Astronomy
A day of exactly 86,400 SI seconds is the astronomical unit of time (the second is not preferred in astronomy).
For a given planet, there are three types of day defined in astronomy:
- stellar day - an entire rotation of a planet with respect to the distant stars
- sidereal day - a single rotation of a planet with respect to the vernal equinox
- mean solar day - average time of a single rotation of a planet with respect to the sun as the central star
For Earth, the stellar day and the sidereal day are nearly of the same length and about 3 minutes 56 seconds shorter than the solar day. Relative to the fixed stars, the Earth spins just over 366 times upon its axis during one complete orbit. The Earth's orbit around the Sun reduces (by one) the number of transits the Sun makes across the Earth's sky in a sidereal year.
Read more about this topic: Day
Famous quotes containing the word astronomy:
“Awareness of the stars and their light pervades the Koran, which reflects the brightness of the heavenly bodies in many verses. The blossoming of mathematics and astronomy was a natural consequence of this awareness. Understanding the cosmos and the movements of the stars means understanding the marvels created by Allah. There would be no persecuted Galileo in Islam, because Islam, unlike Christianity, did not force people to believe in a “fixed” heaven.”
—Fatima Mernissi, Moroccan sociologist. Islam and Democracy, ch. 9, Addison-Wesley Publishing Co. (Trans. 1992)
“It is noticed, that the consideration of the great periods and spaces of astronomy induces a dignity of mind, and an indifference to death.”
—Ralph Waldo Emerson (1803–1882)