Digital Universe Guide - Hayden Planetarium

108 3. THE MILKY WAY ATLAS
3.3.19 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. ◦ 5 to this plane, the Sun appears to move in declination
throughout the year (see “Equatorial Coordinates Sphere” 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. ◦ 5 north and 23. ◦ 5
south latitude, respectively.
Ecliptic coordinates are described by ecliptic longitude and latitude measured in degrees (labels
provided every 10 ◦ ). Longitude is measured from the vernal equinox [(RA, Dec) = (0, 0) ◦ ] and the
ecliptic north pole is in the constellation Draco (23. ◦ 5 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. ◦ 5.
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 Milky Way all-sky image (mwVis) 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 60 ◦ to each other (62. ◦ 87 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 60 ◦ , orbiting in the Galactic plane at 220 km/sec (or just less than 500,000 miles/hour).
We now see there’s a more significant horizon to obey, that of our home Galaxy. Are there any horizons