Unexpectedly! This place on earth is the best place to see the stars

"Sitting on the ground, I can travel eighty thousand miles a day, and survey the sky and see a thousand rivers in the distance."

The vast starry sky has always been the source of human beings’ infinite imagination. With the advancement of astronomy, astronomical observation technology has also been developing continuously.

On December 25, 2021, the successor to the Hubble Telescope, the Webb Telescope, was successfully launched into space. It will take over the work of its "predecessor" and continue to serve as the "eye of the universe." At the same time, China's space telescope plan is also being prepared intensively. China will also launch its own "Hubble Telescope" - a survey telescope , to further promote the development of deep space exploration.

In fact, in addition to space telescopes, astronomical observatories on the ground are also an important way for humans to understand the universe. Today, let us take a look at what we need to know when observing the starry sky.

The history of human beings looking up at the stars

In ancient times, when people conducted astronomical observations, they could only observe with their naked eyes because there were no auxiliary tools. As a result, a large number of "observatories" with good locations and wide fields of view appeared, such as Langya Terrace, Taosi in my country, and Yueshen Terrace in the Mesopotamian region.

Ancient Chinese Observatory

In 1608, Dutch optician Robert Löwen accidentally discovered that stacking two lenses could enable one to see far away, and he created the first telescope in human history.

Inspired by this, Galileo invented an optical system with a convex lens as the objective lens and a concave lens as the eyepiece in 1609, which was called the Galilean telescope . Galileo used this telescope to create a large number of astronomical observation results, discovered a large number of craters on the moon, drew the first sketch of the moon in history, discovered four satellites of Jupiter, and observed the "halo" of Saturn... Astronomy has entered the telescope era since then.

With the development of technology, the defects of telescopes are being corrected, and they are also developing in the direction of large-scale and giant. In 1938, people captured radio waves from celestial bodies in the direction of the center of the Milky Way, and the radio telescope was born, which is another important member of astronomical observation.

Radio telescopes are basic equipment used to observe and study radio waves from celestial bodies . Unlike well-known optical telescopes, radio telescopes do not have optical lenses. They are composed of antennas and receiving systems. The antennas are like the eyes of radio telescopes, which are used to collect weak radio signals in the universe, and then amplify the signals through the receiving system, and finally separate useful signals from noise for scientists to study. The completion of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) means that China's radio astronomy has reached the forefront of the world.

There are some special places to “watch the stars”

Take the world's largest "eye in the sky" mentioned above as an example. FAST was built in Pingtang County, Guizhou Province. The reason why this place was chosen was also after a series of selections from all over the country. First of all , the rock structure, hydrological conditions, major and minor axis ratio, and cut-fill ratio of the construction site are appropriate; the closure of the ring beam and whether the geometric shape meets the standards; the overall geological disasters, earthquake risks, meteorological conditions, radio environment, etc. are all considered in the site selection.

FAST is located in a karst landform area , where the rock is highly permeable, which is conducive to the infiltration of surface water and the overall drainage. In addition, the area is sparsely populated and surrounded by high mountains, which can effectively shield the surrounding radio and electromagnetic wave interference . These requirements are also aspects that need to be comprehensively considered when selecting a site for a radio telescope.

Panoramic view of the "China Sky Eye" taken on December 19, 2021

(Photo taken during maintenance, drone photo) Photo by Xinhua News Agency reporter Ou Dongqu

Of course, it is not just the radio telescopes that have such stringent requirements for site selection; the site selection for optical astronomy bases is also very strict .

The first thing to consider is the meteorological factors . The amount of cloud directly affects the length of observation time, and the absorption of the atmosphere will also weaken the starlight. In addition, the refractive index changes caused by the fluctuations in atmospheric temperature and density will affect the image quality. Therefore, the location of astronomical observation needs to be dry and rainless, with a thin atmosphere, and preferably "no grass" to avoid the water vapor produced by plant transpiration affecting the quality of astronomical observation.

Secondly, light pollution caused by artificial light sources will brighten the night sky, smoke and dust in the air will increase atmospheric absorption, and the radio waves emitted by radio transmitters will also affect radio observations. Therefore, astronomical observations are best avoided in densely populated areas.

When it comes to the site selection criteria for professional observatories, there are two concepts that we must understand: atmospheric turbulence and seeing .

Atmospheric turbulence is an irregular random motion of the atmosphere, and the physical properties of the flow velocity, pressure, temperature, etc. at each point of the turbulence are constantly changing . In astronomy, random fluctuations in atmospheric temperature or atmospheric density will cause turbulence in the refractive index field. These changes will seriously affect the observation quality of the optical imaging system. Therefore, atmospheric turbulence is a crucial indicator for astronomers and a crucial basic work in the construction of observatories.

The intensity of atmospheric turbulence affects the clarity of images displayed by telescopes, and the physical quantity used to describe the degree to which astronomical observation targets appear blurred and flickering under the influence of atmospheric turbulence is seeing . Places with low seeing not only make it easier to capture high-quality astronomical images, but also effectively reduce the cost of astronomical observations.

Therefore, when selecting a site for an observatory in China, a series of testing indicators such as weather, percentage of clear nights, astronomical atmospheric seeing, and skylight background need to be taken into consideration . For example, China's famous Purple Mountain Observatory is located on Purple Mountain in Xuanwu District, Nanjing, where the cloud cover is relatively small, the temperature is low, and the air is thin and fresh, which can avoid the impact of environmental factors on the observation effect to a certain extent and meet the observation needs. The Lenghu Astronomical Observatory, located on the Saishiteng Mountain in Lenghu, Qinghai, is even more known as "having the clearest starry sky in the entire Eurasian continent."

The birth of the world-renowned black hole photo was also due to the collaborative observations of the 65-meter Tianma Telescope in Shanghai and the 25-meter Nanshan Radio Telescope in Xinjiang.

Stargazing in the coldest place on Earth

The pace of exploration never stops! In 2009, China selected and built the Kunlun Observatory in the Antarctic Dome A region . Atmospheric turbulence and seeing were fully considered when selecting the site. The atmospheric turbulence in the Antarctic Dome A region is mainly in the boundary layer near the surface, while the free atmosphere above is very stable, so it has good seeing.

The research results published in Volume 583 of the famous international academic journal Nature in 2020 stated that the optical astronomical observation conditions in the Dome A region of Antarctica where Kunlun Station is located are better than any other known ground sites, laying a scientific foundation for China to further carry out Antarctic astronomical research .

In this way, Chinese astronomers discovered the clearest starry sky on the coldest continent on Earth. This achievement will surely leave a significant mark in the history of human exploration of the starry sky.