Saturday, January 14, 2017

 

Issue #526: The Novice Files Part I


It’s 1965 and I’ve just gotten my first real telescope, a 3-inch Tasco Newtonian (a pawn shop refugee). I really don’t know what the heck to do with it other than look at the Moon—which is wonderful, of course. Shortly, though, my first issue of Sky & Telescope arrives. Surely it will clue me in to everything I need to know about this amateur astronomy business. Alas, after looking through it I wind up more puzzled than I was before.

Sure, there was ample amateur astronomy in the magazine (if not as much as today) following the pro/science articles up front. But the writers seemed to think I already knew enough about astronomy to understand them. Heck, even the advertisements were indecipherable due to the jargon. I was getting nowhere in a hurry.

What was a “clock drive”? When the author of an article said an object in the telescope was “30’ northwest” of another, did that mean the object was 30-feet from the other one? How could you tell what was north or south or east or west in the eyepiece anyway? What was a “Meridian” or an “Ecliptic”? What did “R.A.” and “Declination” mean? What was “figuring” a mirror? Why did it have to be “parabolic”? What was a “prime focus”? What in God’s name was a “drive corrector”?

My confusion didn’t last too long. Between the mutual support and advice me and my buddies gave each other in our little proto-astronomy club, the Backyard Astronomy Society, and the wisdom dispensed by some of Patrick Moore’s books, I finally got off square one. It wasn’t easy in the beginning, though, and I sure wished somebody had explained all those puzzling things somewhere clearly, simply, and in one place.

Today, with the Internet, things are easier, much easier, but it would still be nice to have explanations of the basic concepts and terms of practical astronomy in one spot. Certainly astronomy is every bit as foreign and confusing for beginners now as it was back then. There’s a reason for that. At the beginning of the last century, U.S. secondary science educators decided to deemphasize geology and astronomy in favor of biology and chemistry. Most students get very little on astronomy after the occasional middle school “space science” unit, and its ideas and language are a mystery to most.

The sky globe...
So? In hopes of making that learning curve a little less steep, here are some brief solutions to the head-scratching puzzles you, Joe and Jane Newbie, are running into on the Internet and in the astronomy magazines. There won’t be room to give you everything you need to know in one go, so we’ll do a part II (maybe even a part III and IV) in fairly short order.

The Sky Globe

Many beginners have an awfully hard time wrapping their heads around the way the sky works. All those imaginary lines and stuff, and it’s always in motion! There is an easy way to understand it, however. What did the Ancient Greeks think the sky was? They believed it was a great crystalline globe surrounding the earth. The stars were points of eternal fire, or maybe they were holes in the sky globe allowing the eternal fire beyond to shine through. Of course, today we know that is nonsense. The sky isn’t a glass globe. If, however, you can suspend your disbelief for a while, thinking of the sky as a globe makes it easy to understand how it works.

So, we have this great crystal rotating about the earth once every 23 hours 56 minutes and 4 seconds (a “sidereal day,” see below). Yes, I know it’s really the Earth that’s rotating, but remember what I said about "suspension of disbelief"? To our eyes, it’s the sky that is turning.

Lines and Points

Celestial Poles

If you have a basic knowledge of Earthly geography, the globe of the Earth, understanding the lines and points of the sky is easy. Let’s begin with the celestial poles, the analog of the Earth’s poles. Extend the axis of the earth north and into the sky. The point where the Earth’s axis penetrates the sky globe is the North Celestial Pole (NCP). Extend the axis south into the sky globe and you have the South Celestial Pole (SCP). The sky globe appears to be rotating on this axis, which extends from the North Celestial Pole, through the Earth, and into the South Celestial Pole.

Where are the poles in the sky? They are found at an elevation (north or south) equivalent to your latitude value. Down here, I am at 30-degrees north latitude, so I find the NCP 30-degrees above the northern horizon, conveniently marked by the bright 2nd magnitude star, Polaris (which is actually about ¾ of a degree from the true NCP).

