Date: April 11, 2010

Title: Celestial Coordinates

Podcaster: Mark DeVito

Description: This podcast discusses two of the celestial coordinate systems, specifically Altitude/Azimuth and Equatorial. With any hope, it will assist new amateurs in understanding the two systems and provide a good refresher to season amateurs who may have given these systems up for computer driven mounts.

Bio: Mark DeVito is an amateur astronomer from Virginia. His primary areas of interest are astrophotography and spectroscopy.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2010, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at signup@365daysofastronomy.org.

Transcript:

Are you new to amateur astronomy? Do you have a new telescope? Ever wonder what those funny circles are on your mount, one on each axis? For many people these are a quandary? Even for seasoned amateurs, we sometimes need a refresher. Actually, since I began the pursuit of amateur astronomy, way back in Jr. High, this is one area that has haunted me and I always need to review, the celestial coordinate systems. So today’s podcast is a quick review about Right Ascension, Declination, Altitude and Azimuth. Perhaps the old adage of, see one, do one, teach one, will ring true and make this topic firm in my mind.

Welcome to the 365 Days of Astronomy Podcast for April 11, 2010. My name is Mark and I am an amateur astronomer from Fredericksburg, Virginia in the US. For those of you who follow my Twitter feed as VAStargazer, you know I have been suffering from acute podcast topic block, which is why I choose a basic level topic for this podcast.

Lets start with the altitude/azimuth or alt/az system. Since the time of the ancients, the stars have been imagined to exist on a crystal sphere, of no particular radius, with the Earth at the center, and the stars affixed and immovable. We call this crystal sphere the celestial sphere. Each star on the sphere has a specific coordinate, which varies based upon which system you are using. To discuss the alt/az system we have to define a few items. For our discussion, assume the observer is standing in the northern hemisphere and facing the compass direction of North. The point immediately overhead is called the zenith. The point below our feet, which is also below the horizon, is called the nadir. The vertical circle running north to south through the zenith is the observer’s Meridian. Now, place a star on the celestial sphere above the horizon. The altitude of that star is equal to the angle created from the observer to the star with respect to the horizon. This angle will be between 0 degrees and +90 degrees with +90 degrees being the observer’s zenith. The other element to this object’s coordinate is it’s azimuth. The azimuth is measured in degrees either east or west of the meridian where east is defined as 90 degrees E and west is called 90 degrees W. So, if an object is east of the meridian an observer could say the object is altitude 32 degrees, azimuth 45 degrees E. By using a graduated pointing or spotting device, such as a theodolyte, one can measure an objects altitude and azimuth very accurately. It is important to remember that the altitude and azimuth of an object is not universal but varies based on the observer’s geographic location, date, and time. Because of this variance, it was necessary for a system to be devised that is universal for defining the coordinate location of celestial objects and works regardless of geographic location.

To answer this need, the equatorial coordinate system was created. Oddly enough, despite significant research, I was unable to determine when and how the equatorial coordinate systems was conceived and by whom. What is so excellent about this system is that it allows for an objects position to be constant regardless of the observer’s geographic location and assist’s the observer in knowing what his or her observing limits are. For simplicities sake, our example will assume the observer is in the northern hemisphere. My apologies to listeners in the southern hemisphere but hey, at least you get Magellanic Clouds in your night sky.

In the northern hemisphere there is a point around which all the stars rotate. This point is called the north celestial pole or NCP, but this should not be confused with the star Polaris. Although it is very close to Polaris and Polaris makes an excellent pointer star, the NCP is just 48 arc minutes from Polaris in the direction of Eta Ursa Majoris. Now just to make things a bit more challenging, the North Celestial Pole, and consequently the South Celestial Pole, does not remain constant. As the Earth rotates, it rotates like a slowing top, with the pole pointing at a different point in the sky. This effect is called precession. The NCP changes about every 25,700 years. Worry not, for the NCP will remain as it is now for about another 1000 years.

So why is the NCP so important? The NCP’s altitude in the sky is equal to the observer’s latitude. So, an observer at the North Pole sees the NCP overhead at the Zenith. An observer at the equator sees the NCP on the horizon. This is an essential fact when setting up an equatorial telescope mount. Given this, an observer would angle the axis of the mount equal to their latitude. Now return to the celestial sphere. At the center of that sphere is the Earth. Take the Earth’s equator and expand it outward. This creates the celestial equator. The Celestial Equator is 90 degrees from the NCP. Now, here comes the hard part. Since this is a podcast, we don’t have the benefit of drawings and pictures. It is 90 degrees from the observers Zenith to the horizon. The NCP’s altitude is equal to the observer’s latitude. Therefore, the angle created between the NCP and the Zenith is 90 degrees -the observers latitude. Since the Celestial Equator is 90 degrees from the NCP, the angle from the Zenith to the Celestial Equator is equal to the observer latitude, which in turn means the Celestial Equator is 90 degrees-the observers latitude above the southern horizon. To help you visualize this, I will put some pictures in the transcript document for this podcast to illustrate this.

