Celestial Coordinates Sphere
Celestial Coordinates Sphere
| Group Name | radec |
| Reference | -- |
| Prepared by | Carter Emmart, Brian Abbott (AMNH/Hayden) |
| Labels | Yes |
| Files | radec.speck |
| Dependencies | none |
| Notes | Also represents Earth's radio sphere (see below). |
The celestial coordinate system is a projection of our Earth-based coordinate system of latitude and longitude onto the “celestial sphere.” The celestial sphere is an imaginary shell that surrounds Earth upon which all objects in the sky lie. Astronomers describe an object's position in the sky by its right ascension (RA) and declination (Dec). Declination is simply a projection of our latitude on Earth. The point directly above the North Pole (the zenith point) is the north celestial pole and is located at +90o declination. If you're standing on Earth's equator, your zenith would lie on the celestial equator.
Right ascension is based on Earth's longitude but is expressed in hours instead of degrees. Astronomers have split the sky into 24 hours (15o per hour) measured from the vernal equinox. An object's location is then described in hours, arcminutes, and arcseconds. For example, the star Sirius in the constellation Canis Major, the brightest star in the sky as seen from Earth, is located at right ascension 6 hours, 46 arcminutes, and -16o 45 minutes declination.
We use arcminutes and arcseconds for right ascension to remind ourselves that the length of these units depends on your declination. Close to the pole, an hour of right ascension will be quite small, while at the equator, it will be at its maximum. As with lines of longitude on Earth, lines of right ascension are not parallel with one another.
Drawing the Sphere
From Earth's perspective, the wire-frame sphere perfectly represents the celestial coordinates. If you examine the radec.speck file, you'll see four main sections. The first section is the ellipsoid command that draws the main sphere:
0 0 0 ellipsoid -s line -c 1 -r 20.245 -n 24,19The second part contains a mesh statement that draws a line around the celestial equator; this has a color index of 2. Following this is another ellipsoid statement that draws lines every 15 arcminutes in right ascension and every 2 degrees in declination. This ellipsoid has a color index of 3 and is initially set to black in the mw.cf file. If you have zoomed into a small portion of sky by adjusting the field of view, you can make these visible by setting the color index to a color, such as:
cment 3 0 0 0.2
Finally, there is a series of labels for the data group in hours for right ascension and degrees for declination.
The Radio Sphere
We have not chosen the radius of this sphere arbitrarily. The RA/Dec coordinate sphere takes on another role when you're away from the Sun. We call it the radio sphere.
The radio sphere describes the extent of Earth's radio signals in space. In the early 20th century, radio began to take hold after the discovery that certain radio waves bounce, or reflect, from Earth's ionosphere, a region in the upper atmosphere where gases are ionized by incoming solar particles. However, early broadcasts were not powerful enough to penetrate the ionospheric layers and remained confined to Earth.
Before television carrier waves, early-warning radar first used in World War II, and the detonation of atomic weapons, Earth was radio-quiet to the Universe. After the use of these and other radio emitters began, in the late 1930s and early 1940s, signals were able to escape the atmosphere and travel into space at the speed of light (300,000 kilometers per second or 186,000 miles per second). Since then, we have been broadcasting to the Universe and those early signals that left in 1940 have reached the 65-light-year mark.
As we look farther into space, we look further back in time. Turn on the 1-light-year grid and imagine what happened one year ago. Broadcasts from that time, traveling at the speed of light, are now reaching the 1-light-year mark, 5.89 trillion miles from Earth. They will take an additional three years to reach the nearest star to the Sun.
An event that occurred 10 years ago has washed over eight nearby stars; 20 years ago, 72 stars and two exoplanetary systems. At the edge of the sphere are those transmissions of the 1940s: atomic testing, original Honeymooners broadcasts, and echoes of World War II. Looking into space is looking back in time to a younger Universe.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott