Ecliptic Coordinates Sphere
Ecliptic Coordinates Sphere
| Group Name | eclip |
| Reference | -- |
| Prepared by | Brian Abbott (AMNH/Hayden) |
| Labels | Yes |
| Files | eclip.speck |
| Dependencies | none |
Ecliptic coordinates are based on the imaginary line traced by the Sun throughout the year. This line is called the ecliptic and, in three dimensions, also defines the plane that contains the Sun and Earth.
Because Earth is tilted on its axis 23.o5 to this plane, the Sun appears to move in declination throughout the year [see celestial coordinates for definition]. Two days a year, on the vernal equinox around March 21, and on the autumnal equinox around September 21, the Sun crosses the celestial equator. Around June 21, it lies over the Tropic of Cancer (the summer solstice in the Northern Hemisphere), and around December 21, it lies over the Tropic of Capricorn (winter solstice for the Northern Hemisphere). Coincidentally, the Tropics of Cancer and Capricorn are at 23.o5 north and 23.o5 south latitude, respectively.
Ecliptic coordinates are described by ecliptic longitude and latitude measured in degrees (labels provided every 10o). Longitude is measured from the vernal equinox [(RA, Dec)= (0, 0)o] and the ecliptic north pole is in the constellation Draco (23.o5 from the celestial north pole and the north star, Polaris). The ecliptic north pole is the point perpendicular to the plane of the Solar System and would be the north celestial pole if Earth were not tilted 23.o5.
A New Horizon
In the Atlas, the ecliptic sphere is similar to the celestial sphere but is given a radius of 100 light-years. Turn on the Visible Milky Way all-sky survey and notice the tilt of the ecliptic to the band of the Milky Way. These two planes, the plane of the Solar System and the plane of the Galaxy, are tilted about 60o to each other (62.o87 to be precise).
Many of us are used to thinking of the Solar System plane as our cosmic horizon line. All the planets lie roughly within this plane, so it makes sense that this should be the plane from which we measure up, down, over, and under. However, the Milky Way band tells us otherwise. In fact, the Sun and all its planets are orbiting the center of the Galaxy once every 225 million years. Picture the Sun and its planets tipped 60o, orbiting in the Galactic plane at 220 km/sec (or just less than 500,000 miles per hour). We now see there's a more significant horizon to obey, that of our home Galaxy. Are there any horizons beyond this?
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott