Digital Universe Guide - Hayden Planetarium

32 3. THE MILKY WAY ATLAS
You will notice Polaris, the pole star, is right beside the north celestial pole, just where you probably
expect it to be. Orion’s belt, three stars in a line, is just below the celestial equator.
Ecliptic Coordinates are Based on the Solar System Turn off radec, then turn on ecliptic
coordinates group eclip. The ecliptic coordinates are in red and have a brighter line for the “equator.”
What is the physical significance of this coordinate system, though?
The equator in this coordinate system is called the ecliptic. It is the path the Sun makes in the sky
and, therefore, is the plane shared by the Sun and Earth, generally the plane of the Solar System. In
this system, we measure an object in ecliptic longitude and latitude.
Because ecliptic coordinates describe the Sun’s path in the sky throughout the year, the ecliptic
passes through all the zodiacal constellations. If you turn on the radec group once again, you can see
the effect of Earth’s obliquity. The obliquity refers to the degree to which Earth is tipped on its axis of
rotation. If it weren’t tipped, the ecliptic (bright red line) would coincide exactly with the celestial equator
(bright blue line) and the Sun would always be directly over Earth’s equator, resulting in an Earth with no
seasonal climate changes. The amount of “tip” can be seen in the angles between these two systems.
Find where the celestial equator and the ecliptic cross. The angle between them is 23. ◦ 5. Now look
toward either of the poles. The angle between the celestial pole and the ecliptic pole is also 23. ◦ 5. Also,
the maximum distance between the ecliptic and the celestial equator is. . . you guessed it, 23. ◦ 5. This
number appears many times on Earth’s coordinates too. The angle between the equator and the Tropic
of Cancer or Capricorn, the angle between the North Pole and the Arctic Circle, are all 23. ◦ 5. Let’s keep
to the sky, though.
Galactic Coordinates Trace the Galactic Plane Now turn off the ecliptic and equatorial
coordinates and turn on the Galactic coordinates with the galac button. In Galactic coordinates, objects
are expressed in terms of Galactic longitude and Galactic latitude, which are measured in degrees. The
Galactic equator (bright green line) coincides with the band of the Milky Way in the sky. When you look
toward the equator, you are looking right into the disk of the Galaxy. Therefore, when we look to the
Galactic poles, we are looking directly out of the disk of our Galaxy. These areas are windows out of the
Galaxy—with little gas and dust along these lines of sight, we can see much farther into the Universe.
Now turn on the ecliptic coordinates and find where the ecliptic meets the Galactic equator. Once
you find their crossing, switch to Rotate Fly Mode and rotate the view with the right mouse button such