable galaxies in one square degree of the sky, to avoid confusion with units of time, we divide the units into smaller units of minutes and seconds, and use arcs and arcseconds. It is also called. this is where things start Really Puzzle: Declination is divided into 60 arc minutes (usually just called arc minutes), and each arc minute is divided into 60 arc seconds (so 1 degree is 3,600 arc seconds). However, for RA, we split each. time Divide time into 60 minutes and 1 minute into 60 seconds.
This creates the frustrating situation that 1 minute of right ascension in the sky is not equal to 1 minute of right ascension. In right ascension there are 24 hours around the sky, but in declination it is 360 degrees, and the difference is 15 times greater. That is, one hour of right ascension is equal to 15 degrees of right ascension, and one minute of right ascension is equal to 15 degrees of right ascension. . Hmm.
At least (deep sigh) at the equator they are equal. But it’s the physical unit of RA that makes things almost unbelievably unbearable. The closer you get to each pole of the sky, the smaller it becomes.. That’s because stars near the celestial north pole draw small circles in the sky as the Earth rotates, while stars near the equator draw much larger circles. To explain that, astronomers understand that the length changes depending on where the unit of right ascension is in the sky, and take that into account in their calculations (math buffs will tell you that the change in length is (depends on the cosine of latitude). . Yes, this is a huge pain. And don’t get me wrong, not many astronomers love it. But we cling to it because we perceive the sky as a sphere. Other coordinate system units might work, but in my opinion they’re even more difficult to use, so you’ll probably end up reverting to the old coordinate system. It is based on a rotating earth and we live here. So, literally, here we are.
That’s how we find objects in the sky. For example, if a galaxy’s coordinates are 16 hours 34 minutes 3.25 seconds in right ascension and -32 degrees 10 minutes 49 seconds in right ascension, astronomers can use that to easily find the galaxy with a telescope. . At least for now.
The subtle wobble of the Earth’s axis means that the coordinate system moves slowly across the sky, so the coordinates of objects always change slightly. To account for this, we declare an era of observation (e.g. 2000) and use coordinates based on that year to account for drift since then. This is critical for highly accurate observations, like most space telescopes, where you can miss your target if you don’t account for wobble. But for more casual observers, such precision is overkill.
Well, if you’ll pardon the expression, thank heavens. This is hard enough. And we haven’t even mentioned other non-geocentric coordinate systems, such as those centered on the Sun or the center of the galaxy. Astronomers use all of these, depending on the type of celestial object they are pursuing. It’s a mess.
But it works. We can measure the sky and communicate the location of our observations to other astronomers around the world so they can see the sky, too. All it takes is a little adjustment.
