Sun and Moon Letters

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

Why does this assimilation happen? It happens because the points of articulation in the oral cavity (the mouth) for the sun letters and the لْـ are adjacent. Therefore, the لْـ gets assimilated to the sun letter that follows it. The sun letter, as a result, becomes doubled (i.e., a geminate). This doubling is marked by the shadda  ّ on the sun letter. In phonology (the science of speech sounds), this kind of assimilation occurs because the لْـ and the sun letters share many phonological features. As for writing, the 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 the لـ assimilates. This word (i.e., شَمْس) is chosen because it is one of 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 the 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.