This character originates from a picture: a round celestial entity with a dot drawn inside it. The very same symbol that is still used in western calendars and astrology to mark the sun! This pictograph has been forced into a more square shape though, as have the rest of the Chinese characters.

The word 日rì stands for “sun”, as well as for “day”. The sun as a purely celestial object would be expressed with the word 太 阳 tàiyáng, “ultimate yang”.

星 xīng - star
In the character of “star”, 星 xīng, we can also see the sun-part at the top. Below is the phonetic particle 生 shēng, which as such carries the meaning of “birth, growth” As for the twinkling stars up in the sky we'd use the plural form 星 星 xīngxīng, “stars”. The word for “week” is 星 期 xīngqī, and litterally speaking that translates into “star period”.

月 yuè – moon
It is probably not too difficult to see the character 月 yuè as a crescent moon – just imagine the ends a bit more pointed. The character stands for the heavenly object “moon”, and also the period of the lunar cycle, “month”.

Planets are named in Chinese as sort of stars. Let's first get to know the ones that have been named according to the five elements. In the order progressing outward from the sun:

Mercury= 水 星 shuĭxīng, “water star”
Venus = 金 星 jīnxīng, “metal star”
Mars = 火 星 huŏxīng, “fire star”
Jupiter = 木 星 mùxīng, “wood star”
Saturn = 土 星 tŭxīng, “earth star”

Two more heavenly drifters:
Uranus = 天 王 星 tiānwángxīng, “star of the heavenly king”
Neptune = 海 王 星 haǐwángxīng, “star of the ocean king”

...and the one that got cast away from the planetary high society:
Pluto = 冥 王 星 míngwángxīng, “star of the underworld king”