Where on Earth can you see the clearest starry sky?

Author: Xingliang (University of Chinese Academy of Sciences)

The article comes from the Science Academy official account (ID: kexuedayuan)

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Trapped in the city, when we look up to the sky to seek comfort from the stars, we mostly see tall buildings. Occasionally, there are unobstructed areas of the sky that are often filled with colorful lights, and serious light pollution covers the entire night sky. If the timing is good, you can see the moon, but most of the time, you can't see a single star.

Such places are definitely not good places for stargazing, and ordinary astronomy enthusiasts will stay away from such places. If you want to build a new observatory, densely populated areas will almost be directly passed (unless you are very rich). In addition to avoiding urban light pollution, there are many other factors to consider. We must not only evaluate the air transparency, atmospheric turbulence, and the stability of air density, but also consider the annual clear weather in the area. If you are engaged in radio observation, you must also take into account the degree of interference from surrounding radio signals.

Taking all the above into consideration, the sites of observatories are chosen to be in remote areas far away from cities, or in high mountains where the true appearance of Mount Lu is unknown, or in deserts where one can only survive by looking at plum blossoms to quench thirst, or in polar regions where the ground is white and the wind is cold.

European Southern Observatory in Chile's Atacama Desert, the world's "dry pole" (Image credit: National Geographic)

So is there a best place on Earth to observe it, and if so, where is it?

Ice Dome A-the clearest starry sky!

Recently, the international scientific journal Nature published a study by a team led by Researcher Shang Zhaohui from the National Astronomical Observatory of the Chinese Academy of Sciences. Known evidence shows that the best astronomical observation point on Earth is Dome A on the Antarctic continent.

The location of Dome A in Antarctica (Image source: wikipedia.org)

Antarctica can be divided into East Antarctica and West Antarctica according to geography. Dome A is located close to the center of East Antarctica. Dome A is to Antarctica what Mount Everest is to Earth. It is the highest ice sheet in Antarctica. The reason why it is called an ice sheet instead of a mountain is that it does not reach the sky like a mountain and looks like flat land. Its altitude is 4,093 meters, but because the Antarctic continent as a whole is a high-altitude area, its relative height is only 1,639 meters.

"Penguin" landing on Dome A - originated from China's 35th Antarctic scientific expedition mission (Photo source: Xinhua News Agency)

For ordinary people like us, it is really unrealistic to want to see the starry sky here in person. Because the Antarctic alone is far enough away from us, not to mention that Dome A is more than 1,200 kilometers deep inland from the Antarctic, which is equivalent to the distance from Beijing to Shanghai by car. At the same time, the thin atmosphere at high altitudes will also make you retreat, and you will be short of oxygen if you are not careful. What should also be noted is that the severe cold here is simply creepy. The summer temperature is around -40 degrees Celsius. Every winter, the temperature of Dome A is often lower than -80 degrees Celsius, and there have even been extreme temperatures as low as -98 degrees Celsius (currently there is still controversy in the academic community about this low temperature). Except for some artificially built laboratories, this is almost the coldest place on earth. The coldest town in human history is Oymyakon in northeastern Siberia, Russia, with a minimum temperature of -71 degrees Celsius, which is lower than the average temperature of Dome A in winter.

The coldest town for human survival - Oymyakon (Photo source: Daily Headlines)

Therefore, the three factors of distance, altitude and temperature (and perhaps a fourth factor: a few hundred million dollars) make the good observation environment of Dome A only available to a few people. It is a great honor that this wonderful place was first visited by Chinese people. 15 years ago, in 2005, members of the Chinese Antarctic expedition team traveled 1,228 kilometers from Zhongshan Station to Dome A.

So, where is the clear starry sky here?

This requires consideration of an important measurable indicator - seeing!

Why can't cannons hit the stars?

I will keep the seeing in suspense for now, let's have a brain teaser - why can't a cannon hit the stars? Don't say that the speed of a cannonball can't reach the second cosmic speed so it can't hit the stars, etc., playing brain teasers like that will only make you angry to death... I believe everyone is familiar with the answer, it's because the stars "flash".

