PDA

View Full Version : Astronomy Class - Sextant?



Bones01
2007-Jan-25, 04:48 AM
I just started an online astronomy class. One of the projects involves observing the moon at the same time every night and marking down the degrees above the horizon. The teacher suggested we do this by using our fist (10 degrees per fist), but I'm working a shift that can vary unexpectedly at any time (nights+military) and there's a chance that I might not be able to make a sighting on any given night (though I expect to). I asked my wife to take care of it instead, but her fist is smaller than mine, so the observation won't match up with my own.

So, I was thinking of getting a sextant or something so that if she does it for me one night the degrees won't jump in one direction then go back the other way (I dunno if this is even going to happen or not, since I know next to nothing about how the moon is going to move through the sky during these observations).

Any suggestions? Should I buy a sextant? Is the difference between the size of our hands going to make THAT much of a difference? (the teacher said she's not too concerned about being very accurate)

George
2007-Jan-25, 05:43 AM
You can simply compare the difference in angle between your hand and hers.

For instance, stand at a distance from a wall and at a marked spot, then extend your fist. Tell her where to mark the wall. [Use tape or something that will not give her a chance to make you paint the room. :)]

Have her repeat this and note the difference, if any. Does this help?

Note: since you can measure the distance to the wall from your eye, as well as, the distance between the marks, you can calculate the actual number of degrees you are seeing.

Delvo
2007-Jan-25, 07:04 AM
Forget about fists. Get a couple of yardsticks. Hold one vertical and the other horizontal with the horizontal one next to your eye and the vertical one crossing it or out at the end of it, probably somewhere from a foot to two and a half feet away from your eye (the pupil to be precise). Use the horizontal one to make sure that the vertical one's always the same distance from your eye, whatever that distance will be. You could even cut the horizontal one down to the length you want and press one end into your cheekbone while holding them up, and/or attach them to each other with a (possibly hinged) bracket, to eliminate the chance of "holding them wrong".

Then, once you've established that the vertical one's distance from your eye is a known constant, read the moon's apparent height above the horizon in inches or centimeters along the vertical yardstick. That reading divided by the vertical stick's distance from your eye (as measured with the horizontal one) equals the tangent of the moon's angular distance above the horizon. It's essentially a homemade simple sextant. If you can't clearly see a flat horizon where you live, you could also put a bubble-level on it to make sure it's not tilted.

The simplest reasonably precise way to measure such things in modern times is with a clinometer, but it's unlikely that you'd be able to get one soon enough, and they might be too expensive. I used them routinely when I worked in forestry. A clinometer measures angular heights and depths above and below horizontal but is easier to use than a sextant; it's a sealed metal box a couple of inches long and tall by probably less than half of an inch wide, with an eyehole at the back, a light-intake hole at the front, and a rotating compass-like wheel inside that's weighted to always stop in the same orientation with respect to the vertical and horizontal axes. When you hold it right up to your eye and look through it, you see lines and numbers on a tiny internal ruler-like scale that is aligned vertically in your view, showing what angle above or below the horizon each little gradation is, in straight degrees, radians, or grads... and sometimes there's another scale in linear units, like feet, meters, or "logs" for when you're measuring tree heights (but then you have to be standing at the right distance away from the tree for the scale to be correct).

Delvo
2007-Jan-25, 07:18 AM
PS: I just checked the website of a major forestry supply company. Clinometers go for over a hundred and twenty dollars. That's massively more expensive than a couple of yardsticks (and maybe a bracket, some screws, and/or a bubble level if you get fancy). And they don't give you that "do it yourself" feeling. :D

Tog
2007-Jan-25, 08:04 AM
I built a clinometer that would be more than accurate enough for what you describe for about $1.20 US.
[Start MacGuyver Theme]
You need a Bic Pen, a paper clip, a bit of tape, a lovely bit o'string, and a protractor.

Tie a knot in the string on one end. Thread it through the center hole in the protractor. Not the big one in the middle, but the one on the straigth edge side that is supposed to mark the intersection of the two lines beign measured.

