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Astronomy/Temperatures on Mercury


QUESTION: Dear Philip,

My question is about the temperatures on Mercury. The Mercury is really a very interesting case due to its small distance from the Sun and slow rotation and lack of atmosphere.

On the Internet, in many places, one can easily find the information that the temperatures on Mercury range from -173 (C) [night side] to +427 (C) [day side], and that the average temperature is +167 (C). See e.g.:

But everybody knows that statistics is first of all good for lying. (If 9 trains come in time and one is late by one hour, one may say that in average each train was late by 6 minutes. But in fact NO train was late by 6 minutes). So maybe the fact is that there is hardly any place on Mercury where the temperature is approximately +167 (C), at least for a reasonable period of time (say, a couple of hours), not just a minute during a transition from -173 to +427.

I am rather interested to learn what is the actual temperature in certain specific areas on Mercury. I understand that on the Mercury's equator, in the centre of the actual day zone, it is ca. +427 (C), but I am not sure where is the place with the ca. -173 (C). Is it on the equator in the middle of the actual night zone? Or else is it on the Mercury's poles, which never have sunlight? Whichever the answer of the two, what is the actual temperature in the other place out of the two?

Wikipedia says that "The poles are constantly below −93 (C)" - but it does not say how far below.

This website: says that "There are thought to be craters at the north and south poles of Mercury where the sunlight never reaches, and these places might even be colder than the shadowed side of Mercury" - but it does not say how much colder and what is the point of reference (colder than which temperature?).

Which answer is correct? Which is the coldest place on Mercury? I understand that in average, it is the poles, because it never has sunlight whereas the centre of the night zone today will become the centre of the day zone in some time. But which will be the actual coldest place in absolute measures?

If we refer to other areas - What is the temperature 15, 30, 45, 60 degrees latitude from the poles towards the centre of the actual day zone? - and at the same latitudes distance towards the centre of the night zone? What is the temperature on the equator on the "eastern" and "western" terminuses between the day and night zone? (Please correct me if I use wrong terminology, I hope that you intuitively understand what I mean. English is not my mother tongue). And what is the temperature at the 15, 30, 45, 60 degrees longitude distance from the terminus towards the centre of the day zone and towards the centre of the night zone, calculating along the equator? I know that you cannot provide me with a table of answers for all possible points of the surface, like one could do telling the actual temperatures for Berlin, Tokyo, Capetown, Los Angeles, Reykjavik, Santiago, Sydney, Bora Bora, Irkutsk, Ouagadougou and other places.

How are these temperatures on Mercury calculated for other areas? Or estimated? Or maybe you know if there is a table of those temperatures available somewhere. I understand they will be the temperatures of the surface ("ground" or "rock"), because there is no atmosphere to distribute the heat. And that there are no "seasons" in the Earth sense, because the axis of Mercury is not inclined. Yet, do these actual temperatures depend on the actual distance of Mercury from the Sun, due to the eccentricity of its orbit, thus creating a sort of seasons?

One more question related to the temperatures and the picture on the website linked to by me previously. What are those areas coloured in green, forming, let me call it so, a "longitude 0 - 180" belt passing through the south pole of Mercury? I do not understand the picture (the description is to vague).

Too many questions in one post. Sorry for that.

All the best,

ANSWER: Hello.

Your question actually encapsulates a frequent misconception to do with astronomical measurements, whether of distance (say to a nebula like Orion), or temperatures - say of the stars or surface temperatures of planets. That is: what is the "actual" value, say of distance to the star Rigel, or temperature of Mercury.

The problem is inherent in the choice of the word "actual" which means "real" as in "no other values are possible".  But in a manner of speaking this is bogus. The most we can do given the limits of our instrument is assign ranges or when giving a temperature, also provide the uncertainty in measurement, say: 427 C + 50 or whatever. (Which implies a range, i.e. between 477C and 367 C)

That more sources for astronomical information, measurements don't do this is perhaps for reasons of simplicity - keeping the information basic - but it does tend to mislead.  Your very question:

"Yet, do these actual temperatures depend on the actual distance of Mercury from the Sun, due to the eccentricity of its orbit, thus creating a sort of seasons?"

Embodies this kind of yen for "actual" values - which simply don't exist in reality because our measurements are more in the way of estimates. If instead these are given or delivered as single values then one can say the results are "proofy" to use the term of mathematician Charles Seife referenced in his book, 'Proofiness: How You're Being Fooled by the Numbers'.

Which I would strongly suggest reading to get a better perspective on these issues. What he says in there applicable to a range of examples, also applies to astronomical measurements. In this regard, one of the surest signs of proofiness is the failure to provide attached uncertainties to the measurements - any measurements!