However, stars do more than just "flash". The earth has an atmosphere, which gradually becomes thinner from the inside to the outside. If we want to capture distant starlight, we have to pass through these air media with different densities. The law of refraction tells us that light will be deflected when passing through. The problem is that the atmosphere is not like a steel plate. The air with uneven temperature will cause various turbulences, and there are small turbulences in the large turbulences, stirring the air like paste, and this paste is still changing all the time. If we compare the starry sky that is not "polluted" by the atmosphere to a 4K 120-frame video, then what we see under normal conditions is a terrible 480p 30-frame picture.

In this way, the stars will not only "flash" but also "blur", and the result seen in our telescope will be similar to the one below.

This is a photo of a star taken with a long exposure, reflecting the change in the position of the star in the telescope's field of view over a period of time. We can also see that the stars appearing in the eyepiece of an amateur telescope are shaking. This is probably because your tripod is unstable, reminding you to spend money to upgrade your tripod (Image source: Wikipedia entry - seeing)

Comparison of Jupiter viewed from the same telescope at different seeing conditions. It can be seen that under the poor conditions on the right, only the stripes can be vaguely seen, and the Great Red Spot is almost invisible (Image source: britastro.org)

Yes, it has nothing to do with the price of the telescope. No matter how expensive or precise the mirror is, it will be blurry. We use seeing to evaluate this blurriness. The so-called seeing, its scientific definition is a physical quantity that describes the degree to which the target of astronomical observation appears blurry and flickering due to the influence of atmospheric turbulence. It is usually expressed in angles. The smaller the angle, the better the seeing, and we can naturally see a clearer starry sky. Strictly speaking, the study of seeing belongs to the scope of atmospheric science research. Astronomical researchers usually only measure the value, and rarely build meteorological models to specifically analyze it.

Students who have some understanding of the concept of resolution should know that the larger the aperture of the telescope, the higher its resolution, the more it can distinguish two adjacent objects in the distance, and the clearer the picture we see.

The resolution formula of the telescope, λ refers to the wavelength of light, visible light is generally between 300-800 nm, D is the diameter of the telescope, and θ is the minimum resolution angle that the telescope can see.

In this way, those large telescopes can theoretically achieve a resolution of milliarcseconds (1 degree = 3600 arcseconds), but due to the effects of the atmosphere, the theoretical resolution is always unattainable. Currently, the best telescopes in the world are concentrated in excellent sites in Hawaii and northern Chile, where the seeing is generally 0.6-0.8 arcseconds, which is still far from the performance of the telescope itself.

In the face of such a problem, we usually have two options. One is to go to space like the Hubble telescope. Unfortunately, the space route is complicated and expensive. If there is an accident with the instrument, it will be very troublesome to repair it. Otherwise, why hasn't the James Webb telescope been launched yet? The other option is to stay on the ground and use adaptive optics to try to compensate for the errors caused by atmospheric jitter. However, this technology, which has been applied to astronomy since the 1990s, can only compensate, and it is impossible to completely eliminate the errors. Therefore, it is reasonable to continue to explore the best observation points on the earth.

In fact, the concept of seeing is not far away from us, and ordinary astronomy enthusiasts often deal with it. As long as you download an app like Sunny Day Clock, you will see a column of seeing information. The smaller the circle, the smaller the seeing value, and the better the observation effect.

Information about the National Astronomical Observatory's Miyun Station displayed by the software

The investigation behind the scenes

Antarctica is an isolated place with few traces of human activities. It has a high altitude and little light pollution. As long as the problem of equipment operation in severe cold conditions can be solved, it is naturally a good place to build an astronomical observatory. And under high-latitude conditions, using the unique polar night scene, we can track certain areas of interest for 24 hours.

This time, the detection of Dome A found that the lowest seeing at this place can reach 0.13 arc seconds, and the average seeing measured in long-term measurements is 0.31 arc seconds, which is much better than our previous site selection. Before the optimal results of Dome A came out, the seeing data of Dome C was in a relatively leading position, with an average value of about 0.23-0.26 arc seconds.

