Bands in the Sky

The celestial poles and the celestial equator can be used as reference points for identifying the locations of other objects of interest in the sky – like the sun, moon, and visible planets. It is often instructive to crudely describe where these objects can be located in the sky for various latitudes on the earth with “bands” of declination.

The ecliptic is the plane of the earth's orbit about the sun. Because the earth is tilted (its obliquity) about 23.5° to this orbital plane, the sun's declination is always between +23.5° on the summer soltice and -23.5° on the winter solstice. Thus one can picture a band 47° wide centered on the celestial equator. The sun is always in this band.

The orbits of other objects in the solar system are described with relation to the ecliptic. The moon's orbit is inclined by about 5° to the earth-sun orbital plane. Thus the moon is located in a larger region of the sky.

The orbital inclinations of the brighter planets are small, for example Mars' orbital inclination is about 2°. Because orbital inclination is measured from the center of the sun and declination is measured from the center of the earth, Mars can be as much as 6° off of the ecliptic. The only planet that can be very far from the ecliptic is Pluto but it can't be observed without a good-sized telescope. Thus, the location bands for the bright planets is very similar to that of the moon and the two are shown as one 60° band in the Skyband Simulator.