what is a nocturn(小夜曲)?

A nocturne (from the French for “nocturnal”) is usually a musical composition that is inspired by, or evocative of, the night.

The name nocturne was first applied to pieces in the eighteenth century, when it indicated an ensemble piece in several movements, normally played for an evening party and then laid aside. Sometimes it carried the Italian equivalent, notturno, such as Wolfgang Amadeus Mozart’s quadraphonic Notturno in D, K.286, written for four lightly echoing separated ensembles of paired horns with strings, and his Serenata Notturna, K. 239. At this time, the piece was not necessarily evocative of the night, but might merely be intended for performance at night, much like a serenade.

In its more familiar form as a single-movement character piece usually written for solo piano, the nocturne was cultivated primarily in the nineteenth century. The first nocturnes to be written under the specific title were by the Irish composer John Field, generally viewed as the father of the Romantic nocturne that characteristically features a cantabile melody over an arpeggiated, even guitar-like accompaniment. However, the most famous exponent of the form was Frédéric Chopin, who wrote 21 of them. Later composers to write nocturnes for the piano include Gabriel Fauré, Alexander Scriabin and Erik Satie (1919). One of the most famous pieces of nineteenth-century salon music was the “Fifth Nocturne” of Ignace Leybach, who is now otherwise forgotten.

Other examples of nocturnes include the one for orchestra from Felix Mendelssohn’s incidental music for A Midsummer Night’s Dream (1848), the set of three for orchestra and female choir by Claude Debussy (who also wrote one for solo piano) and the first movement of the Violin Concerto No. 1 (1948) by Dmitri Shostakovich. French composer Erik Satie composed a series of five small nocturnes. These were however, far different from those of Frédéric Chopin and John Field, based on minor chords.

The first movement of Ludwig van Beethoven’s Moonlight Sonata has also been considered a nocturne (certainly, Ludwig Rellstab, who gave the piece its nickname, thought it evocative of the night), although Beethoven did not describe it as one.

Nocturnes are generally thought of as being tranquil, often expressive and lyrical, and sometimes rather gloomy, but in practice pieces with the name nocturne have conveyed a variety of moods: the second of Debussy's orchestral Nocturnes, “Fêtes”, for example, is very lively.

J. Field (1782 – 1837), an Irish who created short instrumental music pieces with free style and fill with romantic mood. Chopin and Debussy are the followers who wrote several famous Nocturnes, with some aspect of the night and are usually solemn and contemplative
Remarks: Nocturne always translate in Chinese as 夜曲 and Serenade as 小夜曲