Leave enough string to hang at least an inch past the outside edge of the protractor. Tie the paper clip to the loose end. The string should be soft enough that it will hang more or less straight down with the paper clip used for weight.

Take the ink stick out of the pen so you are left with a hollow tube. Tape, (or glue) the now hollow pen tube to the protractor so that it runs parallel to the straight edge part.

Now, to use it, you look though the pen at the object with the round part of the protractor pointing down. When the paper clip stops swinging, press the string against the angle scale with your thumb. If you are looking at the horizon, you should get a reading of 90 degrees. Just sutract your angle reading from 90 to get the angle. If it reads 75, it means 15.
[/End MacGuyver Theme]

You may need to play with the design a bit. I think my first version used a strait paper clip instead of string, but that may have been the second version. Either way, it will work for what you need it to do, it's cheap to make, and it can be stored just about anywhere.

Disclaimer: Use of this device for navigation is not advised.

Jeff Root
2007-Jan-25, 10:55 AM
I made an astrolabe for this purpose, so long ago that I can't recall
whether it was for the college intro astronomy course I took, or high
school physics, or a junior high school science class, or just on my
own. Cost zero, but accuracy pretty poor and I've forgotten the
construction details.

The deal with measuring with your fist is that most people's fists
are more-or-less proportional to their arm length, so they'll get
similar numbers even if they have very different-sized fists.

Never having seen one, and not knowing how they work, I wonder
if a golfer's rangefinder can be used like a clinometer...

I see that first quarter is today, so you probably want to get
observing immediately!

I also see that my wall calendar has the labels for "first quarter"
and "last quarter" reversed for January through July! The National
Wildlife Federation is going to get an e-mail from me next!

-- Jeff, in Minneapolis

Tog
2007-Jan-25, 11:02 AM
Never having seen one, and not knowing how they work, I wonder
if a golfer's rangefinder can be used like a clinometer...


The ones I've seen have an internal reticle that represents the height of the flag stick at different ranges. I don't think it would be very useful in this situation.

grant hutchison
2007-Jan-25, 03:44 PM
I asked my wife to take care of it instead, but her fist is smaller than mine, so the observation won't match up with my own.Well, the deal with the fist thing is that her arm isn't as long as yours, either. Since the sizes vary in rough proportion, your fist will subtend roughly the same angle at your eye as her fist will at her eye. So spacing out angular distances with fingers, fists and spread hands is something you can teach children to do with some success, and then you find that the measured distances don't change as they grow up!

Grant Hutchison

Hamlet
2007-Jan-25, 07:06 PM
Any suggestions? Should I buy a sextant? Is the difference between the size of our hands going to make THAT much of a difference? (the teacher said she's not too concerned about being very accurate)

Here's a link to building a homemade clinometer (http://www.rondexter.com/professional/sun/home-made_clinometer.htm).

Here's (http://www.geocities.com/Yosemite/Trails/2642/articles.html#Clino) one that is a little more basic.

Good Luck!

Amber Robot
2007-Jan-25, 08:15 PM
Any suggestions?

Yes, have your wife do all the observations and then there won't be an inconsistency. :p

ArgoNavis
2007-Jan-25, 08:29 PM
Here's a link to building a homemade clinometer (http://www.rondexter.com/professional/sun/home-made_clinometer.htm).

Here's (http://www.geocities.com/Yosemite/Trails/2642/articles.html#Clino) one that is a little more basic.

Good Luck!

This looks similar to one I saw in a book many years ago, only it used a drinking straw instead of a ruler.

I was wondering why an astronomy teacher would get their class to measure the altitude of the Moon. I am not sure what this is demonstrating. Actually you could use a planetarium program like Cartes du Ciel, and that would save actually going outside and observing. The Moons motion is fairly complex, as I understand and goes through a 18 or 19 year cycle. I can't see the point of getting anyone to measure altitude over a few nights. :confused:

Tog
2007-Jan-25, 08:43 PM
Maybe to practice using natural measuring tools on a known object to see if they are doing it right?

Bones01
2007-Jan-26, 01:59 AM
Thanks guys! I bookmarked the sites on building the clinometer (working on discrete math homework so I'll get back to that later). The ideas are great and sure give me a lot of options.

