Amplitude Day Ex Meridian Exercises Pole Star
Naut. Almanac    


Celestial Navigation



Solar System


The Solar System

The sun, the most visible celestial body in the sky, is the centre of the solar system. Associated with it are at least nine principal planets. Some planets like earth have moons/ satellites.

Motions Of Bodies Of The Solar System

The two principal motions of celestial bodies are Rotation and Revolution.

Rotation is a spinning motion about an axis within the body, whereas Revolution is the motion of a body in its orbit around another body. The body around which a celestial object revolves is known as that body’s primary.

The gravitational force of the sun holds the entire solar system together

This gravitational force causes the planets to go around the sun in nearly circular, elliptical orbits.

In each planet’s orbit, the point nearest the sun is called the perihelion. The point farthest from the sun is called the aphelion.

The line joining perihelion and aphelion is called the line of apsides. In the orbit of the moon, the point nearest the earth is called the perigee, and that point farthest from the earth is called the apogee.

The distance from the earth to the sun varies from 91,300,000 at perihelion to 94,500,000 miles at aphelion.

When the earth is at perihelion, early in January, the sun’s diameter appears largest, 32.6’. At aphelion, in June the sun’s apparent diameter is a minimum of 31.5’.


The principal bodies orbiting the sun are called planets.

Nine principal planets are known: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Of these, only four are commonly used for celestial navigation:

Venus, Mars, Jupiter, and Saturn.

Except for Pluto, the orbits of the planets lie in nearly the same plane as the earth’s orbit. This plane of the celestial sphere is called the ecliptic.

The two planets with orbits smaller than that of the earth are called inferior planets, and those with orbits larger than that of the earth are called superior planets.

Planets can be identified in the sky because, unlike the stars, they do not twinkle. The stars are so distant that they are virtually point sources of light. Therefore the tiny stream of light from a star is easily scattered by normal motions of air in the atmosphere causing the affect of twinkling. The planets, however, are close enough to present perceptible disks. The light from a planet is not easily distorted unless the planet is low on the horizon or the air is especially turbulent.

The Earth

The earth rotates on its axis and revolves in its orbit around the sun.

The above motions are the reason why the other celestial bodies have apparent motions.

For most navigational purposes, the earth can be considered a sphere. However, like the other planets, the earth is approximately an oblate spheroid, or ellipsoid of revolution, flattened at the poles and bulged at the equator.

The polar diameter is less than the equatorial diameter, and the meridians are slightly elliptical, rather than circular.

Planets useful for navigation

Inferior Planets

Since Mercury and Venus are inside the earth’s orbit, they always appear close to the sun. Over a period of weeks/ months, they appear to go back and forth from one side of the sun to the other.

They are seen either in the eastern sky before sunrise or in the western sky after sunset. For brief periods they disappear into the sun’s glare.

At this time they are between the earth and sun (known as inferior conjunction) or on the opposite side of the sun from the earth (superior conjunction). On rare occasions at inferior conjunction, the planet will cross the face of the sun as seen from the earth. This is known as a transit of the sun.

When Mercury or Venus appears most distant from the sun in the evening sky, it is at greatest eastern elongation.

(Although the planet is in the western sky, it is at its easternmost point from the sun.) From night to night the planet will approach the sun until it disappears into the glare of twilight. At this time it is moving between the earth and sun to inferior conjunction. A few days later, the planet will appear in the morning sky at dawn. It will gradually move away from the sun to western elongation, and then move back toward the sun. After disappearing in the morning twilight, it will move behind the sun to superior conjunction. After this it will reappear in the evening sky, heading toward eastern elongation.

Mercury is never seen more than about 28˚from the sun.

For this reason it is not commonly used for navigation. Near greatest elongation it appears near the western horizon after sunset, or the eastern horizon before sunrise. At these times it resembles a first magnitude star.

The interval during which it appears as a morning or evening star can vary from about 30 to 50 days. Around inferior conjunction, Mercury disappears for about 5 days; near superior conjunction, it disappears for about 35 days.

