This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Celestial Coordinates

by Dr Jamie Love Creative Commons Licence 1997 - 2011

The Zodiac acts as our point of reference when determining Right Ascension.
You will recall that right ascension (RA) lines are the north-south lines running through the celestial sphere and are the sky equivalent to the Earth's longitude. Instead of being measured in degrees, the RA is measured in hours, minutes and seconds. This is more convenient that using degrees because we can use RA to calculate the amount of time that will elapse as we wait for an object to cross our meridian or any other point in our Earth-bound coordinate system. Along with declination, the RA provides the position of any point in the sky. When I first taught you about RA I explained that the 0h (zero hour) RA line, where we begin to measure the RA, runs from Polaris and close to CASSIOPEIA as it heads south. That is true and CASSIOPEIA is a convenient way to get your RA bearings because it is in the northern sky every night (assuming you aren't too far south).

Yeah, but why did they draw the zero RA line there?

Good question. Actually they drew the "line" (RA = 0) through a point in PISCES and their decision was based upon a clear understanding about the ecliptic and the Celestial Equator. I think you have reached that point in your education when you are ready to understand these coordinates in greater detail.

You will recall that the Celestial Equator is an imaginary line we draw by projecting the Earth's Equator into the sky. The Celestial Equator defines zero declination (0o) and provides us the reference line upon which we imagine our declination lines north and south of it. (Just like the Earth's Equator provides us with a reference upon which we draw lines of latitude on a globe.)
The Celestial Equator is actually positioned by the Earth's spin and that is why Polaris is at a declination of 90oN. (That's 90 degrees north of the Celestial Equator.)

The ecliptic is NOT the Celestial Equator!

The ecliptic is the path of the Sun through the Celestial Sphere and because the Earth's axis is tilted (23.5o) from the orbital plane, the Sun's path (the ecliptic) produces a plane that is inclined to the Celestial Equator.

Here's the night sky for the first evening in October. I've added the ecliptic in orange and a few declination lines in green.

Looks like they intersect somewhere in PISCES.

That's right.
That means the path of the Sun (the ecliptic) crosses the Celestial Equator at that point in PISCES. You may recall that there are two occasions each year when the Sun crosses the Celestial Equator - at the Spring and Autumn Equinoxes. The point illustrated here is the Spring, or Vernal, Equinox and shows where the Sun will cross the Celestial Equator as it passes from the Southern Celestial Hemisphere to the Northern Celestial Hemisphere. (Remember, the Sun moves "backwards" through the Zodiac during its annual migration, so it moves from right to left.)

On the other side of the Celestial Sphere (not shown) you would find the ecliptic passes across the Celestial Equator again during the Autumnal Equinox as it enters the Southern Celestial Hemisphere from the north. That means the Sun crosses the Celestial Equator while it moves through VIRGO but at that time of the year it is moving from north to south.
The Autumnal Equinox was last month (remember?) so the Sun should have been in VIRGO making that constellation difficult to see. And it was. Conversely, during that same time, the Sun would have been completely opposite of PISCES. And it was! That means it would have been very easy to watch PISCES during the night of the Autumnal Equinox because the Sun would have been nowhere near it. That is exactly what happened and we are still enjoying that view a month later.

The point where the Sun passes from south to north across the Celestial Equator (during the Vernal Equinox), is called the First Point and astronomers use it as the "zero point" of our RA lines. If you draw a line from Polaris to the First Point, and extend it to the Southern Pole, you have the RA of 0h. As the sky rotates (actually, as the Earth rotates) this point will move from east to west across the sky. By around midnight (in October) the RA = 0h line will be roughly overhead. The First Point itself would not be overhead (unless you happen to be at the latitude 0o, the Earth's Equator) but the 0h RA line will run from Polaris, pass over head, go through the First Point and continue down to the southern horizon (where it will continue to the pole of the Southern Celestial Hemisphere).

Recall that any imaginary line that runs directly over head (your zenith) and from pole to pole is called your meridian.
By around midnight in October the point at which the ecliptic and Celestial Equator meet, the First Point, will lie along your meridian and the 0h RA line will run exactly north-south.
An hour later the 0h RA line will have moved westward and the 1h RA (one hour right ascension) line will be over head running from Polaris, directly over head (through your zenith) and down to the southern horizon on its way to the Celestial South Pole. At that time your meridian will be 1h RA.

As the night progresses each point in the sky crosses your meridian because each RA line becomes (temporarily) your meridian.
Another way to think of it is that over the course of 24 hours every RA line will line up perfectly with your meridian - but you can't see the ones that occur in daylight.

Here is the night sky again with lines of RA added to the declination and ecliptic lines.
I've also labeled the First Point.

Astronomers use these lines to map the position of stars. A star's declination and RA do not change (much).

Astronomers also use these lines to follow and map the movement of the planets, which follow the ecliptic, and comets, some of which move close to the ecliptic and some which do not (depending upon where they came from).

You should become familiar with RA and declination lines. I admit, this is complicated, but it is also understandable if you just give yourself some time to learn it. You've learned a great deal in these lessons about the Zodiac. You now know how astronomers imagine their RA lines and how the ecliptic is used. Along the way you learned the remaining Zodiac constellations and why they are important.

Yeah, but these coordinate systems are tricky. Is it important to understand them?

Yes!
I have been introducing different parts of the coordinate systems throughout these lessons but I think this might be a good time to review them. There are TWO coordinate systems - one is "Earth-bound" and the other is "space-bound".

The Earth-bound system involves the two "A"s - altitude and azimuth.

The altitude of a celestial body (star, planet, etc.) is its angle above the nearest horizon. Think of altitude as the vertical angle for an object. A star on your horizon has an altitude of zero degrees (0o). A star at your zenith has an altitude of 90 degrees (90o).

