In the era of smart phones with GPS and Google Maps, you're probably already familiar with the system of geographic coordinates that's commonly used to describe locations on the Earth's spherical surface. That system is based on latitude, the distance north or south from the Earth's equator, and longitude, which is the distance east or west of the Prime Meridian, an imaginary line that runs north to south through Greenwich, England. The distances are measured in degrees — 90 degrees in each direction for latitude, and 180 in each direction for longitude — and minutes, seconds and fractions of a second. (For more on how that system works, check out these pages from the U.S. Geological Survey and the IBM Knowledge Center.)
GPS on Earth
That system makes it possible for you to come up with coordinates for everything from the Empire State Building (40°44'55.4"N 73°59'08.5"W, according to Google Maps) to the spot in the desert where U2 shot the cover photo for their 1987 album "The Joshua Tree" (36°19'51.00″N, 117°44'42.88″W, according to the Desert Road Trippin' blog).
Okay, so you knew that. But here's something that you probably didn't know, unless you're an astronomer. It's also possible to describe the location of a celestial object in the night sky, using what basically is an extension of geographic coordinates to create an imaginary sphere surrounding the Earth.
Coordinates in the Sky
"The purpose is to be able to uniquely specify a location in the sky. It's just like latitude and longitude on Earth," Rick Fienberg, press officer for the American Astronomical Society, explains in an email. "If you tell someone to meet you in Littleton, Colorado, at 39°36'47.9484''N, 105°0'59.9292''W they will not only know which town you're in but also which street corner you're waiting at.... It wouldn't be very useful to someone else to know only which town you're in — you need to be more precise if you have any hope of being found by the other person."
Similarly, he explains, if an astronomer discovers a supernova or asteroid and wants others to observe it, providing the celestial coordinates is a way to make sure that everyone is looking at the same thing.
It's a system that's been around since ancient times. "The idea of celestial coordinates does assume that the sky is a sphere that surrounds the Earth. This is the idea that traces back to the early belief that the Earth was the center of everything," Christopher Palma, a teaching professor and associate head for undergraduate programs in the Department of Astronomy and Physics at the Pennsylvania State University, says in an email. "Even though we now know that to not be true, it is still true that the sky appears to be a sphere surrounding us, so we can use spherical coordinates to identify any location on the sky."
Declination and Ascension
That said, the celestial coordinates system has some differences. Instead of latitude, for example, it uses something called declination to describe the distance north or south of the celestial equator, and instead of longitude, right ascension describes the east-west orientation.
"Like any coordinate system, it needs a zero point/calibration," Palma explains. "For celestial coordinates, we project the Earth's equator onto the sky, and so just like latitude measures degrees north or south of Earth's equator, declination measures an angle north or south of the celestial equator. So, for example, the star Spica, which is very prominent in the southern sky tonight from most locations in the U.S., has a declination of -11 degrees 10 minutes, so it is actually south of the celestial equator.
"For longitude on the Earth, we arbitrarily assigned Greenwich, England as the Prime Meridian," Palma says. "The Prime Meridian for the right ascension system is called 'The First Point of Aries,' and it is defined as the position of the Sun on the sky as it moves from south to north along the Ecliptic" — an imaginary line that denotes the path of the Sun — "and passes through the celestial equator. When the Sun is at that location, it is the vernal (or March) equinox on Earth. Right ascension increases to the East from there. So, a star on the sky that is exactly halfway around the sky from the Sun on the vernal equinox would have a right ascension of 180 degrees."
"Because the sky rotates, though, we don't often use degrees to measure it," Palma continues. "Instead, we express angles in hours. So, 180 degrees equals 12 hours of right ascension. The same star I mentioned above, Spica, has an RA of 13 hours, 25 minutes. Which can be interpreted as the point on the sky that is [13h25m* (180 degrees/12 hours)] = 201.25 degrees on the sky eastward from the location of the Sun on the vernal equinox."
Celestial Navigation for Mariners
As Fienberg explains, astronomers aren't the only ones who employ celestial coordinates. "Anyone who navigates by the stars uses them too," he says. "Even though all modern ships and boats have GPS systems onboard, sailors and other mariners are required to learn celestial navigation in case the GPS fails. If you can see Polaris, i.e., the North Star, you'll know which way is north — and, by extension, which ways are south, east, and west too. You'll also know your latitude, as the altitude of Polaris above the horizon is equal to your latitude. And if you have an accurate clock, you can also find your longitude by consulting a table of which stars are due south at the time you're observing."