Sunrise

[Graham_Wright]

Plodding through the explanations of astro navigation, I wondered why I have found no reference to sunrise.
Spotting the lower limb on the horizon and making allowance for height of eye, I would have thought the time of observation would be a useful factor.
I have found many reference to the determination of time of sunrise based on lat and long values but not the reverse.
I would have thought timing sunrise could be an input to position fixing.

 

[Skylark]

Tables of sunrise and sunset can be found in the Almanac.

Very commonly used for compass checking, an alternative method to using its azimuth derived from a sun sight.

 

[neil_s]

In theory, you are correct. In practice, errors due to refraction are large - according to my table, even at one degree above the horizon, refraction is nearly one half of a degree to be subtracted. This implies that when you see the sun just kissing the horizon, you have already missed it!

 

[capnsensible]

I was taught not to take sunsights if it is 10 degrees or less above the horizon due to those innaccuracies.

 

[Skylark]

In theory, you are correct. In practice, errors due to refraction are large - according to my table, even at one degree above the horizon, refraction is nearly one half of a degree to be subtracted. This implies that when you see the sun just kissing the horizon, you have already missed it!

That’s interesting, Neil.

Your comment had me reaching for my Bowdich. He discusses checking compass error by azimuth and amplitude in chapter 17. He presents a table of “correction of amplitude as observed on the visible horizon” This is to account for the slight change in bearing as the body moves between the visible and celestial horizon.

The arguments are latitude and declination. The correction is <1 deg for lat up to around 50 deg and up to around 20 deg declination.

For a typical winter Atlantic crossing the correction is shown at around 0.5 deg.

To which table are you referring?

 

[neil_s]

I am using the page of corrections from the online sun almanac provided by www.backbearing.com.

 

[Skylark]

I’m not following, sorry, Neil. Which table within the sun almanac