Celestial Equator

Do the above with Earth’s equator, extend it into the sky as a flat plane, and you have the Celestial Equator. The Celestial Equator is the imaginary line that divides the sky globe into a Northern Celestial Hemisphere and a South Celestial Hemisphere, just as the earthly equator separates the globe into northern and southern hemispheres.

Latitude (Declination)

Look at the globe of the Earth. How do you find your position north and south of the equator? Simple: there are imaginary lines of latitude. We have the same thing on the sky globe, lines of latitude. They perform exactly the same job; they allow you to find your position north or south of the Celestial Equator.

As on earth, latitude is measured in degrees, minutes, and seconds beginning at the equator, which is 0-degrees. For some odd reason some beginners tend to think the equator should be 90-degrees, but 90 degrees north or south of the Celestial (or terrestrial) equator brings you to the poles. The equator in the sky or on earth is 0-degrees. Latitude is measured in (angular) degrees, minutes and seconds.
Conventions for stating a latitude value? Degrees are indicated with a degree symbol, a single quote (‘) is minutes, and a double quote (“) is seconds, just like in your high school geometry or trigonometry courses.  A minute is a distance equal to 1/60th of a degree, and a second is a distance that’s 1/60th of a minute. North thirty degrees, thirty minutes, and thirty seconds is written as 30°30’30”. If you don’t have a degree symbol in your font, a lowercase “d” will do.  If the latitude in question is a south latitude, a latitude south of the Celestial Equator, a minus (-) sign is placed in front of the value. You can put a plus (+) sign before a latitude to indicate “north,” but the lack of a minus sign is taken to mean it’s a north latitude.

There is one difference between latitude on the Earth and latitude in the sky:  in the sky this north-south measurement is called “declination” (abbreviated “dec”) but that is the only difference. “Declination” might sound forbiddingly technical to you, but it’s not; it just means “latitude on the sky globe.”

Longitude (Right Ascension)

Just as the sky globe has lines of latitude, declination, that allow us to locate points north and south of the Celestial Equator, there are also lines of longitude that enable us to find positions east and west. Just as on Earth, the combination of heavenly latitude and longitude allows us to find anything we want—stars, planets, and deep sky objects.

Celestial longitude is actually simpler than earthly longitude. On earth, longitude is in east and west values. Just as latitude is stated in relation to distances north and south of the equator, longitude on Earth is stated in terms of how far east or west (+/-) you are from the Prime Meridian (the zero line of longitude, which runs through Greenwich England). Longitude in the sky is simpler in that it begins at the sky’s Prime Meridian and runs east, increasing in value, until it comes back around. There is no east/west in celestial longitude.

When talking latitude, it’s easy to see where you measure from. On Earth or in the sky, it’s obvious you begin at the equator. But for longitude, you must choose a starting place. There’s no really obvious place to place the 0 line. On Earth, that line runs through Greenwich, England. Why? Britain was the world’s preeminent naval power when navigation was being sussed, and led the world in the quest to figure out how to determine longitude at sea (not so easy). But where to put the 0 line of longitude in the sky?

Ecliptic

Before talking about the sky’s Prime Meridian, you need to understand another line, the Ecliptic. The Ecliptic Is the Plane of Earth’s Orbit. The major planets are in orbits that are almost in the same plane as the Earth's. As such, they always appear close to the ecliptic. In terms of what you see in the sky with your eyes, the Ecliptic is the apparent path of the Sun through the sky. This path does not remain in the same positon throughout the year, however.

The Vernal Equinox
You may have noticed that the Sun’s path is farther north in the (Northern Hemisphere) in summer, and farther south in the (Northern Hemisphere) winter. The path of the sun moves north and south over the course of the year. When the path is farthest north, it is summer in the Northern Hemisphere. Farthest south and it is winter (reverse that if you live in the Southern Hemisphere). The Sun’s moving path across the sky and the change of the seasons are due to the tilt of the earth’s axis. That’s why we have seasons. It’s not because, as some astronomy newbies (and other people) imagine, the Sun is closer in the summer thanks to the Earth’s elliptical orbit—the reverse is actually true in the Northern Hemisphere.

