|
Author
|
Topic: Mark 3 Sextant use
|
John Pinto
|
posted April 27, 2021 10:10 AM
I just got my Mark 3 Sextant and something just doesn't seem right to me. I'm sure I'm doing something wrong.
When I look at the sun and adjust the scale to the approximate altitude of the sun (checking it with my astronomy app on my iPhone which gives the current altitude of the sun at my location) so I can see the sun in the mirror, the illustration (from the manual) is not what I see (attached is a snip from it).
I see my horizon to the left of the mirror and I see the sun in the mirror but I don't see the horizon in the mirror.
Do I need a brighter horizon?
Is the illustration misleading?
Am I doing something wrong with how I am using this sextant?
One thing I noticed on the Mark 3 is the horizon mirror has a plastic back to it so there is no way for the horizon to be viewed through the horizon mirror --- or again am I not understanding how this particular sextant works?
From: Orlando
|
|
David Burch
|
posted April 27, 2021 12:09 PM
First, I understand that you purchased the sextant at Starpath, so we know they have been individually inspected, so that is not an issue.
Next what we see on a phone app might not be dependable. Best bet is use the data you can get at www.starpath.com/usno That will get you Hc, which is about what the sextant would read... assuming you are measuring relative to a true sea horizon or the shoreline that is a mile or so across from you in the direction of the body.
You will not see the horizon in the mirror. Horizon is only in the open view, the other side is all sky and sun. Looks like the image sent was a schematic illustration of a full-view horizon mirror, which is not what the Mk 3 has, nor is it what you want.
I do not see any reason to think anything is wrong. I understand you ordered the book GPS Backup with a Mark 3 sextant. That is the most detailed source available for this instrument. Suggest using it and discarding the stock manual.
From: Starpath, Seattle, WA
|
|
|
|
David Burch
|
posted April 27, 2021 12:50 PM
Wow! I am totally embarrassed. That picture is wrong! Mirror should be only sun and sky. I will see if we have any others like that.
From: Starpath, Seattle, WA
|
|
David Burch
|
posted April 27, 2021 01:06 PM
That is the only example. Others in the book are right. We will post an errata and fix it in the next printing.
Thanks for pointing this out. You were completely right to be confused by that, and to indeed rightfully question it.
From: Starpath, Seattle, WA
|
|
John Pinto
|
posted April 27, 2021 05:18 PM
I take it from this errata illustration the dotted lines are supposed to represent where we should imagine the left half of the sun would be if we had a full-view horizon mirror. But in actuality we will not see anything in that position other than sky. Am I interpreting that illustration correctly?
From: Orlando
|
|
David Burch
|
posted May 21, 2021 09:57 AM
Yes, you can think of it that way for the Mark 3... ie what it would look like if it were there.
This image is a bit of a hangover from similar images from other sextants that have a clear glass on that side, rather than open air as the Mk3 has, and even with clear glass there is a bit of a reflection from that so this schematic is less of a stretch.
Best approach is take a few sun sights to see what it actually looks like during the process.
Take a look at Figure 3-2 to see if that helps. The Figure 2-2 insert you refer to was intended to just give a rough overview of what is seen.
We will add a note on this to the next printing in fig 2-2 to look at 3-2 which has some discussion. Thanks for pointing this out.
From: Starpath, Seattle, WA
|
|
John Pinto
|
posted May 21, 2021 11:24 AM
Yes figure 3-2 is somewhat helpful David but it still has the dashed outlines of the sun over the sea horizon which could be confusing to the first time user (whom the text is intended for) without some further explanation as to what it represents. Otherwise like me you might think there is something wrong with the sextant or how I am using it! Which is what led to this discussion.
I realized early on that these graphics were probably taken from a text for a sextant that used clear glass to look over the sea horizon (the Mark 15?) and the graphic was meant to illustrate the potential reflection of the sun in the clear glass.
But for this Mark 3 sextant focused book I think it is just confusing. Maybe those dashed outlines of the sun over the sea horizon should just be removed from Figures 2-2 and 3-2 in the interest of clearness for someone who has never used a sextant before.
From: Orlando
|
|
David Burch
|
posted May 21, 2021 11:55 AM
Thanks.
From: Starpath, Seattle, WA
|
|