Venus can reach a distance of 47˚from the sun, allowing it to dominate the morning or evening sky. At maximum brilliance, about five weeks before and after inferior conjunction, it has a magnitude of about –4.4 and is brighter than any other object in the sky except the sun and moon.

At these times it can be seen during the day and is sometimes observed for a celestial line of position.

Superior Planets

They can pass behind the sun (conjunction), but they cannot pass between the sun and the earth. Instead we see them move away from the sun until they are opposite the sun in the sky (opposition). When a superior planet is near conjunction, it rises and sets approximately with the sun and is thus lost in the sun’s glare. Gradually it becomes visible in the early morning sky before sunrise. From day to day, it rises and sets earlier, becoming increasingly visible through the late night hours until dawn. Approaching opposition, the planet will rise in the late evening, until at opposition, it will rise when the sun sets, be visible throughout the night, and set when the sun rises.

Observed against the background stars, the planets normally move eastward in what is called direct motion.

Approaching opposition, however, a planet will slow down, pause (at a stationary point), and begin moving westward (retrograde motion), until it reaches the next stationary point and resumes its direct motion.

The superior planets are brightest and closest to the earth at opposition.

Mars can usually be identified by its orange color. It can become as bright as magnitude –2.8 but is more often between –1.0 and –2.0 at opposition.

Jupiter, largest of the known planets, normally outshines Mars, regularly reaching magnitude –2.0 or brighter at opposition.

Saturn, is at opposition becomes as bright as magnitude +0.8 to –0.2.

The Moon

It revolves around the earth once in about 27.3 days, as measured with respect to the stars. This is called the lunar month.

When the moon is in conjunction with the sun (new moon), it rises and sets with the sun and is lost in the sun’s glare.

From day to day, the moon will rise (and set) later, becoming increasingly visible in the evening sky, until (about 7 days after new moon) it reaches first quarter, when the moon rises about noon and sets about midnight. Over the next week the moon will rise later and later in the afternoon until full moon, when it rises about sunset and dominates the sky throughout the night.

During the next couple of weeks the moon will rise later and later at night. By last quarter (a week after full moon), the moon rises about midnight and sets at noon. As it approaches new moon, the moon becomes an increasingly thin crescent, and is seen only in the early morning sky. Sometime before conjunction (16 hours to 2 days before conjunction) the thin crescent will disappear in the glare of morning twilight.

At full moon, the sun and moon are on opposite sides of the ecliptic. Therefore, in the winter the full moon rises early, crosses the celestial meridian high in the sky, and sets late; as the sun does in the summer. In the summer the full moon rises in the southeastern part of the sky (Northern Hemisphere), remains relatively low in the sky, and sets along the southwestern horizon after a short time above the horizon.

At the time of the autumnal equinox, the part of the ecliptic opposite the sun is most nearly parallel to the horizon. Since the eastward motion of the moon is approximately along the ecliptic, the delay in the time of rising of the full moon from night to night is less than at other times of the year.

The Earth’s elliptical orbit, and Approximate perihelion and aphelion distances and dates

The earth travels round the sun in an orbit, which is slightly elliptical. Its plane being called the Ecliptic, because eclipses happen only when the moon crosses it.

The above shows the earth in its orbit, viewed from an elevation above the plane of the ecliptic, the sun is not exactly at the centre of the ellipse and the earth is shown at the four extremities of the ellipse.

The ecliptic is thus an imaginary plane that cuts the earth and the sun in halves.

The earths distance from the sun varies, It is maximum when it is farthest away – aphelion (about 4th July) – about 93.5 million miles, and nearest – perihelion (1st January) - distance is 90.5 million miles.

Due to the above we see the sun as slightly larger at perihelion and slightly smaller at aphelion. This is the reason that the semi diameter of the sun changes from max. of 16.3’ to 15.8’ of the arc.

Eccentricity of the earth’s orbit

The force of gravity determines the way the planets and the sun move. As a result of gravity, bodies attract each other in proportion to their masses and to the inverse square of the distances between them. This force causes the planets to go around the sun in nearly circular, elliptical orbits.

The sun is not exactly at the centre of the orbit of the earth, which is an ellipse; this is the reason why the earth swings from perihelion to aphelion.

