The Sun (Heb.: שֶׁמֶשׁ shemesh; Greek: ἥλιος helios) is the local body in the heavens that provides light and heat to the earth. It was set in place by the Creator on the fourth day of the creation week and there referred to as "the greater light" (lit. luminary). Its responsibility was to "rule the day" With the moon and stars, it was to serve as a sign for seasons and years.
The setting and rising of the sun set the bounds of the day, being marked by the evening and morning, periods of lesser light greater than that of the moon. The period during which the sun was seen in the sky was designated the day.