How’s Your Altitude (and Azimuth)?

geograph-3960153-by-JaggeryAstronomers use several different coordinate systems to specify the location of objects in the heavens. Some are useful for use with small telescopes, but can be hard to visualize. The oldest and simplest, called Altitude and Azimuth, is no longer generally used by professional astronomers except in some very specific applications. However, because it is easier to visualize, it is the system used for my courses.

Azimuth is all about what direction (“cardinal point direction”) you are facing when you observe something. Analogies for azimuth usually compare it to a compass or clock face. Imagine that you have a large compass, maybe 10 or 15 feet across, laid out on the ground and aligned to North. The compass needle points North and the dial is turned properly to align the compass dial with the needle. Now imagine that you can stand right at the very center of the compass. If you align yourself with the North side of the needle, you will be facing North. In azimuth, we call that 0 degrees North, or more usually, just 0 degrees azimuth.

Then if you turn one quarter turn to the right (“clockwise”), you will be facing East. A one-quarter turn in a circle is 90 degrees, so we call this “90 degrees azimuth.” (As an aside, we say that the sun rises in the East, but only a couple of times during the year will it be exactly at 90 degrees azimuth as it rises. Here in Colorado it runs from about 60 degrees azimuth on the first day of summer to about 120 degrees azimuth on the first day of winter.)

Another quarter turn (90 degrees), brings you to due South. Now from North, that is 90 plus 90 or 180 degrees, so we are at 180 degrees azimuth.

Keep in mind that there are many intermediate values. For example, you could face 40 degrees or 55 degrees or 135 degrees 160 degrees and so on. I will expect your estimates to be reasonably accurate to the nearest 5 degrees, so something like 87.3978 degrees is simply beyond anyone’s ability to estimate just with the eye, and if I were to see an estimate like that, I would know that it did not come from this method.

OK, from due South, another quarter turn brings us due West, which is 90+90+90 or 270 degrees azimuth.

As our last turn, another quarter turn of the circle or 90 degrees, brings to 360 degrees azimuth back to due North. Now, a circle has only 360 degrees, so by convention we don’t refer to it as being 360 degrees azimuth, but 0 degrees azimuth. Every time you come back around to North, it reverts to 0 degrees azimuth.

So what does this mean? When we specify the position of something in the sky, we need four bits of information. Azimuth or distance along the horizon is one of them. Let’ say that you go out some night to observe the Moon, you will be turned a certain number of degrees away from North. Let’s say that when you are facing the Moon, the point on the horizon that is directly beneath the Moon is where you want to estimate the azimuth. If that was halfway between due East and due South, it would be 135 degrees azimuth (90+45 = 135).

Now the Moon at this point is not directly on the horizon, but up in the sky, so we have to specify how far. That’s what we call “altitude.” (Sometimes it is also called “elevation.”) while azimuth counts 360 degrees clockwise from the North, altitude counts degrees from the horizon (0 degrees) to the point directly overhead, the zenith (90 degrees). You can’t go higher than directly overhead, so altitude cannot exceed 90 degrees. Let’s say that you estimate that the Moon is one third of the way from the horizon to the zenith. That’s one third of 90, or 30 degrees. So at this time, the Moon is at 30 degrees altitude, 135 degrees azimuth.

But that is not all. Notice that I mentioned “at this time”? The third piece of information that you must include in a position estimate is time. Why? At a particular time on a particular day, the Moon rises. Then (very) roughly 6 hours later it is in the southern sky. And roughly 6 hours after that, it is setting somewhere in the western sky. The Moon, the Sun, planets and stars all move across the sky constantly, so a position at 8:35 pm will not be valid at 9 pm. So you have to supply the time as well as altitude and azimuth.

So if you made your estimation of the position of the Moon at 8:35 pm on April 22, 2018, the description would be:

April 22, 2018, 8:35 pm, 30 degrees altitude, 135 degrees azimuth

Oh, wait. Sorry, we’re not quite there yet. Actually, “there” is important. You have to specify your location on Earth. That’s because the sky, and the position of things in it, vary according to your location on Earth. The positions of the Sun, Moon, planets stars are different for say, Quito, Ecuador is different from Denver. Normally for your class I would assume that you make your observation in the Denver area, but since I have students in faraway places, you do have to specify your location. So, finally, the correct description of your observation of the Moon would be:

Location: Denver, Colorado (you could use GPS coordinates, but they are not necessary for this activity.)
Date: April 22, 2018
Altitude: 30 degrees
Azimuth: 135 degrees

Obviously you would not always have to state it quite like that. It could be formatted differently (as in a table), but this is the complete description.


Image used under Creative Commons Licence
http://www.geograph.org.uk/photo/3960153

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About Starman

Cosmic Awareness Facilitator. Astronomy, space, physics, science, planets, cosmology, reason, logic, clouds, sky phenomena, the environment, dogs and other animals, and other interesting stuff.
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