Sun and Moon Letters

Arabic letters are classified into two categories: sun letters and moon letters. The definite article الْـ 'the' triggers this classification. الْـ precedes indefinite nouns and adjectives (i.e., prefixed to them). Basically, the لْـ in الْـ assimilates to certain subsequent letters. These letters are called sun letters (also solar letters). And it (لْـ) does not assimilate to the remaining letters. These are called moon letters (also lunar letters).

Why does this assimilation happen with sun letters? It happens because the points of articulation in the oral cavity (the mouth) for the sun letters and that of لْـ are adjacent. Therefore, لْـ gets assimilated to the sun letter that follows it. The sun letter, as a result, becomes a geminate (i.e., doubled). This doubling is marked by the shadda (  ّ  ) on the sun letter. In phonology (the science of speech sounds), this kind of assimilation occurs because لْـ and the sun letters share many phonological features. As for writing, لْـ of the definite article الْـ is not affected; that is, it is written. 

Sun Letters:

Why are these letters called sun letters? They are called sun letters after the word شَمْس "sun", which begins with the letter (sound) ش /sh/ to which لـ assimilates. This word (i.e., شَمْس) is chosen because it is one of a kind. Also, it sharply contrasts with the word قَمَر "moon." 

If you look at the Arabic diagram of speech sounds presented in a previous lesson, you will notice that the letters that are produced / articulated from the vicinity of the لْـ are:

ت ، ث ، د ، ذ ، ر ، ز ، س ، ش ، ص ، ض ، ط ، ظ ، ل ، ن

In the following table, you can see that لْـ under the pronunciation column is dropped and replaced with shadda over the subsequent letter (i.e., the sun letter). 

Moon Letters: 

Why are these letters called moon letters? The moon letters are named after the word قَمَر "moon" which begins with the letter (sound) ق. The word قَمْر sharply contrasts with the word شَمْس; moreover, it is one of a kind. Assimilation of the لْـ of الْـ to the moon letters is impossible because their points of articulation is far from that of the لْـ. In other words, they do not share any phonological features with the لْـ. These letters are: 

 أ ، ب ، ج ، ح ، خ ، ع ، غ ، ف ، ق ، ك ، م ، و ، هـ ، ي

In the table below, you can see that the لْـ does not assimilate to the subsequent moon letter, as shown under the pronunciation column. 

Thus, Arabic letters are classified into sun letters and moon letters. This classification is caused by the لْ of the definite article الْ, a morpheme that is prefixed to indefinite nouns and adjectives to make them definite. This لْ is always saakin (i.e. has a sukuun over it). The first letter of the word to which it is attached is always mutharrik (i.e., vowelized).

The Rule: 

If the vowelized letter to which الْـ is prefixed is one of these ت ، ث ، د ، ذ ، ر ، ز ، س ، ش ، ص ، ض ، ط ، ظ ، ل ، ن, the لْـ is assimilated to it resulting in a doubled sound (a geminate). This doubling is marked by the shadda along with the vowel of the letter.