How can we tell someone else where something is located in the sky?
We have solved that problem on Earth by creating the coordinate system of longitude and latitude; can we use a similar system in the sky? The answer is yes, and we do it with right ascension and declination. I have alluded to this system in earlier columns; today I’ll explain how it works.
Let’s imagine a large hollow ball with the earth at the center and the stars projected onto the inside surface of this ball. This is what astronomers call the “celestial sphere.”
Next, we can project some references onto the sphere. If we project the Earth’s axis of rotation into the sphere, the point above the Earth’s north pole is the north celestial pole, while the point above the Earth’s south pole is the south celestial pole. We can also project the Earth’s equator onto the sphere, which becomes the celestial equator.
As a result of the Earth’s orbiting the Sun, the Sun appears to move across the celestial sphere during the course of the year. This apparent annual path is called the ecliptic. Because the Earth’s rotational axis is tilted with respect to the plane of the orbit, half of the ecliptic is north of the celestial equator and half is south.
On about March 21 each year, the Sun crosses the celestial equator from south to north. The point on the celestial sphere where the ecliptic and celestial equator cross is called the vernal (spring) equinox. This year it will occur March 20 at 4:03 a.m. PDT — the first day of spring. The same thing happens around Sept. 21, except that the Sun crosses from north to south — the first day of autumn.
Using these references, we can define a coordinate system to specify the position of any object on the celestial sphere. The most common system used by astronomers is the system of right ascension and declination.
This system is similar to the system of longitude and latitude used to identify a position on the Earth; right ascension is the celestial version of longitude, while declination corresponds to latitude.
The declination of an object is the angular distance between that object and the celestial equator. It is measured in degrees, arcminutes, and arcseconds. It is a positive value if north and a negative value if south. An object on the celestial equator has a declination of 0 degrees, while one at the north celestial pole has a value of 90 degrees.
The right ascension of an object is the eastward angular distance to the object from the vernal equinox. This is analogous to our definition of longitude, which is the east-west angular distance of a location from the zero longitude line that passes through Greenwich, England.
Astronomers measure right ascension in time units of hours, minutes and seconds. The right ascension of an object is the amount of time required for the celestial sphere to rotate from the zero hour line of right ascension, the one that passes through the vernal equinox, to the line of right ascension that passes through the object.
However, the time used to measure right ascension is not mean solar time, the time on your wristwatch, but sidereal time. Mean solar time is based on the Sun, but sidereal time is based on the stars. Midnight local sidereal time is when the vernal equinox is due south at your location, and a sidereal day is the time it takes for the celestial sphere to rotate until the vernal equinox returns to a due south position. The sidereal day is about 3.5 minutes shorter than the mean solar day.
With this system I can tell you that tonight the planet Saturn will be at right ascension 14 hours 39 minutes 12 seconds with declination minus 12 degrees 48 minutes 51 seconds, or that the Andromeda Galaxy is found at right ascension 0 hours 42 minutes 42 seconds with declination of plus 41 degrees 16 minutes 0 seconds.
And that’s how astronomers map the sky.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.