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Disposition of Celestial Bodies
The Sun is the basis for all life on Earth. It is a small star (a yellow dwarf star) generating light and heat and it is the body around which the Solar System orbits. The nine planets in the system orbit in concentric circles (really ellipses) in a more or less flat plane and our earth is the third one out from the sun.
Those between the Earth and the Sun are called inferior planets (Venus and Mercury). Those further out than the Earth are superior planets (Mars, Saturn, Jupiter. Uranus, Neptune, Pluto)
All planets shine reflected sunlight; so, like the moon, the inferior planets will have Òfull moonÓ and Ònew moonÓ characteristics and will therefore vary in brightness. The superior planets will always be in the Òfull moonÓ phase and their brightness will depend on their distance from Earth. Some planets are enormous; Jupiter is 1320 times the volume of Earth whereas tiny Pluto is 0.1 times and is so far away that it is several thousand times fainter than the smallest object visible to the eye. The figure below shows the relative sizes of the planets within our solar system.
Orbiting Earth is the moon. To give proportion to this, the moon is about 250,000 miles from earth and the sun about 93,000,000 miles. It might help to consider the Sun to be a ball one metre in diameter and the Earth the size of a pea; in these proportions the Earth would be the length of a rugby pitch away from the Sun. Due to a combination of size and distance only four planets can be easily seen and these are the ones used for navigation. Venus, the brightest of all stars and planets, is 67,200,000 miles from the Sun; Mars 141,700,000 miles; Jupiter, 483,700,000 miles and Saturn 885,200,000 (see table below). Their distance from Earth varies with their dispositions in their orbits.
Miles (millions) |
km (millions) |
Astronomical unit (AU)1 |
36.0 |
57.9 |
0.387 |
3.2 |
67.2 |
108.2 |
0.723 |
6.0 |
93.0 |
149.6 |
1.000 |
8.3 |
0.238 |
0.383 |
0.003 |
1.3 |
141.7 |
228.0 |
1.524 |
12.7 |
483.7 |
778.4 |
5.203 |
43.3 |
885.2 |
1424.6 |
9.523 |
79.5 |
1785.5 |
2873.5 |
19.208 |
159.7 |
2796.8 |
4501.0 |
30.087 |
250.4 |
3694.6 |
5945.9 |
39.746 |
332.1 |
1. An Astronomical Unit is a unit of length used in measuring astronomical distances within the Solar System equal to the mean distance from Earth to the Sun, approximately 150 million kilometres (93 million miles).
Although light is considered, for all practical purposes, to be instantaneous, it has a finite speed of travel of c.162,000 nautical miles per second and thus light emitted by the Sun takes nearly 9 minutes to arrive at Earth. Compare this distance with those of the stars: the light of the nearest star takes 4 years to travel here and that of the furthest star used for navigation 5,000 years. So it is possible that some stars may have ceased to exist thousands of years ago but their light is still travelling so we can still see it!! For example, if the North Star (Polaris) had disappeared in 630AD we would have only noticed in 2003!
In normal distance terms a light year is 5,2000,000,000,000 n.miles These vast distances defy comprehension by normal minds and, fortunately, have little to do with their use for navigation apart from one important point. With the exception of the moon, all other celestial bodies are so far removed from Earth that rays of light emitted by them can be considered to travel in parallel lines. It is of fundamental importance that this is understood.
