Coordinates in the Sky

Goals: Familiarize yourself with the different coordinate systems; discover what these systems tell us about the Galaxy and our position within it.

Before starting, turn on: stars, mwVis

You will be using: radec, eclip, galac, altLbl, cment and fov commands

Coordinate systems are used primarily to locate objects in the sky. There are several Earth-based systems that astronomers use to describe the position of an object. One is based on Earth's system of longitude and latitude, while another is based on the plane of the Solar System. Yet another is based on the plane of our Galaxy.

Celestial Coordinates Resemble Earth's Coordinates in the Sky

With the Milky Way Atlas running and from the home position, turn on the radec group . These are the celestial, or equatorial, coordinates. This system is based on Earth's longitude and latitude; the brighter blue line is the celestial equator, the line projected on the sky from our equator on Earth. Pan around and you will see the north and south celestial poles, those points directly above the North and South poles on Earth.

In the sky, the equivalent to Earth's latitude is called declination (Dec) and is measured from +90^o at the north celestial pole to -90^o at the south celestial pole. Longitude is called right ascension (RA) and is measured from 0 to 24 hours. You can find more information on this system in “Celestial Coordinates Sphere.” The coordinates are labeled in 10-degree increments for declination and every hour for right ascension.

You will notice Polaris, the pole star, is right beside the north celestial pole, just where you might expect it to be. Orion's belt, three stars in a line, is just below the celestial equator.

The celestial coordinates are labeled in 10-degree increments for declination and every hour for right ascension.

Ecliptic Coordinates are Based on the Solar System

Turn off radec, then turn on ecliptic coordinates . 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.^o5. Now look toward either of the poles. The angle between the celestial pole and the ecliptic pole is also 23.^o5. Also, the maximum distance between the ecliptic and the celestial equator is, you guessed it, 23.^o5. 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, all are 23.^o5. Let's keep to the sky, though.

Galactic Coordinates Trace the Galactic Plane

Now turn off the ecliptic and celestial coordinates and turn on the Galactic coordinates . 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 that the Galactic equator is level (parallel to your tabletop). The angle between these two lines demonstrates the inclination of the plane of our Solar System with respect to the Galaxy. At about 62^o, the planets revolve around the Sun steeply inclined to the Galactic disk. Of course, the planetary orbits are so small that they are lost in the Galactic disk along with the Sun, but it is fascinating to think we are not orbiting the Sun in the same plane that we orbit the Galaxy. One often thinks of the plane of the Solar System as a cardinal plane, defining how we think of up and down in space, but now we have a new horizon, the Galactic plane.

Digital Universe Atlas

Digital Universe Guide

Coordinates in the Sky