Vernal Equinox

Back to celestial longitude. The chosen point, the place the 0 line of longitude, the prime meridian in the sky, runs through is the Vernal Equinox. The Vernal (spring) Equinox is the point where the ecliptic intersects the celestial equator. As winter ends, the path of the ecliptic moves north, and eventually runs into the Celestial Equator. When the path of the Sun reaches this point, when the Sun hits the Celestial Equator, it is spring. The zero hour line of celestial longitude passes through this point. The Vernal Equinox point is also known as "the First Point of Aries" (the Vernal Equinox no longer lies in Aries, but don't worry about that right now).

Right Ascension

Since, as above, there is no east/west value for celestial longitude, that makes it simpler to work with. Two things make it more complicated, or at least complicated sounding to novices, however. The first is its name. Just as celestial latitude is not called “latitude,” celestial longitude is not called longitude. It is “right ascension” (abbreviated R.A.) That’s scary sounding, but just remember right ascension = longitude. What hangs most newbies up is not celestial longitude’s name, but the way it is measured.

Rather than being given in degrees, minutes, and seconds as latitude is, right ascension is measured in hours, minutes, and seconds. What you have to understand is that these hours, minutes and seconds do not really describe time; they describe distance. One hour of right ascension is 15 angular degrees. 1-minute is 1/60th of that and 1-second is 1/60th of that.

The seasons...
Why “hours” instead of degrees? Since the sky is always in motion, it makes a certain amount of sense. Let’s say you go out one evening and look to the eastern horizon. You notice the bright red giant star, Aldebaran. “Pretty!” you think. But you want to watch a rerun of your favorite program, Jersey Shore, on TV. You hop inside and enjoy Snookie’s antics. Afterwards, you wander back outside and immediately notice that in one hour Aldebaran has risen 15-degrees in the sky. It has moved a distance equal to one hour of right ascension (multiply 15 times 24-hours and you will come out with 360-degrees).

If you just understand that R.A. = distance, 1h = 15-degrees, you will do OK. The convention for stating R.A. is a lowercase “h” denoting hours, an “m” for minutes, and an “s” for seconds as in: 19h17m00s.

Zenith

The Zenith is the point in the sky that is directly over your head. It never moves.

Nadir

The Nadir is like the Zenith, but is the point that is always directly beneath your feet. Like the Zenith, it never moves.

Local Meridian

Yet another imaginary line you need to know is the Local Meridian. It is the line that runs from the North Celestial Pole to the Zenith, through the South Celestial Pole, through the Nadir, and back around. It never moves. As time passes, celestial objects—the Sun, the Moon, planets, stars, deep sky objects, everything—hit and cross this line. When an object touches the Local Meridian, it is said to be “culminating” or “transiting.”

When an object culminates is an important thing for a sky watcher to know. When a star, for example, is on the Local Meridian, it is as high as it ever will get in your sky. If it’s located very far north or south in declination, that might not be very high, but it is still as high as the star will get. And that’s the best time to observe it, when it is as far from the thick, dirty air on the horizon as possible.

Solar day...
Local Sidereal Time (LST)

How do you know when an object will transit the Local Meridian? You check the local sidereal time. When a line of right ascension is straight overhead on the Local Meridian, that is the current LST. Say the 11-hour line of right ascension is on the Meridian. That means the LST is 11:00. Any object with a right ascension, a celestial longitude, of 11h is culminating.

How can you find out what the LST is? Most astronomy program, especially planetarium programs, will give LST. Some, like Stellarium, will display this value as “Mean Sidereal Time” and/or “Apparent Sidereal Time,” but for our purposes that's the same as LST. Right now, it’s 19:17 and globular cluster M56, which has a right ascension of 19h17m, is on the Local Meridian and high in the sky. Typically planetarium program, including Stellarium and Cartes du Ciel, display LST in the information window that comes up when you select an object.

