The size of the solar system –

A few years ago, a more perfect method than parallax was discovered to measure the distances of celestial bodies.

Image result for solar system

It consists of emitting microwaves to space. Microwaves are very short radio waves, like those used in radar. When the waves reach a celestial body, they bounce off it and are picked up and detected on Earth again. The speed at which the microwaves move is known; The time between the emission and reception can also be measured with great precision. Therefore, this technique allows to know, with greater precision than the parallax method, the round trip distance traveled by the microwaves and, therefore, the distance of a celestial body.

There are four ways to express distances, all of them very interesting to know.

They can be expressed in millions of miles. This unit is very common in Great Britain and the United States to measure great distances.

They can also be expressed in millions of kilometers. The kilometer is the unit that is normally used in civilized countries, except for the Anglo-Saxons, to measure great distances. It is also used by scientists from all over the world, including the United States and Great Britain. One kilometer is 1093.6 yards or 0.62137 miles. We can also say that a kilometer is 5/8 of a mile.

If we want to avoid millions of miles or kilometers, we can establish that the average distance from Earth to the Sun is worth an “astronomical unit”, which is abbreviated UA Thus, we can express the distances in UA, where 1 AU is worth 92,950,000 of miles or 149.588.000 of kilometers. Normally, it is said that 1 AU equals 150,000,000 kilometers.

And finally, distance can also be expressed in terms of the time it takes for light to travel. In a vacuum, the light moves at a speed of 299,792.5 kilometers per second, although this value can be rounded up to 300,000 kilometers per second without making an excessive error. It also equals 186,282 miles per second.

Thus, we can define a distance of 300,000 kilometers as “a second-light”, that is, the distance traveled by light in a second. Sixty times that amount, that is, 18,000,000 kilometers is “one minute-light”, and sixty times this, 1,080,000,000 kilometers, is “one light-hour”.

Average distance from the Sun

Planet Millions of miles Millions of Km Astronomical units Light hours
Mercury 35.9 57.9 0.387 0,0535
Venus 67.2 108.2 0.723 0.102
land 92.9 149.5 1,000 0,137
Mars 141.5 227.9 1,524 0.211
Jupiter 483.3 778.3 5,203 0,722
Saturn 886.1 1428.0 9,539 1,321

Therefore, from the time of Cassini it was already known that the diameter of the solar system, from one end of Saturn’s orbit, to the other, measured almost three billion kilometers.

This figure was also overcome with the passage of time. In 1781, this diameter increased twice as much, when William Herschel , the German-English astronomer, discovered Uranus. This diameter was doubled again twice, in 1846, when the French astronomer Urbain Jean Joseph Leverrier discovered Neptune, and in 1930, when the American astronomer Clyde William Tombaugh discovered Pluto.

Average distance from the Sun

Planet Millions of miles Millions of Km Astronomical units Light hours
Uranus 1782 2872 19,182 2.66
Neptune 2792 4498 30,058 4.26
Pluto 3671 5910 39,518 5.47

As the outermost orbit is that of Pluto, and not that of Saturn, we see that the diameter of the solar system is not three billion kilometers, but twelve billion. A beam of light would take almost half a day to traverse the solar system.

The English scientist Isaac Newton formulated the law of universal gravitation in 1684. This law explains in a direct mathematical way, the existence of the Keplerian model of the solar system and allows to calculate the orbit of a celestial body around the Sun even if it is only visible during part of said orbit.

This also made possible the study of comets. Formerly, and also in medieval times, astronomers believed that comets arose at irregular intervals and that they followed trajectories not subject to any natural law. People thought that comets predicted some kind of disaster.

The English astronomer Edmund Halley , friend of Newton , applied the gravitational calculations to the comets, and observed that some appeared in the sky every seventy-five or seventy-six years. In 1704, Halley formulated the hypothesis that all comets were really a single body that moved around the Sun in a regular ellipse, but so elongated, that most of the orbit was very far from the Earth. In that case, it was not visible, but every 75 or 76 years it passed closer to the Sun and the Earth and then it could be seen.

Cometa Halley

Halley calculated the comet’s orbit and predicted that it would be visible again in 1758. Sixteen years after Halley’s death, the comet reappeared, and since then the comet is called “Halley’s Comet.” The first appearance of this comet dates from the year 240 a. C.

At the time of its closest approach to the Sun, the Halley comet is only ninety million kilometers away from it, reaching across the orbit of Venus, although at the moment of its maximum distance from the Sun, the Halley is about three times and average Saturn’s orbit, about 5300 million kilometers. This means that in the year 1760, astronomers had already realized that the solar system was much greater than the Greeks had imagined.

Comet Halley is one of the closest to the Sun. There are others whose orbits around the Sun are so elongated that they are only visible every many centuries and even millennia. They move away from the sun, not billions of kilometers, but hundreds of billions. In 1950, Jan Hendrik Oort, Dutch astronomer, formulated a theory according to which it is possible that a great cloud of comets exists with orbits very distant from the Sun and therefore never visible.

Therefore the solar system could have a diameter of one billion kilometers or more. A beam of light would need forty days to cover this distance, so the diameter of the solar system could be estimated at more than 1 light-month.

On the other hand, the insignificance of the Earth is not only a question of distances. Through a telescope, the four outer planets, Jupiter, Saturn, Uranus and Neptune, become fully measurable spheres. But any of them is a giant compared to the Earth, although they are dwarves if we compare them with the Sun.


This is the ‘PaleBlueDot’ photograph of Earth taken by Voyager 1 on July 6, 1990. Earth is the relatively bright particle inside the blue circle.

Each of these giant planets has a satellite system, and next to them the Earth is insignificant. Of the outer satellites, the first to be discovered were the four largest of Jupiter, observed by Galileo through his first telescope in 1610. Of the great satellites, Triton, the satellite of Neptune, was discovered last. It was detected in 1846 by the English astronomer William Lassell. Later smaller satellites were discovered.

Equatorial diameter

Body Miles Km Earth diameter = 1
land 7929 12753 1,000
Neptune 27700 44600 3.50
Uranus 29200 47000 3.68
Saturn 75100 121000 9.5
Jupiter 88700 143000 11.2
Sun 864000 1392000 109.0