Oh, and Tog... http://www.mania.com/image/374719/250_large.jpg

Jeff Root
2007-Jan-26, 04:47 AM
I was wondering why an astronomy teacher would get their
class to measure the altitude of the Moon. I am not sure what this
is demonstrating. Actually you could use a planetarium program like
Cartes du Ciel, and that would save actually going outside and
observing. The Moons motion is fairly complex, as I understand
and goes through a 18 or 19 year cycle. I can't see the point of
getting anyone to measure altitude over a few nights. :confused:
OH MY, OH MY, OH MY!

The purpose of observing the motions of the moon is to acquire
some idea of what the hell you are studying. Astronomy is not
about the locations of points of light on a computer monitor.

My oh my oh my.

-- Jeff, in Minneapolis

Jeff Root
2007-Jan-26, 04:55 AM
The deal with measuring with your fist is that...


Well, the deal with the fist thing is that...
So, were you unconsciously (or consciously) influenced by
my wording, or is this yet another instance of great minds
thinking alike?



the measured distances don't change as they grow up!
Good added point!

-- Jeff, in Minneapolis

ArgoNavis
2007-Jan-26, 05:53 AM
OH MY, OH MY, OH MY!

Astronomy is not about the locations of points of light on a computer monitor.


What!? Really? Are you sure?

grant hutchison
2007-Jan-26, 12:10 PM
So, were you unconsciously (or consciously) influenced by
my wording, or is this yet another instance of great minds
thinking alike?This is an instance of my not having read your post, I think. :o I wouldn't have duplicated your content if I'd known you'd said it already, since endless posts saying the same thing annoy the hey out of me when other people do it. So the similar choice of wording must be fortuitious.

Apologies for not giving you due attention - I don't know how I missed it.

Grant Hutchison

Jeff Root
2007-Jan-26, 12:35 PM
Another possibility is that I've been unconsciously influenced by
your posts in other threads...

-- Jeff, in Minneapolis

SolusLupus
2007-Jan-26, 07:39 PM
What!? Really? Are you sure?

I'll have to agree with this.

Machines have greatly enhanced our abilities. Using the "old skool" tools are good for history lessons, and understanding how people did it in the past, but you might as well show someone how to swing a sword in order to use a firearm. ;)

BioSci
2007-Jan-26, 08:00 PM
I'll have to agree with this.

Machines have greatly enhanced our abilities. Using the "old skool" tools are good for history lessons, and understanding how people did it in the past, but you might as well show someone how to swing a sword in order to use a firearm. ;)

Perhaps, if you are looking at distant stars (rarely visualised other than on screens) - but if you are starting with the motion of the moon across the sky - real visual observation is just so much more real !
as indicated in the OP: "I dunno if this is even going to happen or not, since I know next to nothing about how the moon is going to move through the sky during these observations"

SolusLupus
2007-Jan-26, 08:03 PM
Perhaps, if you are looking at distant stars (rarely visualised other than on screens) - but if you are starting with the motion of the moon across the sky - real visual observation is just so much more real !
as indicated in the OP: "I dunno if this is even going to happen or not, since I know next to nothing about how the moon is going to move through the sky during these observations"

In my astronomy class, I'd give the sky hardly any credence if it wasn't visually shown on the projetor screen, via computer simulation, the different constellations and star patterns in the sky, and how they revolve around Polaris. It was then I truly realized how the celestial sphere worked (visually).

Either way, I do not agree that the only way to handle learning the experience is the "old skool way", but I would have to say that going out at least once in a while and actually viewing it for yourself DOES help and IS a good idea. I just don't think that it's necessary, and that simulations and programs can teach a lot more than simple visual at night.

Jeff Root
2007-Jan-28, 03:22 PM
Some things can most readily be learned by reading or listening to
what others have to say about them and by viewing photographs or
films or computer simulations. Some things can only be learned by
direct experience. Until you do both for any one bit of knowledge,
you can't be sure which category it falls under.

-- Jeff, in Minneapolis