All measurements of space distances are based on astronomical unit – average distance from Earth to the Sun. But how to define distance to an object which any measuring tool will not reach?
Instruction
1. Attempts to measure distance from Earth to the Sun were made in Ancient Greece (Aristarkh Samossky), but it was difficult to call them exact. In the 17th century this distance was measured by means of parallax method (the difference of provision of an object concerning a remote subject depending on position of the observer). Horizontal parallax of the Sun – a corner under which from the Sun which is on the horizon the Earth radius perpendicular to a sight beam is visible was defined. Further all researches were based on Earth radius length.
2. In 1672 the distance from the earth to Mars which was located in opposite to the Sun to a point at this time was defined. The trigonometrical laws allowing to calculate relative distances of planets, expressed in distance shares Earth Sun were known, and with their help the distance from the Sun to Earth was calculated actually. At that time it was the most exact value – 138.5 million kilometers.
3. Subsequently astronomical unit was tried to be defined many times, taking as a basis of calculations distance from Earth to Venus, but as there is a lot of observers, and measurements differed in high complexity, the disparate in the received values was very big. At the end of the 19th century by means of measurement of shifts of visible provisions of stars more exact value – 149, 5 million kilometers was calculated.
4. The second half of the 20th century brought with itself a scientific and technological revolution, and with it development of radio engineering. Radar method (at which towards a celestial body send a short-term impulse, accept the reflected signal and on the basis of the speed of distribution of radio waves and time of passing of a signal to both directions define distance to this body) allowed to calculate distance from Earth to the Sun most precisely at various times year and to remove the average value equal of 149,597,870 km.