Calculating 'Dip Short' ?

[sneuman]

How does one calculate dip short for taking a sight on anything less than a natural horizon? I can only find height of eye dip in the Nautical Almanac.

Also, once the above correction is calculated, assume it is applied to the Hs in the same manner as height of eye correction ...?

Thanks.

[nigel1]

I have never done this, but have read about it I have the theory in an old Admiralty Navigation Manual Vol 3 (1938 ed), but not with me, I found this on another site
In answer to your question, you aim your telescope at the reflection of the obect in the artificial horizon. You then adjust your index so that the image from the index mirror is superimposed on the reflection. That way you are measuring exactly double the angle from the horizon to the object. You will see that by rocking the sextant you can take out the vertical error. You could get the same effect by laying a mirror on a table if you could guarantee that the mirror was perfectly horizontal. But you can't - so you use a liquid mirror, which is perfectly horizontal.

The angle is then halved, and apparently dip is not relevent

[sneuman]

Originally Posted by nigel1

I have never done this, but have read about it I have the theory in an old Admiralty Navigation Manual Vol 3 (1938 ed), but not with me, I found this on another site
In answer to your question, you aim your telescope at the reflection of the obect in the artificial horizon. You then adjust your index so that the image from the index mirror is superimposed on the reflection. That way you are measuring exactly double the angle from the horizon to the object. You will see that by rocking the sextant you can take out the vertical error. You could get the same effect by laying a mirror on a table if you could guarantee that the mirror was perfectly horizontal. But you can't - so you use a liquid mirror, which is perfectly horizontal.

The angle is then halved, and apparently dip is not relevent

Nigel, you're absolutely right on the method for using an artificial horizon. I've done that with great success.

What I'm talking about is a little different. I'm referring to shooting a sight to a landmass that is short of a natural horizon.

In other words, the other side of the river is 1nm away, but at my height of eye, a natural horizon is 3nm away. So, I do a correction for using the "dip short" of the opposite river bank instead of a real horizon.

[zydecotoad]

I don't think sneuman was talking about an artifical horizon, but a shortened one.

If you are not on the open ocean, but a small bay or lake, you use the far side of the water for your "horizon" but since it is not far enough away, it is not at the same angle as the distant horizon.

There is an adjustment table in Bowditch and most sight reduction tables to account for this.

[sweetsailing]

Hi there,

As zydecotoad stated, there are formulas in Bowditch. However, let me share what I learned on this topic from our recent celestial navigation class.

Several formulas can be used in determining the dip correction. These vary depending on the units used for height of eye and distance. Use caution in selecting the proper formula for the units of height of eye and distance. Avoid dip corrections greater than 200 feet as these formulas result in greater inaccuracies. If such sights must be used, determine the dip short correction using the formula in Bowditch.

Formulas

d in yards, h in feet
Ds = 0.0002052 d + 1146 h/d

d and h in meters:
Ds = 0.0002244 d + 3438 h/d

d in nautical miles, h in meters
Ds = 0.4156 d + 1.856 h/d

d in nautical miles, h in feet
Ds = 0.4156 d + 0.5658 h/d

d in statue miles, h in feet
Ds = 0.3611 d + 0.6511 h/d

Also, once the above correction is calculated, assume it is applied to the Hs in the same manner as height of eye correction ...?

Yes, correct

[sneuman]

Originally Posted by sweetsailing

Hi there,

As zydecotoad stated, there are formulas in Bowditch. However, let me share what I learned on this topic from our recent celestial navigation class.

Several formulas can be used in determining the dip correction. These vary depending on the units used for height of eye and distance. Use caution in selecting the proper formula for the units of height of eye and distance. Avoid dip corrections greater than 200 feet as these formulas result in greater inaccuracies. If such sights must be used, determine the dip short correction using the formula in Bowditch.

Formulas

d in yards, h in feet
Ds = 0.0002052 d + 1146 h/d

d and h in meters:
Ds = 0.0002244 d + 3438 h/d

d in nautical miles, h in meters
Ds = 0.4156 d + 1.856 h/d

d in nautical miles, h in feet
Ds = 0.4156 d + 0.5658 h/d

d in statue miles, h in feet
Ds = 0.3611 d + 0.6511 h/d



Yes, correct

excellent. many thanks.