How far are we from the sun? If we live to 80 without sleep, our 54th generation descendants will be able to reach it

We all know that common units for expressing distance are meters and kilometers. However, these units are inconvenient to use in astronomy, because the distances in astronomy are too far to be measured with common units, and even kilometers are only applicable to celestial bodies that are very close.

Therefore, in astronomy, people established some new distance units, the smallest and most basic one of which is called "astronomical unit", abbreviated as AU.

1 astronomical unit refers to the average distance from the Earth to the Sun, which is about 150 million kilometers. Let us now introduce the origin of this unit.

1

How is the distance from the Earth to the Sun calculated?

Let's first talk about how far the Earth is from the Sun. People have been thinking about this question for more than 2,000 years. In his book On the Size and Distance of the Sun and Moon, the ancient Greek scientist Aristarchus calculated that the distance from the Sun to the Earth is 18 to 20 times the distance from the Moon to the Earth (the actual ratio is about 389 times) based on the angular diameters of the Sun and the Moon and the angle between the Sun and the Moon during the half-moon.

Ptolemy's calculations on the size and distance of the sun and moon, image source: wiki

Ptolemy first measured the parallax of the moon and calculated that the distance between the moon and the earth is about 64 times the radius of the earth. Then, he compared the visual sizes of the sun and the moon and concluded that the size of the sun is approximately equal to that of the moon.

Finally, based on the diameter of the shadow cone of the moon passing through the earth during a lunar eclipse, the distance from the sun to the earth was calculated to be 1210 times the radius of the earth (the actual magnification is about 23455 times). This distance is about 19 times the distance between the earth and the moon, which is basically consistent with Aristarchus's result.

In the Middle Ages, many Arab astronomers also calculated the distance to the sun, and their results were similar to Ptolemy's.

It was not until the 16th century that the invention of the telescope made astronomical observation data more accurate, and astronomers found a new observation method, which was to use the transit of Venus to measure the parallax of the sun.

By measuring the transits at two different locations, the parallax of Venus can be calculated. Based on the relative distances of the Earth, Venus and the Sun, the solar parallax can be obtained, and the distance to the Sun can be calculated.

British astronomer Jeremiah Horrocks used the transit of Venus in 1639 to calculate the solar parallax to be 15″, which gave a Sun-Earth distance of 13,750 times the Earth’s radius.

After comparing the sizes of Venus and Mars, Dutch astronomer Christiaan Huygens believed that the distance should be even greater. He estimated the solar parallax to be 8.6″, so the distance between the sun and the earth should be 24,000 times the radius of the earth, but due to the lack of observational evidence, this statement was not recognized.

Venus crosses the surface of the sun, image source: wiki

In 1672, French astronomer Jean Richer and Italian astronomer Giovanni Domenico Cassini remeasured the parallax of Mars when it was in opposition. The corrected solar parallax value was 9.5″, from which they calculated the distance between the Earth and the Sun to be 22,000 times the radius of the Earth, a distance that was generally recognized at the time.

The method of using the transit of Venus to measure the distance to the Sun also began to be used long-term and widely.

Schematic diagram of observation using Venus transit, image source: wiki

By the 19th century, people had more methods and better instruments to conduct observations, and by the 20th century, thanks to the application of radar and telemetry technology, the measurement accuracy of the distance from the sun to the earth had reached the meter level.

2

Redefining the astronomical unit

The Earth's orbit around the Sun is an ellipse whose semimajor axis is defined as half of the straight line segment connecting perihelion and aphelion, with the center of the Sun located on this straight line segment but not at the midpoint.

Since the ellipse mathematically defines the exact shape, calculation of the entire orbit and predictions based on this are possible.

The major semi-axis of the elliptical orbit also becomes the maximum distance that the Earth travels in a year. This distance is very important for our observations because this line segment must be used as the baseline when observing other celestial bodies.

Baseline for measuring celestial bodies, Image source: wiki

In the 20th century, astronomical measurements became more and more precise, and the requirements for benchmarks became higher and higher. In 1976, in order to establish a more precise measurement for the astronomical unit, the International Astronomical Union (IAU) officially adopted a new definition and used the symbol A to represent the length of the astronomical unit.

By definition, an astronomical unit is a particle with an infinitesimal mass that orbits the Sun at an angular frequency of 0.01720209895 radians per day, with an orbital radius of one astronomical unit.

Under this definition, the specific value of the astronomical unit depends on the heliocentric gravitational constant, which is the product of the gravitational constant and the mass of the sun. If these two values ​​are measured separately, the measured values ​​are not accurate enough, so the accuracy of this definition will not be very high.

With the development of space measurement technology, the accuracy of the positions of celestial bodies we obtain is getting higher and higher, and some errors that were previously negligible have begun to be taken seriously. As a result, people have to redefine the astronomical unit.

In August 2012, the International Astronomical Union redefined the astronomical unit. This time it was no longer expressed in physical quantities, but was directly defined as 149597870700 meters, and its symbol was also changed to AU.

Although the astronomical unit was redefined, its importance was reduced and it was only used to facilitate certain applications.

Schematic diagram of astronomical unit, image source: wiki

Currently the AU is used only for scale descriptions within planetary systems and as a baseline for triangulation.

And as the sun continues to radiate energy, it will continue to lose mass, so the orbits of the planets are also constantly expanding outward. According to observational data, the increase in the unit distance of the planetary orbit is about 15 meters per century, which is negligible relative to the distance between celestial bodies. However, some people still call for abandoning the astronomical unit as a measurement.

In the future, we may use other units or methods to express this minimum basic distance, but for now, astronomical units are still used frequently within the solar system.

END

Review expert: Lu Yang Xiaoyi, PhD in planetary physics.

Tadpole Musical Notation original article, please indicate the source when reprinting

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