Sun’s altitude and logarithm tables at sea

On this page Sun’s altitude and logarithm tables at sea, we will find a fully worked-out example of how to calculate the height of the sun (Hc) using logarithm tables at sea.

Calculated sun's altitude and logarithm at sea, image calculated circle and observed circle

Observers who see the sun at the same altitude are on the same position circle. (Red Circle)

We can also calculate the sun’s altitude from our DR position, resulting in a position circle for the same calculated altitudes (Blue Circle)

On this page, we focus on how to calculate the height of the sun with our D.R. position (Blue circle) using logarithm tables.

Note: Under normal navigation circumstances, the circles are so large that locally we can regard them as straight lines

For further information, please refer to the principles of the sun sight.


 Hurricane Igor (Bermuda). Author image: Lee & Chantelle McArthur

Sun’s altitude and logarithm tables at sea: using the tables

In fact, the inputs obtained in the first part of the worksheet necessary to find the calculated height (Hc) are:

L = 16° 09′ N (D.R. latitude)

D = 19°21′,9 S (declination of the sun)

P = 42°15′,1 (NE) (polar angle)

Calculated sun's altitude and logarithm at sea, image part of the worksheet

See: same name/not same name

L and D not same name: ( L+D ) = 16° 09′ + 19°21′,9 = 3 5° 30′,9


Sun’s altitude and logarithm tables at sea:

Table 1

The tables below are not very readable since they have been transformed into jpeg or png files.

You can always view the original PDF files that are available for download.(see below)

let's start with sextant-sun-sight-observations and exercises
Calculated sun's altitude and logarithm at sea, table 1

log cosine 16°09′ (latitude)

log cosine 16° 09′ on our scientific calculator gives us -0.017485.

It is nevertheless much more convenient to put these sorts of logarithms in the form of positive quantities.

American tables are then presented in the following form:

10 + (-0,017485) = 9,98251

Log cosine 19° 21′,9 (declination)


Sun’s altitude and logarithm tables at sea:

Table 2

Calculated sun's altitude and logarithm at sea, table2

Log versine 42° 15′,1 (polar angle)


Sun’s altitude and logarithm tables at sea: Table 3

Calculated sun's altitude and logarithm at sea, table 3

L and D not same name: ( L+D ) = 16° 09′ + 19°21′,9 = 3 5° 30′,9

cosine 35° 30′,9 ( L + D )


1T 1………….LOG COS L = 9,98251

2T 1………….LOG COS D = 9,97470

3T 2….LOG VERSINE P = 9,41461

_______________________________+

………………LOG 2e Term = 29,37182


Calculated sun's altitude and logarithm at sea, table 4, logarithms of whole numbers

Indeed, Table 4 can be used to convert the logarithmic result (log 2e Term) back to a natural number (Nat 2e Term)

part of the worksheet.

Firstly, with the Mantissa we will search for the whole number in table 4 (logarithms of whole numbers.)

Calculated sextant height with the tables table 4
table 4: logarithms of whole numbers.

Finally, after finding the whole number (2354), we have to put the comma!

natural 2e term = 0,2354


Sun’s altitude and logarithm tables at sea

Table 5

Calculated sun's altitude and logarithm at sea, table 5

T 3………..COS (L +/- D) = 0,81395

T4………………..NAT 2eT = 0,2354

____________________________-

………………………..SIN Hc = 0,57855

T 5………………………….Hc = 35° 21′

SIN Hc = 0,57855

T 5………………………….Hc = 35°21′

In fact, with calculator: arcsin (0.57855) = 35° 21′


Nove Scotia, hurricane Katia. Author image: Fiddler from Canada

Sun’s altitude and logarithm tables at sea:

Brief summary

Firstly, in celestial navigation, navigators measure the angle between the horizon and the sun using a sextant to determine the observed height or the observed altitude.

Additionally, celestial navigators obtain the calculated height, which is determined using trigonometric formulas considering the observer’s geographic coordinates, the sun’s declination, and other relevant parameters.

comparing the observed height with the calculated height.

Furthermore, navigators can correct any potential errors and find their Line of Position by comparing the observed height with the calculated height.

Moreover, this comparison between the observed height obtained from a sextant and the calculated height plays a crucial role in celestial navigation, allowing navigators to refine their calculations and improve the accuracy of their dead reckoning position.

By comparing the observed height with the calculated height, navigators can provide a reliable method for determining a ship’s location when other navigational aids are unavailable.