Celestial Sun Declination Navigation Aid

Celestial Sun Declination Navigation Aid: this section of our self-instruction website underscores the significance of comprehending the theory of the sun’s Declination, a pivotal factor in understanding  the line of position (LOP), also known as the Sumner line.

Celestial Sun Declination Navigation Aid, image relation polar star and axis of the earth
relation polar star and axis of the earth
Coming home. author image: Michael Rapp

Celestial Sun Declination Navigation Aid:

The theory of the sun’s declination: Earth’s axial tilt, about 23.5 degrees, causes seasons as it orbits the sun. This tilt results in varying sunlight exposure, creating summer and winter conditions in different hemispheres throughout the year.

There is indeed a page on how to calculate the Declination with the help of the nautical almanac and the exact time of the observation at sea.


Celestial Sun Declination Navigation Aid:

point PG

To clarify a thread stretched between the sun center and the earth center strikes the earth’s crust at point PG

Celestial Sun Declination Navigation Aid: image point PG
point PG
Celestial Sun Declination Navigation Aid, image GHA and D

GHA and D represent the coordinates of the sun on Earth for point PG

Indeed, it can be concluded that D is equivalent to the latitude and GHA is equivalent to the longitude at that moment.

However, the declination of the sun (latitude) can never exceed 23° 44′ N or 23° 44′ S.

The nautical almanac gives us for each day and each whole UT hour this Declination and GHA.


Yacht in St Tropez FRANCE. Author image: Liliane Paingaud

Celestial Sun Declination Navigation Aid:

explanation of the solar declination

In fact the declination of the sun is due to the axial tilt of the Earth.
The angle between the earth’s axis and a line perpendicular to the earth’s orbit is called the obliquity.

Also always think of the point PG:  A thread stretched between the sun center and the earth center strikes the earth’s crust at the point PG.

equinox and solstice
Celestial Sun Declination Navigation Aid, image axis always point to the polar star
Celestial Sun Declination Navigation Aid, image solstice/équinoxe

Imagine looking from the sun to the Earth. At the beginning of summer, you will mostly see the northern part of the Earth, and at the start of winter, you will mostly see the southern part. This is because the axis of the Earth always points towards the polar star, even when it is orbiting around the sun.

The Earth exhibits three primary movements: it rotates daily around itself, orbits yearly around the Sun, and maintains a nearly constant axis angle with only minor variations.

Celestial Sun Declination Navigation Aid, image winter solstice
winter solstice
Celestial Sun Declination Navigation Aid, image winter solstice and equinox
Celestial Sun Declination Navigation Aid, image equinox
equinoxe

Celestial Sun Declination Navigation Aid:

To summarize, the Sun’s declination varies due to Earth’s axial tilt as it moves around the Sun in its annual elliptical orbit, with this angle never exceeding 23.5°.

In other words the declination of the sun is increasing from the winter solstice to the summer solstice  

( -23° 26’ au 23°26’ )

and decreasing from the summer solstice to the winter solstice.

( 23°26’ au -23°26’ )

Passing through the equinoxes where the declination is 0°

Celestial Sun Declination Navigation Aid, image tropic of cancer/capricorn
The sun’s declination throughout the year.

Of course the declination of the sun is important to us since we use them in our formulas for solving the spherical triangle.

That is to say these two data ( D ) and ( GHA ) are used with the time (TU) in our mathematical formulas (resolution of the spherical triangle)

Tornadoes on the Tyrrhenian sea in Italy. Author image: Tulumnes
The nautical almanac
Celestial Sun Declination Navigation Aid, the use of the nautical almanac and the daily pages

While, the nautical almanac gives us for each day and each whole UT hour this Declination and GHA.

Therefore, it is up to us to interpolate to have the Declination and GHA at the exact UT time of the observation. That is, down to the second!

The theory of the sun's declination,image nautical almanac

daily page of the nautical almanac

Celestial Sun Declination Navigation Aid, image daily page nautical almanac
Icebergs in the Lemaire Channel, Antarctica. Image Author: Liam Quinn

Celestial Sun Declination Navigation Aid:

Latitude and Solar Declination:

An Analysis of Seasonal Variations:

These seasonal changes are due to the inclination of the Earth’s axis of rotation relative to its orbit around the Sun. This inclination leads to seasonal variations in the angle of incidence of solar rays at different latitudes throughout the year. The farther one is from the equator, the more pronounced these variations become.

Celestial Sun Declination Navigation Aid:

High latitudes

As one gets closer to the North Pole, seasonal variations become more pronounced. At higher latitudes, such as those near the North Pole, four distinct seasons are generally observed: spring, summer, autumn, and winter. These seasons are characterized by changes in temperature, day length, and the availability of sunlight.

Spring: Spring marks the beginning of the warming period after winter. Temperatures rise, snow melts, and vegetation begins to awaken.

Summer: Summer is typically the warmest season, with long days where the sun remains high in the sky for a greater part of the day. It is the period of maximum vegetation growth.

Autumn: Autumn is characterized by a gradual decline in temperatures and the changing color of tree leaves before they fall. Days become shorter.

Winter: Winter is the coldest season, with shorter days and longer nights. Snow and ice may be present, and vegetation enters a dormant phase.

Celestial Sun Declination Navigation Aid:

The tropics:
The theory of the sun's declination, image RUISUI tropic of cancer marker
YangChen(TW) Ruisui Tropic of Cancer Marker

In the tropics, the concept of seasons is often different from that in temperate regions. The tropics are generally characterized by two main seasons: the rainy season (or wet season) and the dry season. These seasons can vary in intensity and duration depending on the specific location within the tropics.

Rainy Season (Wet Season): During this period, there is usually an increase in precipitation. The rains can be abundant and regular, contributing to lush vegetation growth. This season is often associated with high temperatures.

Dry Season: This is a period when precipitation is less frequent, or even absent for several months. During the dry season, temperatures can be high, and there may be an increase in drought conditions.

Celestial Sun Declination Navigation Aid:

The equator

It’s important to note that the variation between these two seasons can be more subtle in some parts of the tropics, while in other regions, the changes can be more pronounced. The proximity of a location to the equator also influences how seasons manifest.

The theory of the sun's declination, image of golden equator on map
Chuttersnap Golden equator

The closer one is to the equator, the less pronounced the seasonal variation