Celsius and Fahrenheit are names, just like Mexico and U.S.
So is Watt.
As an addendum to this, Ireland, the only English-speaking country that has the euro, uses the €3.50 format. (And pluralizes euro as “euro” and cent as “cent”.)
To get the figure on most keyboards, use “Alt Gr” to the right of the space bar, then hit 4 (i.e. make a dollar symbol, but use Alt Gr instead of shift). And any professional font version younger than about six years old will contain the symbol.
And so does “Celsius.”
“Nautical” is not a proper noun.
Yes, but that’s entirely consistent with the proposed rule: Lower-case a unit (even if it is based on a proper name), but preserve the capitals in an adjective modifying or specifying the unit, if it is based on a proper name.
‘No flag, no country. That’s the rules which…I’ve just made up.’ 
Sunspace didn’t present it as a proposed system, but as the correct way to do it.
In my experience, itis. Perhaps your experience is different. 
Well, the SI itself proposes these capitalisation rules. I’m just proposing a rational justification for them.
OK, checked a couple of other style guides. There’s no consensus:
European Union - agrees with you, that proper names used as adjectives retain their capitals, and gives the examples ‘Richter scale’, ‘Mach number’ and ‘degree Celsius’.
US GPO - seems to agree with you on Celsius, but at various points in a list of abbreviations mentions ‘degree Rankine’ and ‘degree rankine’
The Economist - is silent on the issue
I like the Guardian’s summary the best: celsius: scale of temperature invented by a man named Celsius 
It would help if any of my three computers (just over 1 year old) actually had an “Alt Gr” key. Apparently, it seems to be a mostly European thing, or for US users with an international layout keyboard. Apparently, the substitute for AltGr is “Control-Alt” and, on my computers, control-alt-e gives me the euro symbol. So does holding down alt and typing 0128 on the numeric keypad.
For the Mac, using the U.S. keyboard layout, it’s Option-Shift-2. British layout, it’s Option-2.
:smack:
Thanks for the correction - consider ignorance fought.
On capitalisation of Celsius, I agree with acsenray regarding the SI rules; The names of all SI units are written in lower case (to distinguish them from the name of the scientist honoured) with the symbol normally the upper case initial. The point is that there is no SI unit the “Celsius”, the fundemental unit of thermodynamic temperature is the kelvin (K) and by definition the degree Celsius is equal to 1 K.
There’s at least one other unit which must be capitalized: The Calorie, to distinguish it from the calorie. 1 Calorie = 1000 calories. Yes, it is screwed up, but we’re stuck with it.
Agreed it’s a screw-up: am I right in thinking this is the main reason food labelling (in the UK and presumable Europe-wide) lists the energy as ‘kJ’ and ‘kcal’, meaning the latter will be correct for anyone counting Calories?
Aren’t both units officially deprecated under SI?
(Yes, joules and even BTUs are much tidier than calories. Or Calories.)
Come to think of it, the British Thermal Unit is capitalised. But that’s because it’s the proper name of one particular thermal unit, not a run-of-the-mill noun, like euro or watt.