Do NOT confuse our use of "astronomical altitude", which is measured in degrees, with its more common use as an indication of height, which is measured in feet, miles, meters or kilometers. This use of the word "altitude" in two different ways might seem odd but it sort of makes sense both ways. In both meanings of the word, we are concerned with the vertical position of an object. The astronomical way is an attempt to use altitude in a similar way to its more common use but it's impractical to think of the "height" of a star because its distance from us is measured in light-years! So we use the angle, measured in degrees, above the nearest horizon as our astronomical altitude.

Azimuth is the horizontal direction or bearing of an object in the sky and it sets the line along which the altitude is measured. The azimuth is designated as the angle created by that imaginary line and true north. True north has an azimuth of 0o, east is at 90o, south is at 180o and west is at 270o azimuth. This numbering is counter clockwise in this picture of the sky (looking up) but is clockwise (and easier to understand) if you remember that it's meant to be imagined as a circle drawn on the Earth's surface.

Polaris always has an azimuth near zero (0o).

Your meridian is a special line that runs directly north-south. Your meridian line has an azimuth of 0o when you are facing north but an azimuth of 180o when facing south. Indeed, all azimuth lines change by 180o when they cross your zenith. Your zenith is the point in the sky directly overhead so it has an altitude of 0o and any azimuth that you want to give it would be correct!

Both the altitude and the azimuth of an object are determined by how that object appears to YOU! That means altitude and azimuth are Earth-bound systems. Your position on the planet (your latitude and longitude) and the orientation of the Earth with respect to the Sun (your local time) play as much a role in determining an object's altitude and azimuth as the object's celestial coordinates.

Celestial coordinates are the "space-bound" coordinates of right ascension (RAs) and declination. Unlike the Earth-bound system, celestial coordinates are fixed. That is, a star's position is defined by its celestial coordinates and they do not change. (Actually, over many, many years a star's position might change but that's a topic for next month.) We use the Earth based system of longitude and latitude as a guide to visualizing the celestial coordinates.

The Celestial Equator divides the night sky into two hemispheres - north and south. It's an imaginary line projected from the Earth's equator into the sky and objects along it are said to have a declination of 0o. Objects north of the Celestial Equator are given a positive declination and measured in degrees away from the Celestial Equator. Objects south of the Celestial Equator are given a negative declination but otherwise are measured in the same way.

If you live at the North Pole you would only be able to see stars with a positive declination.

If the Earth's axis was not tilted with respect to its orbit, the path of the Sun, the ecliptic, would be along the Celestial Equator, at a declination of 0o and astronomy would be simpler. (Also, if the Earth's axis was not tilted with respect to its orbit, our North Star would not be Polaris.) Planets, the Sun and anything else that moves along or near the ecliptic are still measured in declination and Right Ascensions (RAs).

Of course, now you know exactly how RAs are positioned. It all has to do with the First Point.

Speaking of "point", I thought the point of an astronomy course was to learn how to identify stars and stuff like that.

Yes, but it's also important to know the coordinate system so we can communicate with other astronomers. If you discovered a nova you would want to report its position in RA and declination, not altitude and azimuth. On the other hand, when using a telescope you often have to set it up in such a way that your altitude and azimuth are used to get the device correctly oriented.
However, your "point" is well taken so let's finish this month with some observational astronomy.

Below is another image of the October evening sky looking a little to the east and south so you can get a good look at the part of the sky you have just learned. I've drawn in a few tiny constellations. They don't have a lot to offer but you may have been wondering if they have names.

DELPHINUS, the Dolphin, is a tiny kite-shaped constellation on the east edge of the Great Summer Triangle. It has such a striking pattern that it draws the attention of many amateur astronomers.

Between Deneb and Altair is the tiny constellation of SAGITTA, the Arrow.

There are several other small and unimportant constellations around here but we'll leave them alone.

Use this image to practice identifying stars and constellations. Here's a reversed (white/black) image you might want to print out.

Start with the obvious stars and constellations and use them to help you find the others. First find the three stars and constellations that make up The Great Summer Triangle. Then use those stars as pointers to SAGITTARIUS (the "teapot") and CAPRICORNUS (the "twisted triangle"). You know that AQUARIUS is the dim "stick figure" that is reaching towards CAPRICORNUS. After AQUARIUS comes PISCES which starts as a loop of stars and then follows a long arc under the Square of Pegasus as it dips below the ecliptic and then back up again. (Watch that bright star near the bend. It isn't part of PISCES!). After PISCES comes ARIES. You should be able to find its brightest stars but I had to trim off the star at the far east side because this image is getting awfully big.

Now that you have the bottom stars, why not complete some of the stars and constellations above it. You learned them in earlier lessons. The Square of Pegasus is obvious standing above PISCES and you can add the extra "legs" that are on the west (right) side of the Square. You'll recall that ANDROMEDA shares the alpha star from the Square and goes on to form a string of stars in a kind of wide "W" as if CASSIOPEIA was drawn out as it was pulled south. If you're real good you can complete ANDROMEDA by adding in the "hour-glass" and her long arm that points towards Deneb. Speaking of CASSIOPEIA, you should be able to see most of her near the top of this image.

Below is that same image with the constellations drawn in and major stars labeled.

Here's a reversed (white/black) image of the "answers" that you might want to print out.

I hope you were able to find all five Zodiac constellations and maybe recalled some of the others.

It isn't important to get every line right but be sure you know roughly where each constellation resides. And you should know the major stars. Use these last two images to remind yourself about the layout of this part of the sky and then get outside this month and have a good look at the real thing.

See you next month.
Wishing you "Clear Skies".
Jamie (Dr Love)




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.