Sadly then, you will not find - nor can I provide - any special "table" for "actual temperatures" across the planet Mercury because our data simply isn't at the level of refinement to allow that. The best we can do is provide values as generalities, as your Wikipedia searches disclose.

As for the planet locations yielding coldest and hottest temperatures, these would be on the night side and day side, respectively. The thermal map shown on this site:

gives some idea of the times spent in the sunlit area for a particular region  (south pole) of the planet.

To understand the image, imagine holding a globe (say of Earth)  so you are looking at it from below. The gradation in color (as in the above map) from orange to green to blue to black gives an idea of the times spent outside the sunlit region. The greenish-spotted blue 'belt' - with black spot at the very center at S. pole - is then representative of the temperature changes over time in the S. pole region betweeen (-180) and +180 planetary longitude.

Hopefully this helps, and I hope you can also get the book by Charles Seife!

---------- FOLLOW-UP ----------

QUESTION: Dear Philip,

thank you for such a prompt answer. I admit, that I am mixed with some issues. Yet when I have several times used the word "actual" in my question, I have always had in mind the meaning "present, current" (The Concise Oxford Dictionary, p. 11), and not, as you write, '"real" as in "no other values are possible"'. Of course other values are possible, but they happen in some other time. Maybe a better word would be "simultaneous": 'While in the middle of the day zone there is +427, at the same time on equator in the terminus point East (twilight zone) there is only N degrees.'

Of course I understand that it might be that everything is just based on some kind of estimation, therefore no ready table of results is accessible. Yet it seems to me that every estimation in science is based on a certain kind of calculation from some source data, therefore I thought you might know how these values are calculated (or could be calculated).

As far as the image concerned, before I wrote to you I had exactly imagined what you have now written to me to do, and I couldn't - and still cannot understand. Maybe I will explain my question like that: Why is not the whole surface of Mercury (excluding the polar areas and certain holes in the ground) sunlit more or less evenly? Why is there a certain regular belt visible? There must be a reason. Is this related to the the shape of the orbit of Mercury? If it were on Earth I would suppose that those are the areas that are regularly covered with clouds, let say, like the areas along the Andes mountain range in South America (I have just invented the example, as a kind of thought experiment, I don't know if such a phenomenon takes place there, but it is not impossible, therefore it might happen). What is the reason of such a phenomenon on Mercury?

With greetings,

ANSWER: Hello,

First, according to my Websters Encyclopedic Dictionary - so we are clear -  the primary definition of "actual" is:

Existing in act or fact, REAL

Hence, my meaning holds.  (The def. you gave from your concise Oxford is given as a secondary definition)

So, in the frame of my answers the primary Webster's definition is the one to which I will use and refer. Again, the primary misunderstanding of many newcomers to astronomy is that we know these values measurements absolutely and they are therefore the only real values.

But not taken into account are the various limitations that emerge say in the course of measuring, say for assorted space craft like the Mariner 10 or the Messenger to Mercury. Time limitations, reception of signals, location of the craft all may play roles in delimiting what can be found out.

As for temperatures, they can be estimated in various ways and there is a vast amount of source material. To fully have addressed your questions then would have gone far beyond the scope of a reasonable answer so I will provide some "resource" material for you to plumb through - also sparing me from having to "re-invent the wheel".

You can find details of thermocouple -type measurements of temperatures in this paper - which is at a very readable level:

It also provides a useful history of such efforts.

You can find further details on planetary temperatures here:

Re: your set of questions in the last paragraph, they are all nicely answered in a site from UCLA which ties the orbital extremes (perihelion and aphelion) to the temperature distribution. You can find it here:

Unfortunately, I do not have the time to explain it all - but using the terms present and adequate Googling you can likely find the answers.

Re: your other remark:

"the picture would be just lying, pretenting to be a faithful representation of the surface of the Southern hemisphere of Mercury. If the picture is really taken from NASA sources, I am inclined a priori to reject such an explanation and serach for another explanation. "

Again, it isn't a matter of "lying" or "misrepresentation" but rather in being able to obtain data -results within the scope of the observational or experimental limits. Again, I have a problem with use of the term "really" taken from etc.  Yes, it was taken from those JPL etc. sources, and hence doesn't bear any "a priori rejection".

In future please give no more than two questions at one time.


---------- FOLLOW-UP ----------

QUESTION: Dear Philip,

Thank you once again for your answers. Pointing to some available resources is a good answer, indeed. I am really grateful both for the book indicated, as for the web links, as also - for showing me the flaws in my understanding of the subject. Had I understood everything well from the beginning, I wouldn't have needed to ask AllExperts, would I?