Sidereal Day and Solar Day

Yes, yes, I know back in first grade your teacher, kindly Miss Franklin, told you a day is 24-hours long. But that’s not exactly true. Not always. The actual time it takes the Earth to rotate once on its axis (as measured by the time it takes a star to make two transits of the Local Meridian) is 23 hours, 56 minutes, and four seconds.

So what’s with the “24-hours”? That’s a Solar day, the time it takes for the Sun to transit the Meridian twice. Why the difference? The Sun is close compared to the stars, and the fact that the Earth is moving along in its orbit in addition to rotating, means a bit of parallax error comes into play. As you can see in the picture, when the Earth has rotated once on its axis, it’s moved along in its orbit (greatly exaggerated here) as well, and, so, has to turn a little more on its axis to put the sun back overhead. That extra time is the nearly four minute difference in the two varieties of day.

Measuring Distances in the Sky

All this degrees and minutes stuff is well and good, but how do you judge distances in the sky? Luckily, nature has provided you with a convenient measuring tool. Your outstretched fist covers about 10-degrees from thumb to pinky. Your index finger is approximately 30’ across. “But Rod,” you might say, “I have small hands.” Nevertheless, this should still work. Most people with small hands have correspondingly short arms, and your outstretched fist will still span 10-degrees.

Whew! That was a lot. Nearly too much for one sitting, so we will stop here. Go over these concepts until you are clear on them; these are things every astronomer needs to know. Even in this day of do-everything computerized telescopes, I believe it is still vital—for understanding and enjoyment—that you know how the great sky globe works.

Next time? I am not sure. I’d say that if the weather continues to be as lousy as it is right now, we might go on to Part II of the novice files. Or I may talk about the new Stellarium. Or something else may come into my mind (such as it is). We shall see.

Comments:
Hi, Unk' Rod! Just a few questions:
1) My forefinger is about 3/4" wide and about 26" from my eye, giving roughly 2 degrees across, not 1/2. Yes?

2) "As winter ends, the path of the ecliptic moves south" - isn't it moving north, towards the higher summer sun transits?

Good posting - I hadn't realized the definition of "culmination" before. I've only been observing and reading S&T, etc. since the late '60s. Does that make me a long time newbie?
 
HI Jim:

1. No...my index finger tip at arms length just covers the Moon.

2. Yep. Got that backwards. Thanks.
 
Unk Rod
Thanks for a nice summary of the basics. Want you to know I, and I am sure many more, appreciate your writing and service to the Astronomy community.

Best Regards,

Ed Gill
 
Mr Horns calculations are correct for a 20mm wide finger. At 26 inches (660 mm) his finger would subtend an arc of 1.6 degrees. Finger width would have to be around 7 mm to subtend a 31 arcminute angle.
Also, if the fist covers 10 degrees and the flat palm is close to the same width at the knuckles, each finger would be 2 degrees.

Do to the lack of consistancy in finger wides and arm lengths one would need to experiment and determine which finger works. My thumb subtends twice the angle my pinky does. I of course never use my middle finger to measure anything, to much chance for misinterpretation. 😁
 
Thanks Rod. Very usefull!!!


 
Thanks a lot for this, I really appreciate it. As a complete newbie, I learned a lot.

However, I did not understand one thing. The celestial equator passes through the ecliptic twice right? Then how did we choose one of those points to be the vernal equinox? Sorry if this is a completely stupid question :p
 
The point that is the Vernal Equinox is, by definition, the beginning of spring in the Northern Hemisphere. The Autumnal Equinox means fall as begun in the Northern Hemisphere. ;)
 
Yeah that is correct, perhaps I should have instead asked that why is the vernal equinox chosen as the zero hour and not the autumnal equinox. Is there a reason for that?
 
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