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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation > Celestial navigation (quiz)
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Topic Review (Newest First)
03-02-2010 01:31 AM
AdamLein
Quote:
Originally Posted by magnusmurphy View Post
Thanks a lot. So, does the RA have more of an association with SHA then (which is measured from the Meridian of Aries)?
RA = 360 - SHA.
03-01-2010 03:28 PM
magnusmurphy Thanks a lot. So, does the RA have more of an association with SHA then (which is measured from the Meridian of Aries)? This seems to be academic anyway, if it's not being used by navigators. Now I have to wrap my head around all these meridians....(prime, Aries, geo, celestial...not to speak of the ecliptic etc.) Very confusing, but fascinating. Wish I started this long ago.
Those navigators of old were a very clever lot. Puts most of us to shame.

M Murphy
03-01-2010 12:44 PM
AdamLein
Quote:
Originally Posted by magnusmurphy View Post
What is the difference between GHA and RA (right ascension)? It sounds like the same thing to me. Is it?
The two coordinates are similar in that they measure east-west angular distances, but beyond that they are quite different.

The first difference is that GHA is an angular distance relative to the terrestrial prime meridian. The GHA of a star changes throughout the day, but the GHA of a geostationary satellite is constant.

RA, however, is measured relative to the celestial prime meridian or first point of Aries. The RA of a star barely changes at all during the day (it may change over a period of centuries because stars do move).

The second difference is the direction in which they are measured. GHA, like all hour angles, is measured westward from the Greenwich meridian to the body---think of an hour angle as representing the amount of time since a celestial body has passed a given point. RA is measured eastward (probably because that's the direction angles are measured in in righthanded coordinate systems).

The last distance is that mariners don't use RA; astronomers and maybe rocket scientists do

Quote:
Also, is the celestial meridian (defined as the line extending from the North Celestial pole through the vernal equinox to the South Celestial Pole), the same thing as the Aries meridian?
There are many celestial meridians and one celestial prime meridian. Aries, a constellation, doesn't really have one meridian, but the first point of Aries does, and it is indeed the point where the celestial equator crosses the celestial prime meridian.
02-28-2010 11:55 PM
magnusmurphy Guys, as mentioned earlier, I'm just starting to learn CN and also this also launched me into a general reading of basic astronomy. One question that came up today is this:

What is the difference between GHA and RA (right ascension)? It sounds like the same thing to me. Is it?

Also, is the celestial meridian (defined as the line extending from the North Celestial pole through the vernal equinox to the South Celestial Pole), the same thing as the Aries meridian?

M Murphy
02-23-2010 10:02 AM
AdamLein Omo: I've never heard the term "ground point" before either but I can see it making sense as it's the point on the ground under the celestial body.

But okay let's move on to a new question:

The nautical almanac lists Greenwich hour angles (GHA) for the sun, moon, planets, and first point of Aries, and sidereal hour angles for the stars. For navigation, we need the local hour angle (LHA) for each.

1. Why the difference?
2. How do you compute LHA in each case?
02-23-2010 02:25 AM
Omatako
Quote:
Originally Posted by AdamLein View Post
I would add, for intuition purposes, that a body will get higher in the sky as you approach its GP (geographical position), and lower in the sky as you get farther away. So if computed altitude Hc is greater than the observed altitude Ho, that means that the assumed position is closer to the GP than you are.
Yes and conversely if the Ho is greater than the Hc then it is further from the GP than you are. Hence towards and away. If it closer then your LOP is towards, if it is further then the LOP is away.

I'm interested in the concept of GP being a "ground point". I've always know it to be the geographcal position. I guess either one works, I've just never heard GP referred to as ground point before.

The best description of the GP is:

"If a line could be drawn between the celestial body and the centre of the earth, the point at which the line passes through the surface of the earth is the GP". That puts it into clear perspective (for me anyhow )
02-22-2010 10:40 PM
AdamLein I would add, for intuition purposes, that a body will get higher in the sky as you approach its GP (geographical position), and lower in the sky as you get farther away. So if computed altitude Hc is greater than the observed altitude Ho, that means that the assumed position is closer to the GP than you are.
02-22-2010 08:34 PM
btrayfors The GP is not your DR or assumed position. It's the Ground Point of the celestial body under observation. That is, it's the point on earth which at the precise time of the sight lies directly under the celestial body you're using.

After you've determined the true sextant altitude of that celestial body (using your sextant and applying corrections), and you've determined the calculated altitude for that celestial body at the time of the sight, you compare the two and come up with a difference in minutes or nautical miles (nm).....say 8nm.

Now, you draw a line FROM your DR or assumed position TOWARDS or AWAY from the GP of the celestial body, using the calculated Azimuth. Then, you draw a perpendicular to that line and you have a line (part of a huge circle) on which the boat is somewhere located.

Here are the steps as listed on the BBC website:

"You would actually proceed as follows:

1. Measure the true altitude of the Sun.

2. Determine the tabulated altitude of your DR position.

3. Take the difference between them (in minutes, which is also nautical miles). Let's say it is eight miles.

4. Plot your dead reckoning position on the chart.

5. Draw a line towards (or away from) the Sun's geographical position using the azimuth from the DR position.

6. Measure off eight miles, or whatever your difference was.

7. At this point draw a line at right angles. This is a small part of the boat's position circle."

Bill
02-22-2010 07:44 PM
Boasun Ground point is the assumed position you are using for that celestial body.

What is the meaning of toward or away and how do discern the difference?

This is part of your routine math problem that you have with each celestial body...
02-22-2010 01:15 PM
AdamLein
Quote:
Originally Posted by Boasun View Post
I don't see looking for the little known bits of celestial science will help.
But then you do need to understand working a celestial problem.
Not sure exactly what you mean here, but I think understanding what's going on "behind the scenes" helps give you (i.e., me) a better intuition for the whole operation, which is invaluable when solving problems (which is often done by rote).

Quote:
Such as What do they mean with "toward" or "Away" from the ground point?
Group point = GP = geographical position, right?
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