Sun Solar in Chinese / Japanese...

日 is the word for sun.

It also means day and can refer to the day of the month when expressing the date.
Example: October 1st would be “10 Moons, 1 Sun.”

日 is also the first Kanji for the title of Japan (in Chinese, Japanese Kanji, and Korean Hanja). Thus, this character is used as an adjective for things that are Japanese.
Ever heard of Japan being called, “The land of the rising sun”? Well, that's what the full title of Japan means.

Depending on the context, this character can mean Sunshine or Sunlight.

Note: In Japanese, this Kanji has a variety of possible pronunciations. The pronunciation changed depending on context and how this Kanji is combined with other Kanji. When used alone, this is usually "hi" (pronounced like "hee") but sometimes it’s "nichi." When combined, it can be "tsu," "ni," "ka," and a few others.

太陽 is a two-character title for the Sun.

This refers specifically to Sol, the star at the center of our Solar system.

In Japanese, this is often romanized as Taiyou or Taiyo but can also be pronounced as the names Minami, Hiroaki, Hinata, Hikaru, Tsubasa, Tahi, Takayasu, Takaharu, or Soru.

This is how to write “day” in Chinese, Japanese, and Korean Hanja.

This can also mean “Sun,” the star in the middle of the Solar system in which we live. In Japanese, it can also mean “sunshine” or even “Sunday.”

When writing the date in modern Chinese and Japanese, putting a number in front of this character indicates the day of the month. Of course, you need to indicate the month too... The month is expressed with a number followed by the character for the moon. So “three moons ten suns” would be “March 10th” or “3/10.”

Note: This is also the first character for the proper name of Japan. Remember that Japan is “The land of the rising sun”? Well, the first character for Japan means “sun” and the second means “origin” so you get the real meaning now. Sometimes, in China, this sun character can be a short name for Japan or a suffix for something of or from Japan.