This is the 8483rd most frequent Russian word.
нептун
Neptune (Roman god or planet).
The word 'Нептун' refers to the planet Neptune in this context.
Нептун — восьмая планета от Солнца в нашей Солнечной системе.
Neptune is the eighth planet from the Sun in our Solar System.
Here, 'Нептун' is used as the name of the Roman god of the seas.
По мифологии, Нептун был богом морей у римлян.
According to mythology, Neptune was the god of the seas among the Romans.
In this sentence, 'Нептун' is again the planet, referring to its observation.