Astronomical Unit
Light-year
Parsec
Comparison of sizes
Curiosities
General Informations
Titius-Bode sequence

 

• A remarkable coincidence: the apparent sizes of the Sun and Moon are almost the same.

 

The Sun is 400 times larger than the Moon, but looking from Earth, the two seem to have the same size. This happens because the Sun is about 400 times farther from us than the Moon. The difference between these reasons is less than 3%, which we can not see with the naked eye.

 

• The sky that we see is actually a picture of different moments of the past.

 

The nearest star to us, Alpha Centauri, is located at a distance of about 4.3 light-years. This means that light takes about 4.3 years to travel the distance that separates us from that star. So we see the star 4.3 years ago. Amazing! Imagine that each bright in the sky is a different moment of time. The more long distant means that we are looking at an increasingly distant past.

 

• See how we are insignificant in size.

 

            Rounding up the numbers, today we know that the biggest planet in our solar system has a diameter 11 times greater than ours. The Sun has a diameter 109 times larger than Earth. The star VV Cephei has a diameter about 1600 to 1900 times greater than our Sun.

 

• Human technology farther from our planet.

 

            In 1977, to Jupiter and Saturn, the man launched the Voyager 1. Today it is located more than 13 billion kilometers from Earth and continues moving away about 17 thousand kilometers per hour equivalent to 1 billion km every 10 years. Even with that speed it would take approximately 76 thousand years to reach Alpha Centauri.

 

 

General Informations

 

 

Comparison of sizes between the Earth, the Moon and the Sun.

            To begin to talk about astronomical distances, is appropriate and sensible to talk first about the size of Earth, the Moon and the Sun, to take an initial idea of which  distances we are referring.

 
            It started in ancient Greece, when Aristarchus of Samos, a Greek mathematician and astronomer, calculated the diameter of the Moon on the Earth, based on the shadow of our planet during a lunar eclipse. Using revolutionary tools for the period, came to the conclusion that the moon was 3 times smaller than the Earth, very close of the current that is approximately 3.7 times.

 
            Knowing this, Aristarchus deduced that the solar diameter was 20 times larger than the Moon, and 7 times greater than Earth's. Today, we know that the diameter of the Earth do not reach one hundredth the diameter of the Sun. While the results showed deviations, the errors occurred because of lack of precision of its instruments, and not because of its method of calculation, which was quite consistent.

 

            For comparison purposes, if the sun were a ball of 23 cm in diameter, the Earth would be a grain of coriander (2 mm), and Moon, a poppy seed (0.6 mm).

            Some comparisons of planets, dwarf planets and stars.

 

            The planet of the Solar System with a smaller diameter is a dwarf planet called Ceres, with about 950 km in diameter. Already the smallest planet is Mercury, with 4,880 km in diameter. The largest planet in the Solar System is Jupiter, with 142,984 km in diameter. The sun has a diameter of 1392500 km and it is 109 times larger than Earth. But today we know that stars, such as MY Cephei (or MU Cephei) and VV Cephei, which are more than 1400 times larger than the Sun. VV Cephei, considered one of the biggest stars in the firmament, its size is estimated between 1600 and 1900 times the size of the sun, and is larger than the orbit of Jupiter!

 

The Titius-Bode sequence

 

            The sequence number discovered by Johann Daniel Tietz (1729-1796) known by his romanize name of Titius of Wittenberg and the German astronomer Johann Elert Bode (1747-1826), represents a good approximation of the values of the semi-major axes of planetary orbits and impressed many astronomers of the period.

            We started with a natural sequence of numbers start by 0 (zero), then 1 and from it a geometric progression of ratio 2:

 

 

0, 1, 2, 4, 8, 16 e 32

 

Multiply each term by 3, we obtain:

 

0, 3, 6, 12, 24, 48 e 96

 

Finally, adding up to 4 units each term and dividing the result by 10, we obtain:

 

0,4 – 0,7 – 1,0 – 1,6 – 2,8 – 5,2 - 100

 

These numbers represent approximately the average distance of the planets to the Sun, taking as unity the mean distance Earth-Sun:

 

Planet	Mercury	Venus	Earth	Mars	Asteroids	Jupiter	Saturn
Distance	0,4	0,7	1,0 = AU	1,6	2,8	5,2	10

 

Thus, the average distance of Mercury to the Sun is four tenths of the Earth, that of Venus is seven tenths of the Earth and of Saturn is 10 times the distance from Earth to the Sun.

 

So for the average distance to the Sun the planet Mercury, we have:

 

d = 0,4.AU = 0,4 x 150.000.000 km = 60.000.000 km

 

            Note the precision comparing actual distances between the Sun and the planets and sequence Titius-Bode: