27 THE NAVIGATIONAL PLANETS

102. THE PLANETS IN THE SOLAR SYSTEM.

The Solar System comprises the Sun., which is a star, and the planets which revolve around it, their orbital movements being controlled by the Sun’s gravitational force. The planets are not, like the Sun, self-luminous: they are rendered visible by reflected sunlight, and appear as small discs of light with no appreciable apparent size. Because of their proximity to the Earth compared with the remote fixed stars they move comparatively rapidly relative to the stars. It is for this reason that they are known as planets – the word planet being derived from the Greek word meaning wanderer. The planets, in order of distance from the Sun, are illustrated in fig. 27-1. Several of the nine planets have satellites: the Earth has one (the Moon) but Jupiter, which is the largest of the planets, has at least nine.

The satellite or satellites of a planet revolve around the parent planet in much the same way as the planets revolve around the Sun. In addition to the planets and their attendant satellites other members of the Solar System are the minor planets or asteroids, the orbits of which lie-mainly between those of Mars ♂ and Jupiter. Comets and meteors (shooting stars) also belong to the Solar system.

The planets Uranus, Neptune and Pluto were discovered in comparatively recent times; the other planets, therefore, are often referred to as the ancient planets.

The two planets Mercury and Venus ♀are known as inferior planets because their orbits lie within the Earth’s orbit. The other planets whose orbits lie outside the Earth’s are known as superior planets.

The path of a planet round the Sun is known as the orbit of the planet, the plane of which tends to be fixed in space and is an ellipse hang the Sun at one of its focal points.

The time taken for a planet to make one revolution round the Sun is known as the planet’s period of revolution. The gravitational force on a planet varies directly as the mass of the planet and inversely as the square of the distance between the planet and the Earth. The nearer planets, therefore, regardless of their masses, have smaller periods than the more remote ones. The Sun, whose diameter is about 865.000, has a mass equal to about 750 times the mass of the remainder of the Solar System combined.

The table in fig. 27-2 summarises the more important Astronomical data of the planets. The point in a planet’s orbit which is nearest to the Sun is called perihelion and the most remote point is called aphelion. The Earth is at perihelion at about 03rd Jan and at aphelion at about 03rd Jul each year.  If then a planet and the Sun lie in the same direction opposite to that of the Sun it is said to be in opposition to the sun. When the angle at the Earth between a planet and the sun is 90º, the planet is said to be in quadrature with the Sun.

When a planet lies to the W. of the Sun it will normally set before sunset and rise before sunrise. In this circumstance, it will be visible for some time before sunrise and is therefore called a morning star. When a planet lies to the E. of the Sun it will rise after sunrise and set after sunset and is therefore called an evening star.

The apparent path of a planet as observed from the Earth is the resultant of its own orbital motion and that of the Earth’s around the Sun. The planets trace out complex paths often forming a series of successive loops known as retrogressive loops. When an inferior planet is at inferior conjunction its illuminated hemisphere is directed away from the Earth. When at superior conjunction it appears as a disc of light, – its illuminated hemisphere then being directed towards the Earth. Because the proportion of the illuminated hemisphere varies with time, inferior planets exhibit phases as does the Moon.

103. THE NAVIGATIONAL PLANETS.

The planets, Venus ♀, Mars ♂ Jupiter♃and Saturn♄are, at times, suitable for navigational purposes. They are, therefore, called the navigational planets and Astronomical data for them is tabulated in the N.A. The remaining planets are not bright enough to be of use (with the exception of Mercury, which is quite a brilliant body, but which is too near the Sun for navigational purposes). The planets Venus ♀and Jupiter are sometimes visible during the hours of daylight and are therefore, most useful for navigational purposes. The declinations of the four navigational planets rarely exceed the limits of 26º N. and 26º S. and therefore the navigator has a general idea about their positions in the sky.

104. POSITIONS OF THE PLANETS & PLANET NOTES FOR 197X

VENUS ♀is never more than 47º removed from the Sun and for this reason, it cannot be seen at night in temperate latitudes, being either a morning or evening planet. Its magnitude varies slightly but is on average -3.4. No other star or planet is so brilliant. It is an evening star until the beginning of April. From mid-April until mid-December it is a morning star. For the rest of the year, it is too close to the Sun for observation. Venus ♀is in conjunction with Saturn♄ on June 11 and with Jupiter on November 4.