The inclination of the earth’s axis to the plane of the orbit and the stability of the axis (ignoring precession) – Cause of seasons

If the earth’s axis were perpendicular to the plane of the ecliptic the sun would be over the equator throughout the year and then we would not have any seasonal changes.

But, the earths axis is not perpendicular to the plane of the ecliptic, it is tilted at an angle of 23˚26’. The axis of the earth may be assumed to be pointing fixedly in one direction in space, about the direction of the pole star.

Due to this tilting of the earths axis from the perpendicular, the sun bobs up and along from the earths equator.

Thus when the sun reaches its maximum northerly travel along the ecliptic, when the declination is 23.5˚N, it is summer in the Northern Hemisphere. And when the sun reaches its maximum southerly travel, when the declination is 23.5˚S, it is winter in the Northern Hemisphere.

At 23.5˚N the sun therefore is overhead on earth, the 23.5˚N latitude on earth where the sun is overhead is called the Tropic of Cancer.

Similarly the Southern 23.5˚S latitude is called the Tropic of Capricorn.

Please note that actually the sun does not travel but it is the earth that does the travelling, but we from earth view the sun to be moving apparently.

Declination may thus be defined as the angular distance of the sun North or South of the equator.

Dates of the solstices and equinoxes

The solstices are two points in the ecliptic where the sun reaches its maximum (23.5˚) North or South declination. The summer solstice is on June 21st and the winter solstice is on December 22nd.

Concept of the earth’s axial rotation giving day and night

As the earth spins on its axis, only one half of the earth’s surface faces the sun. Therefore only that half receives the light and heat of the sun, this half is experiencing daylight and the half that is on the other side - the dark side – it is Nighttime.

As the earth rotates more areas come within daylight and other area on the day’s edge move into darkness or night.

Varying length of daylight through the year

If the earths axis were not tilted, then the sun would be always above the equator, in that case the sun would rise and set at the same time everywhere throughout the year, However, when it is summer in the Northern hemisphere the sun is overhead or nearly overhead at 23.5˚N, the footprint of the sun therefore is more encompassing the Northern Hemisphere and the days are longer in the Northern Hemisphere.

But when the sun moves down south and is at 23.5˚S the place in the Northern Hemisphere are not at the centre of the footprint but at the edge of it, the days are therefore shorter.

Daylight and darkness conditions in various latitudes at the solstices and equinoxes

Due to the tilt in the axis of the earth, the sun apparently moves along the ecliptic and at 23,5˚N declination more of the Northern Hemisphere is facing the sun, more heat and light are received by the northern hemisphere, summer is then being experienced in the northern hemisphere, with more daylight and less darkness.

But with the earth continuing its journey, the apparent sun starts to move southward and on September 28th Autumnal Equinox (equal nights), the sun is shining is overhead at the equator, the days and night are of equal duration.

On the 22nd of December the sun is overhead on the Tropic of Capricorn - latitude of 23.5˚S, it is summer in the southern hemisphere and winter in the northern hemisphere. The days are shorter and nights longer in the Northern Hemisphere.

After this the earth continues its journey and on March 21st the sun again is overhead at the equator – Vernal Equinox – and the days and night are of equal duration.

The significance of the tropics of Cancer and Capricorn and of the Arctic and Antarctic Circles

Due to the tilt in the axis of the earth, the sun’s apparent motion in the sky during the year is from latitude 23.5˚N to 23.5˚S. Thus the sun is overhead at latitude 23.5˚N on June 21st and overhead at latitude 23.5˚S on 22nd December. Between these two latitudes lie the area which receives the maximum heat and light from the sun and they are termed as the Torrid Zone, the 23.5˚N latitude is known as the Tropic of Capricorn and the 23.5˚S latitude is termed the Tropic of Cancer.

The parallels about 23.5˚ from the poles, marking the approximate limits of the circumpolar sun, are called polar circles, the one in the Northern Hemisphere being the Arctic Circle and the one in the Southern Hemisphere the Antarctic Circle. The areas inside the polar circles are the north and south frigid zones. The regions between the frigid zones and the torrid zones are the north and south temperate zones.