¡SkyCaramba! Weekly astronomy blog for the week ending March 16, 2013
For most of us, telling what direction a star or planet is in and what other objects are near it are enough to communicate how to find it. But if you’re making a map of the heavens or making calculations concerning the positions of objects, you need to know exactly where to put each dot. There are two systems that give us coordinates as precise as latitude and longitude for objects on Earth.
One system is the ecliptic coordinate system. Thousands of years ago, people who watched the skies observed that the sun, moon, and planets traveled somewhat closely along a common path. We now 
understand this path to be in the plane of the earth’s ecliptic, or its orbital path around the sun. It’s a coincidence that most of the planets aren’t orbiting at great angles from Earth’s ecliptic. Comets, many asteroids, and Pluto can be very far from the ecliptic at times. But by definition, the sun is always on the ecliptic. That’s the object the earth goes around, so anywhere you draw a line showing the ecliptic must be in the same plane as the sun and Earth.
The entire dome of our sky is half a sphere. It and the other half we can’t see are divided into 360 degrees. Each degree is divided into 60 minutes of arc. And each minute is divided into 60 seconds of arc. Even the seconds can be represented by fractions or decimals for more accuracy still. Astronomers often find it convenient to say where an object is by comparing it to the ecliptic. For example, Jupiter could be 2° south of the ecliptic. To say how far east or west it is, you measure the distance from a north-south line that goes through the point where the sun is at the March equinox. The coordinates are called celestial latitude and celestial longitude.
Another coordinate system is the equatorial system. The imaginary line drawn in the sky for the equatorial system is right over the earth’s equator. Since the earth rotates on a tilted axis, the celestial equator is at an angle to the ecliptic. While the sun is always on the ecliptic, it’s only on the celestial equator twice a year where the two lines intersect. The planets and moon may be on the celestial equator more often, especially in those parts of the sky where the two lines are close.
As with the ecliptic system, the equatorial system is also divided into smaller units for specifying where an object is. It can be divided into degrees. And it always is when you measure how far something is north or south of the celestial equator. But it’s usually divided into hours, minutes, and seconds when you go east of the north-south line that goes through the sun’s March equinox position. That measure is called right ascension. Just like the day, we have 24 hours of 60 minutes each, and those minutes are divided into 60 seconds each. Seconds can be divided even further as needed. The measure of how far north and south the object is from the equator is called declination. A new comet could be found at 18° south and 21h, 42m, 18.2s of right ascension. We don’t give hours and minutes west of the 0 mark.
If the two lines intersect at the March equinox, it shouldn’t be any surprise they also intersect on the opposite side of the sky at the September equinox. On March 20, 2013 at 11:02 UT, the sun will be right at one spot where it’s 0 degrees from the celestial equator and 0 degrees from the ecliptic. It will be at 0 degrees in celestial longitude at the same time. And it will be at 0h, 0m, 0s of right ascension. Then on September 22 at 20:45, it will be at 0 degrees in celestial latitude and declination again. But it will be at 12h, 0m, 0s or right ascension or 180° in celestial longitude.
If you use astronomy software or look at star maps, you could see either kind of coordinate system shown as a grid just like latitude and longitude lines on a map of Earth. I find knowledge of these coordinate systems useful for computing. But if you’re just a casual observer, that’s fine too. You’ll know what someone’s talking about if terms like celestial longitude and right ascension come up in conversation. But the view up there is just as good without worrying about them.
¡SkyCaramba!