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A '''day''' is the amount of time it takes for a given [[planet]] turn once around its axis.
A '''day''' is a period of [[rotation]] of a celestial body. It is also a standard cycle on [[Earth]]. But today its meaning is the subject of controversy in [[Christianity|Christian]] circles and even [[creationist]] circles.
On [[Earth]], a day is 24 hours. It can also be defined as 86,400 [[SI]] seconds.<ref>[http://asa.usno.navy.mil/SecM/Glossary.html#day The Astronomical Almanac]</ref> According to the [[Bible|Biblical]] book of [[Genesis]] [[God]] created the heavens and the earth in six days, and rested on the [[Sabbath|seventh]].<ref>Genesis 1:1-2:24</ref> It should be noted that a day within the Bible is not always a 24 hour period. Sometimes, to God, a day is as a thousand years. <ref>Psalms 90:4, II Peter 3:8</ref> Also, the Hebrew word used for day in Genesis, ''yom'', can also mean a longer, unspecified period of time. <ref>http://www.answersincreation.org/yom_hebrew.htm</ref>
== Day in astronomy ==
In [[astronomy]], the word ''day'' has as many as three specific meanings:
=== Sidereal day ===
A ''sidereal day'' (from a [[Latin]] word associated with the stars) is the period that a celestial body requires to make one full turn around its axis with respect to the [[star]]s.
=== Solar day ===
A ''solar day'' is the time that a celestial body requires to present the same face toward the [[sun]], or toward its own star in the case of an [[extrasolar planet]]. On [[Earth]] the solar day is 24 hours, or 86,400 [[SI]] seconds.<ref>[http://asa.usno.navy.mil/SecM/Glossary.html#day Definition of 'Day'] The Astronomical Almanac, [[United States Naval Observatory]], n.d. Accessed June 27, 2008.</ref>
=== Synodic day ===
A ''synodic day'' is the time that a celestial body requires to present the same face toward its own particular primary. For [[planet]]s and [[dwarf planet]]s in orbit around the sun, the synodic day and the solar day are the same. For [[moon]]s, the synodic day is the day between "rises" of a moon's primary in that moon's sky.
A celestial body in [[tidal lock]] with its primary does not have a synodic day.
== Day in the Bible ==
According to the [[Bible|Biblical]] book of [[Genesis]] [[God]] created the heavens and the earth in six days, and rested on the [[Sabbath|seventh]].<ref>{{Bible ref|book=Genesis|chap=1}}, {{Bible ref|book=Genesis|chap=2|verses=1-24}}</ref> It should be noted that a day within the Bible is not always a 24 hour period. Sometimes, to God, a day is as a thousand years. <ref>{{Bible ref|book=Psalms|chap=90|verses=4}}, {{Bible ref|book=II_Peter|chap=3|verses=8}}</ref>
However, the [[Bible]] defines the word ''day'' ([[Hebrew]] ''yowm'') most clearly by the context of its use in {{Bible ref|book=Genesis|chap=1}}. In [[Genesis]] 1, the word ''day'' appears six times, preceded by the phrase "evening and morning," clearly indicating a solar day.
Outside of Genesis, the word ''day'' appears more than four hundred times, and never means anything other than an ordinary day.
== The day-age controversy ==
[[Progressive creationism]], in an attempt to compromise Scripture with [[geology|geological]] [[uniformitarianism]] and [[evolution]], states that each of the "days" in [[Genesis]] 1 were actually great ages of the earth. This interpretation fails for a number of reasons. The most salient reason is that Genesis tells us that [[plant]]s were created on Day 3, while the [[sun]] and [[moon]] were created on Day 4. While plants might survive one solar day without light, they would not survive for millions of years without light. Nor does progressive creationism say where the light of the first "day" of Genesis 1 came from. (In addition, most uniformitarians insist that the sun existed before the earth, and in fact date the sun primarily by adding a few hundreds of thousands of years to their value for the age of the earth.)
==References==
==References==
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== See also ==
* [[Orbital period]]
[[Category:Units of Measurement]]
[[Category:Units of Measurement]]
[[Category:Chronology]]
[[Category:Chronology]]