MARS ♂ is a morning star until opposition on May 31; it is then an evening star for the rest of the year and moves through Libra and Scorpius before coming to the opposition in Ophiuchus. After opposition, it moves through Sagittarius and Capricornus and by the end of the year Mars ♂ is in Aquarius. It varies appreciably in brilliance, with an average magnitude of about – 0.2. but is easily distinguished by its reddish light..

There are three conjunctions with Antares; Mars ♂ is 6° N, of it on March 17, 2° N. on June 3 and 1.3° N. on August 12. The reddish tint of Mars ♂ should assist in its identification.

JUPITER♃ is a morning star until opposition on March 21; it is then an evening star until the end of September. It is too close to the Sun for observation until the end of October when it becomes a morning star and is visible as such for the rest of the year. Jupiter is in conjunction with Mercury on September 7, September 19 and October 26 and with Venus ♀on November 4. It ranks next to Venus ♀in brilliance, with an average magnitude of – 2.2.

SATURN♄ is an evening star until the end of March, when it becomes too close to the Sun for observation. From May until opposition on October 29 it is a morning star. It is an evening star for the rest of the year. Saturn ♄ is in Pisces at the beginning of the year, moving slowly into Aries, Saturn♄is in conjunction with Venus ♀on June 11. It is not readily identified and has an average magnitude of 1.4.

MERCURY can only be seen low in the east before sunrise, or low in the west after sunset (about the time of beginning or end of civil twilight). It is visible in the mornings between the following approximate dates: February 4 (+1.6) to March 31 (-1.0), June 7 (+2.3) to July 15 (-1.4), October 6 (+1.3) to November 1 (-0.9); the planet is brighter at the end of each period. It is visible in the evenings between the following approximate dates: January 1 (-0.7) to January 24 (+1.4), April 17 (-1.4) to May 21 (+2.3), July 31 (-1.1) to September 23 (+1.8), December 4 (-0.6) to December 31 (-0.1); the planet is brighter at the beginning of each period. The figures in brackets are the magnitudes.

105. THE IDENTIFICATION OF THE NAVIGATIONAL PLANETS.

In addition to ephemerides for the navigational planets on the daily pages the N.A. contains (in the opening pages) notes to assist the navigator to locate and identify the planets in that particular year together with a Planet Diagram. The Planet Notes and Diagram for the fictitious year 197X are shown in fig. 27-2.

The Planet Diagram shows in graphical form for any date during the year the local mean time of Mer. Pass. of the Sun and of the five planets Mercury, Venus ♀, Mars ♂ Jupiter and Saturn ♄ together with lines showing the L.M.T. of Mer. Pass. of even hour circles (every 30º of S.H.A.). The horizontal argument on the diagram is a date, and the vertical argument is local mean time.

On each side of the line marking the time of the Mer. Pass, of the Sun there is a shaded band, 45m. wide, to indicate that bodies falling within this band on a particular date are too close to the Sun for observation. The lines joining the times of Mer. Pass, of the five planets, are each drawn in a distinctive manner to avoid confusion. The diagram gives, simply by entering the date the following information: –

• Whether a planet is observable on that day or whether it is too close to the Sun (within the shaded area).
• The local meantime of the Mer. Pass, of each body (when a meridian altitude sight will be available).
• Whether the body is a morning or evening star. A body in the bottom half of the diagram is a morning star and one in the top half is an evening star.
• Whether or not it will be at low altitude during twilight when the Mer. Pass is between Oh. and 2h, the planet is observable low in the W. during morning twilight, when the Mer.Pass., falls just below the shaded area, it is visible low in the E. during morning twilight, and when the Mer.Pass., falls just above the shaded area it is visible low in the W. during evening twilight when the Mer.Pass., is between 22h and 24h, the planet is observable low in the E. during evening twilight.
• Whether other planets are in the immediate vicinity when care must be taken to avoid confusion. 

The local meantime of the Mer. Pass, of a star, can be found by inspection, simply by applying its S.H.A. to the diagram and navigators can, if they desire, draw in lines corresponding to particular stars and so determine the Mer. Pass, of these stars immediately.