Starry sky

“Dark-Sky Level”〜for the outreach of The Beautiful Starry Sky Experience.


(Added Aug. 11, 2020)

It turns out that the NSU (Natural Sky Unit; a linear measure of sky brightness when SQM = 21.6, which is considered to be the ideal natural starry sky, is set to 1) has already been clearly defined by researchers. 21.6 is the official IAU Recomendation.

This means that the problem of SQM being difficult to understand and use in defining the origin has already been solved. This paper will be substantially revised in the form of NSU.

Hey Guys! Where do I go to see the Milky Way? What does that place in the starry sky look like? Have you ever been asked that question?

We in the astronomical cluster are the ones who are most aware of the beauty of the natural starry sky, but isn’t there no way to tell the public how well we can see the stars (i.e. how dark the sky is) and share it objectively?

This article is a proposal. Why don’t we establish a scientifically based, easy-to-use and easy-to-understand index to tell the public how dark the sky is = how well the stars are visible? This is the story.

Translated with (free version)

“SQM Value-17” Proposal

This proposal is very simple. Instead of using “SQM(*)” as it is, let’s use an index that is minus 17.

(*)The SQM, as it is well known in the astronomical community, is an instrument called the Sky Quality Meter, which measures the background brightness of the sky. As an instrument, the SQM is simple: point it at the sky and press a button to digitally display a number. The “mag/arcsec2” value displayed on the SQM is a value that has a clear scientific basis and can be accurately measured. Although there may be various errors in measurement conditions and measurement accuracy, they are considered to be “reliable enough” for the purpose of this paper.

An ideal sky (with no atmospheric absorption and no artificial light) is said to have an SQM value of “around 22”. Since there are many stars and galaxies shining in the universe, no matter how dark the sky is, the sky will not be completely dark because of “starlight” (*). This upper limit is “22” (magnitude).

(*) The actual night sky can be darker than “22” due to atmospheric absorption (including clouds). In my experience, when the sky is completely free of light pollution, it becomes really dark because of the blockage of starlight by clouds.

On the other hand, in metropolitan areas such as the suburbs of Tokyo, the SQM value is around 17, even in parks and riverbanks where there is little direct light. When the sky reaches 16, it becomes bright enough to read a book, and is no longer in the category of “night sky”. In other words, almost everywhere on Earth, the SQM value of the starry sky will lie between 17 and 22.

A supreme starry sky with an SQM value of 22.0 class. In Western Australia.

However, I’m not going to tell the public, “It’s an awesome starry sky on SQM 21.9! Usually it’s 21.0 at best”. People who don’t have the prerequisite knowledge don’t understand at all when I say, “Why is 21.9 so great? What does 21.9 mean? If you start answering and explaining the question, you may end up damaging the interest of the general public who are interested in the starry sky.

So, why not use the number “17 minus” the SQM value as a reference point? In the case of the highest starry sky “SQM value = 22.0”, if you subtract 17, you get 5. If we take 17 as the starting point, almost all points on earth will fit into a 5-point scale from 0 to 5. A “five-level scale” makes it easier for the average person to understand. “It’s a 5″ starry sky at the highest level! “The Milky Way is on a 4″ starry night in level 4″, which is pretty straightforward, don’t you think?

What’s more, the scientific basis and objectivity is not lost: if you add 17 to it, it equals the SQM value, and the SQM value can be measured by anyone who wants to, by themselves. Isn’t this pretty cool?

SQM value = 17 means that there are about 17,000 -1 magnitude stars in the entire half of sky. In the case of SQM value = 22, the difference is 1/100 of this magnitude, or 170 magnitude -1 stars, or about 7 Venus at -4.5 magnitude. (Without taking into account the atmospheric depletion near the horizon).

Naming is important

It turns out that “SQM value minus 17” is a pretty useful indicator. However, it is still not enough. We need a catchy and fitting name to share with the general public that this value is an indicator of the beauty of the natural starry sky, in order to make it easy to understand and bluntly share with the public.

I would suggest the name “Dark-Sky Level”. When expressed numerically, it uses a value with a decimal point, such as 4.5 or 3.7, rounded up to the nearest whole number as a five-level “star level”, such as Five Star (*).

(*) When you want to appeal to a small number of people with accuracy, you should write something like “The highest Dark-Sky Level of 4.9″ according to the Ministry of the Environment’s National Continuing Observations of the Starry Skies,” and in advertising copy, you should write something like “This is the highest level of Dark-Sky Level 5-star sky! You can use it as follows.

The bottom line is that it should be an intuitive, plain name that most people can use naturally, and not one that’s already in use for other purposes (*). I know “Dark-Sky Level” is just a suggestion, but if you have any other good ideas, I’d love to hear them!

(*)It’s even better if they have the same nuances, even if they are in different languages.

