If you stick around astronomy for long enough, you are certain to come across stellar coordinates or star coordinates. This is a string of letters and numbers used to locate stars in the sky via a star chart or map. At first, these coordinates will look like gibberish but once you understand how they work, they can be used to quickly find and identify objects in the night sky.
If you’ve never come across coordinates before, don’t worry. They are not as common as they used to be. With the rise of powerful astronomy software and smartphone apps, they have become less and less common in casual astronomy. Serious astronomers and professionals will surely know how to use star coordinates but you might not be at that level yet.
What Are Star Coordinates?
Similar to latitude and longitude coordinates used on Earth, star coordinates are a string of descriptive numbers that tell you the location of a star in the night sky compared to some imaginary axes which have been created by people for the express purpose of navigating the night sky.
Just as latitude and longitude are used here on Earth for GPS, geocaching, and military operations, star coordinates can be used to identify specific spots in the night sky. If you have a set of coordinates and know how to use them, you can easily find objects in question.
The coordinates for the star Sirus for example are RA 6h 45m 9s | Dec -16° 42′ 58″. Just like latitude and longitude, these coordinates use a series of degrees, hours, minutes, and seconds to pinpoint exact points in the sky.
So how exactly do you use these coordinates?
Right Ascension and Declination
The coordinates for stars are presented in right ascension (RA) and declination (Dec). Just like latitude and longitude, these two numbers describe a position along two different axes. Declination corresponds to a star’s position along the latitude and right ascension is used to describe a star’s position along longitude.
These lines are taken from the Earth’s own latitude and longitude lines and projected upwards onto the sky. That means that the equator of the Earth is pushed outwards into a ring around the Earth called the celestial equator.
The celestial equator is used to determine a star’s declination. Anything above the celestial equator is considered a northern declination and is represented by a positive number. If we go back to the Sirius example, RA 6h 45m 9s | Dec -16° 42′ 58″, we can see that Sirius has a negative declination, that means it is below the celestial equator, or south of it.
Right ascension is determined by a star’s position in relation to the vernal equinox line. That is an imaginary line derived from the spot in which the sun rises in the horizon on the spring equinox. Since the sun rises in the east, all right ascension coordinates are derived in a position to the east of this vernal equinox line.
This is called out in terms of hours and minutes based on the 24 hour clock. One hour is equal to fifteen degrees (360 divided by 24). This rotates around in a circular pattern that encompasses the sky. Each slice of fifteen degrees can tell you what part of the sky an object is in. If you want to break it down even further you can divide it into minutes and even seconds for extremely precise positioning.
If you are lost, don’t worry. None of this can be effectively utilized without a key. That would be a map, star chart, or computer program.
Using A Program
If you try to parse this out on your own without any sort of guide, you are going to be completely lost. In the past, these coordinates were used in conjunction with star charts or sky maps. Since the Earth is a sphere, these imaginary lines that we are using for coordinates will look different and be positioned differently depending on where you are in the world.
That is also why some stars look different in different places on Earth. The celestial equator is going to be in a different part of the sky up north by the arctic circle than it is around the actual equator. The star’s position in relation to these lines won’t change but your ability to see them might.
That is why, in 2020, it is best to learn and explore stellar coordinates with the use of an astronomy program. There are plenty available out there. There are many phone apps made by companies like Celestron that will help you navigate the night sky. If you download one of these apps, you will notice that many of them will display the sky with these lines present on your screen. That is because these programs use your location and the stellar coordinates of stars to display them on your screen.
There are also powerful computer programs, such as Starry Night, that can do the same thing on your desktop or laptop. Many of them come included with the purchase of a telescope and allow you to print out your own maps.
Using Star Charts
The old fashioned way is to use a star chart. If you get a coordinate for a star you want to see you can use those coordinates to match them up to a physical star map. Be sure to get a map that displays your section of the sky appropriately. With the star map, which maps out these lines for you in the sky using bright familiar objects, you can then navigate your telescope to the spot using the map and the coordinates to focus on a real object.
This is all very advanced stuff, and like geocaching, it is done more so as a hobby to tease your brain rather than any practical use. Technology has come along far enough to make needing to use a star chart and coordinates obsolete. However, you will come across the coordinates during your studies if you are involved with astronomy long enough.
Now, next time you see them, you will know what they are, how to read them and, if you are feeling up to it, will be able to pick out a faint object using the coordinates for yourself.