Let me express one small reflection more. Please do not be attached to the letter of my questions but rather to their spirit - don't catch me on a supposedly wrong usage of single word(s), because even if a meaning of a word, which I point to you as used by me, is 'only' a secondary one - it is still a VALID one, so when I explain you how I understood that word (what I meant by that) you cannot answer that you don't use that word this way so my question has to be rejected as ill-formulated.

Also, already in the first question of mine I have written to you that English is NOT my mother tongue and in fact I am not even living in an English speaking country, so because of that you might have had pity on me, if not for other reasons.

Because these reflections are not related to astronomy, they do not require your answer - but you are of course welcome to reply. I promise I will not enter into a further argument. It's only because I don't know how to express my thanks and comments otherwise as you don't accept 'ratings', that I have to post a follow up.

Nevertheless don't take me wrong. I am really grateful for your answers. I have learned much from them.

Sincerely yours,



And thanks for your response. Let me touch on several aspects here:

1) You write:

"Had I understood everything well from the beginning, I wouldn't have needed to ask AllExperts, would I?"

This is true, but for future reference please bear in mind that one or two questions at the most is what we look for, not eight or more. You basically peppered the "question" with multiples and my response was generic - touching on what I believed was the central problem - one of perspective to do with what qualified as a "real measurement"

2) You also write:

"Please do not be attached to the letter of my questions but rather to their spirit"

Then the best way to achieve this is, in future, to limit the number of questions you put forth at one time. Do not pepper the expert with multiple questions - especially as this will also skew statistics, i.e. time of response etc. - so one can't expect the same time for response for a single question as for eight or nine packed in one.

3) You also write:

"don't catch me on a supposedly wrong usage of single word(s), because even if a meaning of a word, which I point to you as used by me, is 'only' a secondary one - it is still a VALID one"

I didn't say your usage was "not a valid one", only that it was not the one I had intended, or a primary one as given in an unabridged dictionary-  and as I was the one addressing your question that (I believe) trumps your usage in terms of my response.

Also, when you cited your "Concise Oxford Dictionary, p. 11" and stated emphatically that the meaning , "is not, as you write 'real' as in 'no other values are possible'"  - that invited a corrective response from me. (My first thought was 'Wait! He's correcting me with a concise dictionary?')

Again, the response was not to devalue your word usage but to provide the context for mine, in the answer I'd already given.

Nor was your initial question "ill formulated" necessarily if my meaning is used. It merely means that I have a different take on these sort of issues, based on some four decades of teaching astronomy, and giving astronomy workshops to the public. So, in other words, I have seen and heard similar questions before.

I think the main takeaway again, in order not to get into these problems, is to avoid asking too many questions at one time. Pick one or two, hone them for clarity (as far as you as a non-English speaker can allow) and then send them.

My fault, I guess, was accepting your initial spate of questions when normally I'd have rejected them with advice to cut them down to 1-2 - but I believe the polite way you asked enticed me to respond.

Anyway, I am glad I could have been of some help and I believe you will find the links I sent useful for more study  


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Philip Stahl


I have more than forty years of experience in Astronomy, specifically solar and space physics. My specialties include the physics of solar flares, sunspots, including their effects on Earth and statistics pertaining to sunspot morphology and flare geo-effectiveness.


Astronomy: Worked at university observatory in college, doing astrographic measurements. Developed first ever astronomy curriculum for secondary schools in Caribbean. Gave workshops in astrophysics and astronomical measurements at Harry Bayley Observatory, Barbados. M.Phil. degree in Physics/Solar Physics and more than twenty years as researcher with discovery of SID flares. Developed of first ever consistent magnetic arcade model for solar flares incorporating energy dissipation and accumulation. Develop first ever loop solar flare model using double layers and incorporating cavity resonators.

American Astronomical Society (Solar Physics and Dynamical Astronomy divisions), American Mathematical Society, American Geophysical Union.

Solar Physics (journal), The Journal of the Royal Astronomical Society of Canada, The Proceedings of the Meudon Solar Flare Workshop (1986), The Proceedings of the Caribbean Physics Conference (1985). Books: 'Selected Analyses in Solar Flare Plasma Dynamics', 'Physics Notes for Advanced Level'. 'Astronomy and Astrophysics: Notes, Problems and Solutions'.

B.A. Astronomy, M. Phil. Physics

Awards and Honors
American Astronomical Society Studentship Award (1984), Barbados Government Award for Solar Research (1980), Barbados Astronomical Society Award for Service as Journal Editor (1977-91)

Past/Present Clients
Caribbean Examinations Council, Barbados Astronomical Society, Trinidad & Tobago Astronomical Society.

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