The evolution of the Spanish word lunes ‘Monday’ is analogous to that of martes that we just saw, with only a minor difference. Sp. lunes comes from Lat. diēs lūnae ‘day of the Moon’, by the dropping of the diēs part. Lat. lūnae ‘of the Moon’ was the genitive wordform of the word lūna ‘Moon’. However, as you may have noticed, something is not quite right here, for there is an s at the end of the word lunes which was not there in the Latin phrase. In other words, whereas martes looks just like we would have expected it, the word lunes does not. the word for ‘Monday’ should have been *lune in Spanish, not lunes. The fact that Spanish lunes has an e instead of the Latin diphthong ae, which in Classical times was pronounced [ai̯], was to be expected, for that is what happened to this diphthong in Spanish patrimonial words by regular sound change (and even in loanwords from Latin, by analogy). But the appearance of the final s is not a regular sound change.
The only way that we can explain the final s of lunes is that it must have been added by analogy with the s that existed at the end of the names of other days of the week, also derived from genitive (possessive) cases that had a final ‑s, namely the words that have given us martes from mārtĭs, as well as jueves from iovĭs, and viernes from vĕnĕrĭs, all of which ended in ‑s. The reason that the genitive lunae did not have a final s is that this one was a first declension word and the genitive case in the first, second, and fifth declensions did not end in ‑s, like it did in the third and fourth declensions (cf. Part I, Chapter 8, §184.108.40.206).
We have evidence that already in the Vulgar (spoken) Latin of later centuries, the word for ‘Monday’ was sometimes changed from diēs lūnae to diēs lūnis, with a final s, by analogy with the names of other days of the week. Thus, this is something that happened early in the change from Latin to the Romance languages and not a peculiarity of Spanish.
This kind of sound change is very different from the other, more common type of sound change, such as the one responsible for Latin ae changing to Spanish e, which was a regular sound change, one which could be summarized thus: ‘Whenever there is ai in a Latin word, we find e in a Spanish that descended from it by word of mouth’. The type of change that resulted in the word lunes having a final s is called analogical sound change. This kind is much more sporadic and unpredictable than the regular sound changes that applied to all sounds (in a particular phonetic context) equally. We find analogical sound change in contexts in which a word becomes associated with others in a set, such as a paradigm.
As for the Latin word luna (in the nominative case form), there is evidence that it comes from Old Latin losna, a word that has been traced to Proto-Indo-European *lowksneh₂, derived from the root *lewk- ‘to light, shine; see; be bright’. This PIE root is also found in Lat. lūx (acc. lūcem), the source of Sp. luz ‘light’, and Lat. lūmen (acc. lūmen, gen. lūmĭnis), the source of Sp. lumbre ‘fire, flame’ and, ultimately, of Eng. luminous. This same root is also the source of the native English word light, which comes from Old English leoht ‘light, daylight’.
The Spanish word luna descended orally from Latin without undergoing any changes. All other Romance languages have kept this word too, though in some of those languages, the word has changed somewhat, e.g. It. luna, Fr. lune /ˈlyn/, Port. lua and Gal. lúa /ˈlu.a/ (earlier lũa /ˈlũ.a/), and Cat. lluna /ˈʎu.na/.
Lat. luna was the name of the Earth’s satellite, the moon (the name is sometimes capitalized to Moon), but it was also the name of a moon goddess in ancient Roman religion and myth, a ‘personification’ of the moon itself (there were two other lunar goddesses, Diana and Juno, but only Luna personified the Moon). She is typically represented with a female body with a crescent moon over her head and driving a white two-yoke chariot. Luna was the female counterpart and complement of the Sun (Sol), which was conceived of as a (male) god. That is the reason that the word luna was feminine in Latin and is still feminine in Spanish.
The reason the day of the week was called diēs lūnae ‘day of the Moon’ in Latin was that this day of the week was consecrated to the moon goddess, just like Tuesday was consecrated to Mars, the god of war, as we saw earlier. As we will see later, the ancestors of the Anglo-Saxons calqued this identification of this day of the week with the moon, which is why the day of the week in English is called Monday, from Old English Mōnandæg ‘moon day’ or ‘day of the moon’.
As usual, this Roman goddess had a Greek counterpart, whose name was Σελήνη (Selḗnē). The name has been ported into English as Selene, pronounced /sə.ˈli.ni/, and into Spanish as Selena /se.ˈle.na/, from a Latinized version of the name. Selena is a woman’s name in Spanish and Russian, for example. Luna is also a woman’s name in many languages.
Interestingly, English also has the word luna, though it is quite rare and most English speakers have never heard of it. Besides its use as the name for the Roman goddess of the moon, which is capitalized (Luna), Eng. luna has been used at different times, with different meanings. In alchemy, a pseudo-science practiced in the Middle Ages, luna meant ‘silver’. The word luna, or luna-moth, is used in entomology to refer to a ‘large grey-green N. American saturniid moth, Actias luna, with crescent-shaped spots and long curved tails on the wings’ (SOED).
Sp. luna has other meanings besides ‘moon’. In particular, it is used to refer to certain things made of panes of glass, such as to ‘a large sheet of glass used in mirrors [luna de espejo], shop windows [luna de escaparate], car windshields [luna de coche], etc.’ The source of this second meaning of Sp. luna seems to be the same word luna that means ‘moon’, though it is not clear how the derived meaning came about. In addition to luna llena, there is another word for ‘full moon’ in Spanish that contains the root lun‑, namely plenilunio, a loanword from Lat. plēnĭlūnĭum (plēn‑ĭ‑lūn‑ĭ‑um).