In fact, the academic community has long speculated that Dome A should be the best observation site on Earth. Since 2005, my country's scientific expedition team has set foot in this area, completed the investigation of Dome A, and made preparations for building a station here. In 2009, we established our third Antarctic scientific research station here-Kunlun Station. At the same time, it is also the first inland Antarctic scientific research station in China, ranking the top of the entire Antarctic. Later scientific expedition teams began to build observation equipment here one after another.

The results of this observation of Dome A also rely on the work done by the 35th expedition in 2019. Researcher Shang Zhaohui is in charge of the astronomical part and deployed the new Kunlun telescope (full name: Kunlun Differential Image Motion Monitor, abbreviated as KL-DIMM in English). The main task of the Kunlun telescope is to measure the seeing, which allows us to quantitatively evaluate the characteristics of the Kunlun Station observatory site at Dome A.

Interestingly, it is not directly fixed on the ground, but placed on a tower 8 m high. This is because the atmospheric boundary layer thickness here is low, only about 13.9 m. As long as it is above this height, the turbulence on the ground can be avoided, thus obtaining extremely excellent seeing. At the same time, the tower is also very cleverly erected, which can make the top platform shake less than 3 mm at a wind speed of 8 m/s, so that the stars to be observed will be firmly limited in our field of vision and will no longer "flash away".

A pair of Kunlun telescopes and towers (Image source: Shang Zhaohui)

Just like when we build blocks, the higher the stack, the easier it is to fall. It is difficult for our tower to remain stable at a height of 14 meters, but compared with Dome C next door, it is much easier. The thickness of the boundary layer at Dome C is about 30 meters. To achieve similar observation effects here, we need to build a stable tower more than 30 meters high, which is really a brain-consuming and expensive task.

Previously, we have always believed that Dome A is a good place for stargazing, but we lacked sufficient data to support the specific reasons. Through the seeing data accumulated this time, combined with multiple meteorological observation equipment working nearby, we have enough reason to believe that Dome A is the best, and we are not far from building a telescope there.

The accompanying multi-layer automatic weather station KLAWS-2G (left) and two cloud cover and aurora monitoring instruments KLCAM (pointed by the right arrow) (Image source: Shang Zhaohui)

A little outlook

It is indeed much more convenient to build a mirror in Antarctica than in space, but even if it is convenient for later maintenance, it is not realistic to have people stationed there for a long time in such a low temperature. Therefore, when building a new telescope in Antarctica, especially on Dome A, it is necessary to ensure good stability so that it can adapt to ultra-low temperature work and allow us to operate the terminal for observation at home.

In fact, this is what we are doing now. The Kunlun Telescope does not need anyone to be on duty on site. It was designed to work at minus 80 degrees Celsius. As for power supply, we don’t need to worry. The Antarctic is extremely cold and the sunlight is low. Using solar panels is expensive and troublesome, and the energy may not meet the needs. So we simply put a container full of batteries next to the telescope, which is convenient and stable. A box is used for one year. Anyway, there will be scientific expeditions every year. When the time comes, we can just come here to change the battery, which will not take too much time.

Kunlun Station astronomical work site (Photo source: National Astronomical Observatory)

If we assign people to watch over it, the supply of materials will be a problem, and it will be boring to be with ice and snow all day. After all, no one wants to be frozen all year round, so wouldn't it be better to automate the whole process?

In fact, Dome A is not only suitable for visible optical telescopes, but also for radio telescopes due to its extremely dry climate and clean electromagnetic environment. Environmental measurements in the terahertz band will allow radio telescopes here to observe more electromagnetic bands...

In short, this is a precious land, and it will definitely contribute to our domestic astronomical undertakings in the future.

References:

[1] Ma, B., Shang, Z., Hu, Y. et al. Night-time measurements of astronomical seeing at Dome A in Antarctica. Nature 583, 771–774 (2020).

[2] DOME A – COLDEST PLACE ON EARTH http://suo.im/5DBxbx

[3] Chinese astronomers confirm that Dome A in Antarctica is the best ground-based optical astronomical observatory site - National Science Foundation of China - http://suo.im/5DBxaJ

[4] Astronomical seeing—Wikipedia http://suo.im/5w5jIu

[5] Scambos, TA; Campbell, GG; Pope, A.; Haran, T.; Muto, A.; Lazzara, M.; Reijmer, CH; Van Den Broeke, MR (2018)