Topic 3 - Celestial Navigation

Module: Principles of Navigation MA0539Topic 3: Celestial Navigation

Objectives:

1. Introduction to Celestial Navigation2. Understand the concept of celestial sphere and equinoctial system of

coordinates

Introduction

When out of sight of land the navigator can use the positions of the sun, planets and stars to determine his position. If he is to understand what he is doing, he must at least have an elementary knowledge of astronomy. He must understand the relative movements of the sun, planets and stars and he must understand the tabulated information that he obtains from the almanacs and uses it in the calculation of his position.

The science of celestial navigation has developed with the development of astronomy and mathematics. In determining position at sea by astronomical observations, the navigator is making use of methods developed largely by astronomers and mathematicians who had little association with seafaring or practical navigation. The practice of ocean navigation is a very good example of the practical application of theoretical concepts.

The relative movements of the celestial bodies can be accurately predicted and, when a navigator out of sight of land determines his position he is simply measuring the difference between his position and the predicted positions of the bodies he observes. To calculate his position, he needs to actually measure this difference with a sextant, know the time that he made the observation and use this time to enter the nautical almanac to obtain the position of the body he has observed. He then uses mathematical tables and calculators to do the calculations.

Understanding the concept of the celestial sphere also known as the "celestial concave" helps considerably in simplifying the subject. The observer and the earth are imagined to be at the centre of a sphere of infinite radius and the sun, moon, planets and stars are imagined to move across the inside surface of the sphere, their movements being relative to a stationary earth; ie their relative motion.

It is necessary to have the background knowledge so that you will know the theory and principles of taking sights (sextant altitudes of celestial bodies) at sea in order to determine the ship's position.

True motion

The true motion of a celestial body is its own motion when following its path or orbit around the central body; i.e. the actual movement of a planet around the sun. When considering only the solar system, the true motion of the sun is nil because, relative to the other bodies in the solar system, it is stationary.

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Apparent Motion

The apparent motion of a body is the motion that appears to an observer on the earth and it is due to the combined effects of the movement of the earth and the movement of the body.

Thus the sun appears to rise in the east and set in the west in the course of a day because of the rotation of the earth alone. The revolution or the motion of the earth in its orbit causes the sun to appear to move along its own orbit against the background of the stars and complete one revolution in one year. Such an orbit is called the apparent path of the sun and is known as the ecliptic. The truth, of course is that the sun is the centre of the system and what we see as the movement of the sun and other heavenly bodies is only apparent. The direction of apparent motion is westwards.

Celestial sphere

To an observer on the earth's surface, the sun and all other heavenly bodies, irrespective of their actual distances, appear to be moving along the inner surface of a very large sphere having an immense radius, with the earth at its centre. This sphere is known as the celestial sphere, or celestial concave. It can be defined as an imaginary sphere of infinite radius having the earth at its centre and upon which all celestial bodies appear projected, irrespective of their actual distance from earth. Thus, for navigational purposes, all celestial bodies are considered to have the same distance from earth.


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Celestial Poles

If the earth's axes were extended to meet the celestial concave, the two points P and P1, would be the celestial poles. That which is nearer the observer is the elevated pole and the other is the depressed pole.

Equinoctial or Celestial Equator

It is a great circle on the celestial concave formed by extending the plane of the equator to the celestial concave.

Celestial Meridians

These are great circles which join the two celestial poles.

Ecliptic

The ecliptic, or the sun's apparent path, is a great circle on the celestial sphere and lies in the same plane as that of the earth's orbit, which is an ellipse. The radius of the celestial concave being infinite, it appears to us that the sun follows a circular path amongst the heavenly bodies. The ecliptic cuts the equinoctial at two points, the equinoctial points known as the vernal (or spring) and the autumnal equinoxes.

Obliquity of Ecliptic

This is the angle between the plane of the ecliptic and the plane of the equinoctial and equals the sun's maximum declination of 23½º.


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Apparent Motion of the Sun and the Seasons

As we are not directly aware of the earth's motion around the sun, it appears to us that the earth is stationary and that the sun describes an ellipse around the earth.

First Point of Aries

This is the point where the ecliptic cuts the equinoctial on March 21st when the sun changes its declination from south to north. It was so named because the point was first noted to be in the constellation of Aries.

The northern hemisphere begins when the apparent Sun in its northerly path along the ecliptic crosses the equinoctial. This occurs on March 20, and is known as the vernal equinox and is also referred to as the first point of Aries.


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Summer begins when the apparent Sun reaches its most northerly point on June 21 at the summer solstice. Summer ends when the apparent Sun, moving south, cross the equinoctial on September 22 at the fall equinox. This point is also referred to as the first point of Libra.

The northern hemisphere winter begins when the apparent Sun attains its most southerly point on the ecliptic on December 21 at the winter solstice.


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Rotation on its axis – Daylight and Darkness

The earth rotates around its axis once in one day. The rotation causes the observer to experience daylight and darkness. The distribution of sunlight on the earth is not the same at all latitudes. It will depend not only on latitude but also on the declination of the sun. An observer at the equator will experience 12 hours of sunlight and 12 hours of darkness every day.

An observer at 66.5 degrees North or South will experience sunlight, which could be 24 hours (when at 66.5 degrees North and declination 23.5 degrees same name) and another time darkness for 24 hours (latitude 66.5 degrees North with declination 23.5 degrees opposite name).


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Daylight and darkness will depend on the declination of the sun, in this case as the latitude of the observer is known.


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Topic 3 - Celestial Navigation

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