Words derived from Lat. luna
From the Latin noun lūna (lūn‑a), an adjective was derived by means of the adjectival suffix ‑ār‑. (Actually, the suffix was ‑āl‑, but the l of this suffix changed to r when following a root that had an l in it.) Adjectives derived with this suffix took third declension endings, such as masculine and feminine nominative singular ‑is. Thus, the adjective is typically given its form lūnāris (lūn‑ār‑is). Its meaning was ‘of the Moon’ or ‘lunar’.
As you may have guessed, Lat. lūnāris is the source of the English and Spanish adjectives lunar, pronounced /ˈlun.əɹ/ in English and /lu.ˈnaɾ/ in Spanish. The adjective lunar came into English in the early 15th century through Old French, first with the meaning ‘crescent-shaped’. Its current meaning is ‘of, determined by, or resembling the moon’ (COED) and we find it in phrases such as lunar landing (Sp. alunizaje), lunar surface (Sp. superficie lunar), lunar month (Sp. mes lunar), lunar rock (Sp. roca lunar), or lunar eclipse (Sp. eclipse lunar). In some of these cases we can substitute lunar with moon, used as a modifier in a noun-noun compound, e.g. moon landing or moon surface (cf. Part I, Chapter 5). Other times, it is equivalent to the modifying phrase of the moon, as in surface of the moon (Sp. superficie de la luna).
The Spanish adjective lunar is probably a learned loanword from Latin and not a patrimonial word, that is, one that was not transmitted uninterruptedly by word of mouth but rather borrowed from written Latin later on as a learned (fancy) word (though it is not fancy any longer). In addition, this adjective has also been converted into a noun in Spanish, the masculine noun lunar. The main meaning of this noun is ‘mole’, that is ‘a small dark blemish on the skin caused by a high concentration of melanin’ (COED). An additional meaning derived from this one is ‘polka dot’, that is, ‘one of a number of round spots that form a pattern, especially on cloth used for clothing’ (DOCE). For example, un vestido de lunares is a polka-dot dress.
Another set of cognate words derived from the word luna, are the adjectives Eng. lunatic and Sp. lunático/a. These words are taken from Late Latin lūnātĭcus ‘moon-struck, crazy, insane’ (fem. lūnātĭca). This Latin word was originally used to refer to people affected by neurological or mental diseases, such as epilepsy and bipolar disorder, which at the time were thought to be caused or somehow affected by the moon. As late as the 18th century, it was still thought that the moon influenced a number of diseases. Lat. lūnātĭcus was primarily an adjective, but it could be used as a noun to refer to a person who was moon-struck or crazy. As for the derivation of the Latin word lūnātĭcus, it has the adjectival suffix ‑ic‑ that we have seen before, but it also has the ‑āt‑ suffix associated with the first conjugation verbs’ passive participle (and supine) stems. However, the morpheme complex ‑ā‑t‑ĭ‑c‑(us/a) came to be used as an extended suffix that formed adjectives from nouns indicating some kind of a relation or action to the meaning from that noun.
Sp. lunático/a is first attested in the 13th century and it is an obvious loanword from Latin (if it was a patrimonial word, it would be *lunazgo). English got this word either directly from Latin or through Old French lunatique ‘insane’ as an adjective in the 13th century, and it is attested as a noun a hundred years later. This word replaced the native Old English word monseoc, which literally meant ‘moon-sick’, based on the same principle. The main collocations using the word lunatic in English are lunatic asylum, a dated word for a psychiatric hospital, and lunatic fringe, a phrase coined by President Theodore Roosevelt in his autobiography (1913), which has come to be used to refer to ‘the members of a usually political or social movement espousing extreme, eccentric, or fanatical views’ (AHD).
By replacing the ‑tic suffix of lunatic with the Latinate suffix ‑cy that is associated with abstract nouns, English developed the noun lunacy /ˈlun.ə.si/ in the 16th century. At first it referred to ‘the condition of being a lunatic’, i.e. insanity, though now it refers more to ‘behavior that is stupid or crazy’ (OALD) and is thus a synonym of madness. This word is often found collocated with the modifier sheer. The Latinate suffix ‑cy in English came along French words and it stems ultimately from Latin ‑cia or ‑tia, or its Greek cognates ‑κια (-kia) or ‑τια (-tia), e.g. infancy, accuracy, efficiency, solvency, and redundancy. Unlike in the case of lunacy, the suffix ‑cy typically alternates with the ending ‑t(e) of related adjectives (cf. infant, accurate, efficient, solvent, redundant).
As we said earlier, the English names for the days of the week are mostly calques or loan translations of the Latin names. Eng. Monday /ˈmʌn.deɪ̯/, comes from Old English Monandæg, among other spellings, which meant literally ‘Moon’s day or ‘day of the Moon’. The parts of this compound were monan, the genitive case wordform of móna ‘moon’ and dæg (dæᵹ) ‘day’. This name is an obvious calque of Latin diēs lunae, with the same meaning.
 Thus, for example, from the noun via ‘road, path’ (vi‑a), the word viāticus ‘related to a journey or travel’ (vi‑ā‑t‑ĭ‑c‑us). The neuter form of these adjectives was often turned into a noun, e.g. viāticum, which is the source of Sp. viático (learned) and viaje ‘trip, travel’ (Catalan loanword). Actually, Spanish words in ‑aje and English words in ‑age come from Late Latin words with the extended suffix ‑āticum, cf. Part II, Chapter 16.