Celestial Coordinates

The Celestial Coordinate System

You are probably familiar with using latitude and longitude to identify locations on the earth. Astronomers use a similar coordinate system to refer to locations in the night sky. Right Ascension is the astronomical equivalent of longitude. Just as longitude describes how far east or west some place is from a fixed reference on the surface of the globe, right ascension describes how far east or west something appears on the celestial sphere. Right ascension is measured in hours rather than degrees, with 24 hours bringing you back to the starting point, mirroring the apparent motion of the heavens around the earth in a 24 hour day. Likewise, Declination is the astronomical equivalent of latitude. It measures the number of degrees a heavenly body sits north or south of the celestial equator. Imagine the equator of the earth and project that circle outwards onto the sky. Objects lying on that line have a declination of 0 degrees. The north celestial pole has a declination of 90 degrees, and the south celestial pole lies at -90 degrees. Using this coordinate system, you can accurately refer to any position on the celestial sphere.

The Coordinate Grid

Take a look at the star map on the Messier Observer’s Planisphere. Start at the north celestial pole. It is the point in the center of the metal grommet around which the top dial of the planisphere can rotate. You will find a series of concentric circles printed on the map, centered on this point. These circles are placed at 10 degree intervals. The pole is at 90 degrees declination. The first circle is at 80 degrees declination, the second at 70, and so on. The celestial equator is at 0 degrees declination and is printed in black to make it stand out. Subsequent circles denote -10 degrees, -20 and so on to the edge of the chart.

Also pay attention to the straight lines radiating out from the north celestial pole like spokes on a wheel. These right ascension lines are spaced 1 hour apart starting at 0 and increasing through 23. You will notice that the right ascension lines are aligned with the hours on the clock dial around the perimeter of the chart. You can use the large numbers on the clock dial to quickly and easily find any specific right ascension line on the coordinate grid.

Locating Objects

Now that you are familiar with the coordinate grid on the Messier Observer’s Planisphere, it is time to put that knowledge to good use. Turn the planisphere over and take a look at the table of Messier objects on the back. Let’s practice finding objects on the star map using M1, the Crab Nebula. Look up the right ascension (RAh) value for M1 in the chart. It is 5.6. Now, turn the planisphere back over and look at the clock dial around the edge. We are looking for 5.6 hours, which translates to just a little more than 5:30 AM on the clock dial. Rotate the top layer of the planisphere until the RA grid lines corresponding to 5:00 AM and 6:00 AM are visible in the center of the window. Your target located at 5.6 hours of right ascension should be somewhere on the chart about half way between these two lines.

Refer to the back of the planisphere again to look up the declination (Dec) for M1. It should be 22.0 degrees. That should be just a bit higher than 2 declination circles above the celestial equator on our coordinate grid. Find the celestial equator on the star map, it is the only circle on the grid that is printed in black. Count 2 rings up from there towards the pole, still looking about half way between the 5h and 6h RA lines. Do you see it? You just located M1 on the star map!

While you still have M1 centered in the window, find today’s date on the calendar along the edge of the planisphere. What time is aligned with today’s date? That is the time when M1 will be at that position in the sky today. Alternatively, look for 10:00 PM on the clock dial. On what date will M1 be well positioned for viewing at that time? You can see how the Messier Observer’s Planisphere might be useful as a planning tool before your observing sessions, as well as under the stars.

Practice finding a few more objects from the table. Before too long, you will be able to look up the location of an object on the chart and have that object centered in your eyepiece more quickly than the guy next to you on the observing field with the fancy computer driven telescope.

Other Chart Elements

While you were exploring the coordinate grid on the Messier Observer’s Planisphere, you may have noticed a thick gray circle on the star map that is not exactly centered on the celestial pole like the declination circles are. That circle represents the ‘Ecliptic’, the path that the sun, moon, and planets travel as they move across the sky. Unlike the stars and Messier objects, the planets are not printed on the star map because their positions change. If you see a bright star in the sky that is not printed on the map, but it is located somewhere along this ecliptic circle, it is almost certainly a planet. Point your telescope at it and enjoy the view!

One last element that is printed on the star map that we have not discussed is the Milky Way. From our vantage point within the Milky Way galaxy, it appears as a glowing cloud of stars stretching across the night sky. The Milky way is shown in light blue on the star map. You will need a dark observing site far away from city lights to fully appreciate it with your own eyes.