Side view of Declination lines for an observer at 45° Latitude: - they are all parallel - the circles get smaller towards the Celestial Poles - only the Celestial Equator touches the exact E-W horizon. Courtesy Calgary Centre

http://calgary.rasc.ca/radecl.htm

0 is the Celestial Equator
+90 is the NCP

The first coordinate of the Equatorial Coordinate system is called Declination or DEC. The declination of an object is based upon its altitude above the Celestial Equator and ranges from 0 degrees to +90 degrees above the Celestial Equator and 0 degrees to -90 degrees for objects below the Celestial Equator. It is essential that the sign be included in the declination number because it describes the objects location either above or below the Celestial Equator. Given these facts, if our observer was located at 45 degrees N latitude, the observer could see objects as high as +90 degrees DEC down to -45 degrees DEC. This is calculated by subtracting the observer’s latitude from 90 degrees. This means that objects with a Declination of +90 degrees – the observer’s latitude or greater do not rise and set. They are circumpolar objects.

Now we move on to what I have always felt was the somewhat more illusive coordinate of the EQ system, right ascension or RA. An analogy for RA is Longitude lines on Earth. And therefore measure an East to West direction. But East or West of what? We will expand on this in a moment. Unlike Declination, RA is not measured in degrees, but in hours, minutes, and seconds. Given this fact, and that the Earth rotates one full rotation in 24 hours, it is not surprising that the RA scale is broken up into 24 hours. And like a clock, each hour is broken into 60 minutes and each minute into 60 sec. When you look at an EQ grid imposed on a star map or planetarium, the individual RA “bands” are numbered 1-24 and sub-divided further into minutes. My chart shows them at intervals of 5 minutes. Now, as you know the Earth rotates on its axis every 24 hours. So, as the Earth rotates, it moves 15 degrees each hour and moves one hour of RA. Think of it this way. If a RA band started on the meridian, after one hour the Earth has rotated 15 degrees and to the next RA band. So if you stand outside and look at the sky long enough, you will see everything rotating across the Meridian from East to West in 24 hours. Now, to make matters worse, the Earth is also rotating around the sun. So how does this effect us, and if an object has a cataloged Declination and Right Ascension, how do we know where to find it in the sky if it is constantly rotating? To do this, we need a starting point, a celestial equivalent to Greenwich England if you will, where the lines of Longitude begin. It was decided that the starting point would be the point in the sky where the Sun was at the time of the Spring (Vernal) Equinox. That would be the “0” point for RA. But, not so fast, to completely understand RA and use it there is one more element we need to understand, sidereal time. The Earth rotates once each day; however, we are also orbiting the Sun. Lets assume the Sun is directly overhead at our observer’s location at local noon. After a 360 degree rotation of the Earth, one Sidereal Day, the Sun is not directly overhead yet. In order for the Sun to appear directly overhead at local noon the next day, the Earth must rotate a small amount more than 360 degrees rotation so the Earth can “catch up” with the Sun. This is why we define the Earth’s “24 hour day” relative to the Sun or Solar Time and not the Earth’s actual rotation or Sidereal time. If we did, noon would occur a little later each day until noon eventually occurred at night. A Sidereal Day is actually 24hrs 56min. We add an additional 4 minuets to our “day” to compensate for the additional rotation of the Earth needed to put the Sun overhead at local noon. This additional rotation takes 4 minutes. Thank you to the Wikipedia entry for Sidereal time for the great graphic illustrating this. I have included it in the transcript document for this podcast.

Sidereal Time. Credit: Wikipedia

So, if you want to see an object that is cataloged at RA 8 hours, it will be on your Meridian when the Local Mean Sidereal time is 8 hours. If you would like to confirm this for yourself, calculate your local sidereal time, or Google the US Naval Observatories calculator, and see what is at your Meridian at that time. Get a star chart, locate that Meridian and see what is along it. Now go outside and face north. You should see the same constellations along your Meridian that you saw on the chart. Now you know how to identify your Local Mean Sidereal time, how to identify when a particular RA hour will be on your meridian and how to use that to fin objects.

Hopefully this has helped you in some way to understand basic celestial coordinates. Oddly enough it is something that often confuses beginners and seasoned amateurs alike.

Clear Skies!

Mark
www.stargazersfield.com

End of podcast:

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