Most people may know this, but still sometimes reviews can refresh things :-)
ചിങ്ങം is the തത്ഭവം of സിംഹം, lion, or leo according to zodiac. The സ in Sanskrit gets converted into ച. (It seems to be the case that all the words with ശ, ഷ, സ and ഹ came from Sanskrit and are not Tamil origin, though I am not quite sure.)
The names of the other months closely follow the names of the western counterparts:
Leo - സിംഹം - ചിങ്ങം
Virgo - കന്യ - കന്നി
Libra - തുലാം
Scorpio - വൃശ്ചികം
Sagittarius - ധനു
Capricon - മകരം
Aquarius - കുംഭം
Pisces - മീനം
Aries - മേഷം - മേടം
Taurus -ഋഷഭം - ഇടവം
Gemini - മിഥുനം
Cancer - കര്ക്കടകം
Of these the, word for Taurus is a bit complex in etymology. There seems to be a series of letter transformations, ഋ -> ഇ, ഷ -> ട and ഭ -> വ, but these are kind of not atypical.
Thus, it is obvious that the mallu calendar we now follow came with the Sanskrit sub-culture.
It will be interesting to see what kind of calendar we had when malayalam was in its infancy - as a branch of Tamil. Even the Tamil calendar was based on the Hindu solar calendar.
Does any one know if there were any other calendars in use in ancient times?