# My new Planisphere watch

1. Feb 3, 2004

### one_raven

I bought a cool new watch with a Planisphere face.
Click to see it
All along the outer ring of the watch face it has the 12 months broken down in five day increments.
There is a dial on the face with a solid shape (something like a crescent moon, but larger).
Around the outside diameter of the crescent it has the times from 6PM to 6AM.
On the crystal there is a sky map showing the major constellations.
When you align the time on the dial with the date around the ring of the face, the crescent represents the earth, and everything that is not covered by it is what is visible in the night sky.
Turn the watch to face north (the directions are on the outer dial) and you have a map of the current visible sky with the horizon at the bottom.

It really is a pretty cool watch.

Anyway, onto the questions...

The planisphere built into this watch is designed for the Northern Hemisphere (between 35 - 50 deg. lat.).
How much is the accuracy affected with each degree of lattitude?
Let's say I designed a planisphere that was 100% accurate on the equator.
If I travel to 60 deg lattitude, how far off would it be?
Is the correction that would have to be made linear? In other words, would I have to make the same level of correction going from 20 deg to 40 deg as I would going from 40 deg to 60 deg, or would the correction be less as I head further towards the pole?

It seems that I could tell the exact date and time by the position of the stars in the sky. It doesn't seem too difficult of a task to create a star calendar/clock if it were stationary, say bolted to the ground in my back yard. Let's say, though, that I were travelling by boat and did not know the what lattitude I was at, could I determine the date and time by star positions?
Or is it that I could find my position if I knew the date and time, or I could find the date and time if I knew my position? Do I need one to determine the other?

Basically this has been a too long post to ask, "How do these darned thingies work?"

Thanks

2. Feb 3, 2004

### Cecil

The position of the stars do not uniquely identify a date and time, since the star pattern repeats itself every 23 hrs, 56 mins. If you knew the date, you could determine the time and vice versa, but you can't get both just from the stars. The stars look the same at midnight on Jan 1 as they do at 11:56pm on Jan 1, 11:52pm on Jan 2, 11:48pm on Jan 3, etc. Now if you know where the Sun is along the ecliptic, you can figure out the day, and hence, the time as well.

You can always figure out your latitude (in the Northern Hemisphere) by the angle Polaris forms above the horizon. On the equator, Polaris is on the horizon (ignoring atmospheric refraction), at 45 deg N, Polaris is 45 deg above the horizon, and at the North Pole Polaris is at the zenith.

The big problem when travelling by boat (and the one early explorers had until the invention of accurate clocks) is figuring out your longitude. The star pattern at, say, 90 deg W at midnight is the same as the stars at 105 deg W at 1am, and at 120 deg W at 2am. You need to know the date and time in order to find your longitude.

So it boils down to this: If you know the exact date and time, you can figure out your position. If you know your position, you need to know either the date or the time to figure out the other.

3. Feb 3, 2004

### one_raven

Thank you!

(deleted the rest of this post due to misreasing your last post)

Last edited: Feb 4, 2004
4. Feb 4, 2004

### one_raven

Can you elaborate on that a bit, please?

Also, what about the position of the planets?
For example:
(I know my numbers are probably way off here, but bear with me...)
Let's say that Dubhe is 90 deg above Polaris.
You know that it could be midnight on March 10th.
Or it could be 11:56 on March 11th.
Or 10:00 on April 12th and so on...

If you know where Jupiter is supposed to be througout the year (I just picked Jupiter because it is so easy to spot and my favorite planet ) could you then use that information to determine what the date and time is?

What other techniques are there?

I have always been curious about stellar navigation, but never really started looking into it in depth.
You seem fairly knowledgable (at least) on the topic, and I am hoping maybe you can give me a bit of a boost in my start.

Do you know of any good educational sites about it?
Or mabe a book that you can highly recommend for beginners?

How does This Beginners Kit look to you?
How much does a $156 sextant really differ from a$1500 one?

Last edited: Feb 4, 2004
5. Feb 4, 2004

### one_raven

I should also mention that I plan on using stellar navigation primarily on land.
I know the tools and techinques were designed for seafaring, what things do I need to know?
For example:
Let's say I am on a camping trip in the Pacific Northwest.
If there is a mountain range to my north, it will be difficult to discern where the horizon is.
Is there a trick to that?

6. Feb 4, 2004

### ray b

there is a device to fake a horizen
most stores that sell sextants will also carry the artifical horizens for land use
or there is a way to use a pan of water

cheap plastic sextants are not as accurate or as well made as a real brass one
optics are the other big price difference ie a real glass lens vs no lens or a poor plastic one
sun-moon filters are allso a difference
cheapies are also venner vs micrometer-drum on the better units

GPS is way eazyer and faster and now cheaper way to go

if you must have a sextant look for an aircraft
one they could be found gov surpluss many years ago, good quality for a very low price
try E-bay as NOBODY uses them anymore in aircraft

Last edited: Feb 4, 2004