Who to involve

We are trying to introduce new metrics that have never existed in the world before, so we can’t just spread them by shouting them in our corner of the world. The three key words are “authority,” “commercialism,” and “grassroots”. Unless we get these things involved, it will not spread.

In order to achieve this, we need to get the support of as many companies, organizations, and individuals as possible, including the National Astronomical Observatory of Japan, the Ministry of the Environment, public astronomy clusters such as planetariums and public observatories, the International Dark-Sky Association, the Association for the Promotion of Space Tourism, the National Astronomical Observatory, and astronomy magazines. Not. I would like to ask these people to look at this article first and get their opinions.


Japan as seen on a The darkness of the night sky can now be quantified and visualized with high accuracy. The next challenge is to share a “measure that is easy to understand” with everyone.

What did you think?

The only weakness of the SQM value (mag/arcsec2) (*) is that it is too difficult to use the “origin” setting. It is that the “origin” setting is too difficult to use. Just “subtract 17” from this and you’ll have a “5-point scale” measure that is easy for everyone to understand. Let’s protect the beautiful starry skies and establish a metric more suitable for outreach that concretely shows the beautiful starry skies so that more people can enjoy them!

(*) In fact, there are some problems in SQM values, such as instrumental measurement errors, lack of uniformity in the brightness of the starry sky itself, and clarification of the relationship between SQM values and the effect of air clarity, but these are the only weaknesses for the purpose of this paper.

I would very much like to hear the opinions of many people regarding this issue. We at Tenrif would like to think of ways to involve more companies, organizations and individuals in our activities.


In writing this article, I was greatly assisted by the authors of all the pages I linked to and by the feedback I received on Twitter. I would like to express my deepest gratitude to them.

Reference link

SQM(Sky Quality Meter)


A device to measure the brightness of the night sky, sold by Unihedron. In Japan, Kokusai Kohki is the distributor for this product. The measured results are displayed in “mag/arcsec2“. In the astronomical cluster, the word “SQM” does not only refer to this measuring instrument, but is also used to refer to a unit of brightness (*). The selling price is 20,000 to 30,000 yen.

(*) It is rather problematic that the unit of “mag/arcsec2” does not have a name (common name) defined, although it would be faster if the IAU could officially decide that the unit of mag/arcsec2 shall be the SQ(M).

Borttle Scale

American amateur astronomer John E. Bortle published a nine-point numerical scale in 2001. It is represented by a depiction of the major astronomical objects familiar to astronomy fans, and is an easy indicator to realize for those who have a lot of experience looking at the starry sky. On the other hand, I felt that some of the links to the marginal magnitude (NELM) are slightly out of sync with reality (*), and the “highest level of sky” is too fragmented, which makes it difficult to match for general outreach.

(*) My guess is that you extrapolate the premise that “you can see 6th magnitude stars in a sky with SQM value = 20.5” and think that “if SQM value = 22, you can (and should) see up to 7.5 magnitude stars”. In my opinion, the former is not necessarily wrong, but the latter is a bit harsh.
On the other hand, it is true that skies with SQM values of 21.0, 21.5, and 22.0 are overwhelmingly different from each other, and I thought it was rather significant to describe “the level of the starry sky that cannot be measured by the faintest stars”. In that sense, the “Dark-Sky level” proposed in this article is too rough to describe the real beauty of the natural starry sky. On the other hand, this criterion may be convenient for outreach, since many of the places in Japan that aim for “starry sky tourism” are unquestionably “Level 5”.

Light Pollution Map

Light Pollution Map

The Light Pollution Map site, which has rapidly become a popular and almost de facto site in recent years. It allows you to see the brightness of the world’s skies on a map with SQM values and a vortle scale.

Mutual conversion of SQM values, light pollution maps, and vortle scale(in Japanese)


A Japanese blogger “Starlight_365” has created an interconversion table of SQM, light pollution maps and the Boottle scale, which includes the “how to see” and “how to feel” of SQM values at more than 400 locations, and it is very informative.

Summer Stargazing Survey Results of the Digital Camera Night Sky Brightness Survey

Japan’s Ministry of the Environment, in collaboration with a private Japanese organization, the 星空公団, has been calling for a survey on the brightness of the night sky. The purpose of the survey is to make people aware of light pollution and to deepen their awareness of environmental conservation, with the aim of using the results for tourism and education. (*) The results of the survey are published in the same format as SQM, i.e., “mag/arcsec2”.

Although there are certain limits to the accuracy and reproducibility of the measurement results, it is a significant achievement to have continuously covered the entire country on an objective basis.

AO-ZORA Spot(青宙スポット)

This is a catalogue of Japanese astronomical sites produced by Japanese volunteers(*). There is no description of SQM values, but it is possible to make analogies based on the view of the Milky Way.

(*)I also helps with registration.