Refers to the sun as it sets below the horizon, often with a poetic or literary tone. Can also be used figuratively for decline.
The setting sun sinks behind the mountains.
His decline was obvious to everyone.
旭日 refers to the rising sun in the morning, while 落日 refers to the setting sun in the evening, both being literary terms.
lonely and helpless; desolate isolation
Compound of 落 (raku, 'to fall') and 日 (jitsu, 'sun'), literally 'falling sun'. The reading is on'yomi, suggesting a Sino-Japanese origin.