Rotation period,astronomyAstronomically, a noun refers to the time required for a celestial body to rotate around its axis.Normally, the rotation period is equal to the length of the star day of the object.
For example, equinoxDiurnal alternation, the length of day and night are all inherent in the earthnatural phenomenaAlthough both involve day and night conditions, the causes are different.The equinox of day and night is a static concept, which does not involve the movement of the earth, but is determined by the earth's characteristics of "the earth is an opaque sphere".The alternation of day and night is dynamicconcept, mainly byearth rotationThe light source is from the sun, so the cycle of day and night is aSolar dayThat is, 24 hours.However, the Earth's rotation period is 23 hours, 56 minutes and 4 seconds, also known as aSidereal day, rather than 24 hours, because the Earth's rotation is also in revolution, which will cause the relative position of the sun to the ground to change.Therefore, the rotation period should be measured relative to the sky background.[1]
Chinese name
Rotation period
Foreign name
Rotation period
Subordination
astronomy
Interpretation
Time required to rotate one circle along the rotation axis
Since ancient times, the movement of the earth has naturally provided people with the basis for measuring time, giving two natural time units, namely, day and year."Day" refers to the cycle of day night alternation. In ancient times, people used a gnomon to measure the length of the sun. For example, when the sun was in the south at noon one day, the shadow was the shortest. From this moment to noon the next day, when the sun was in the south again, the shortest time interval of the shadow was one day, that is, one dayTrue solar day。
It is generally believed that the earth rotates one week for one day.However, it is not so easy to make sure that the earth has turned a circle.Here we introduce the concepts of star day and true sun day.The straight line connecting two points of due south and due north of a place is the meridian, and the plane determined by the meridian and the plumb line is the meridian in the direction of due south and due north.The time interval between two times opposite the same star on the astronomical meridian plane of a certain place is called the star day. The star day is the rotation period of the earth with the star as the reference.
If the time unit is defined as the time interval between two astronomical meridians facing the center of the solar circle (that is, the center of the solar circle goes up to the middle of the sky twice), then this time unit is called true solar day, also called true time for short, or apparent time.It is the rotation period of the earth with the sun as reference.[2]
Sidereal day
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Schematic diagram of the difference between stellar day and solar day
Sidereal dayIt is always shorter than the true sun day.This is because the earth is so far away from the star that the earth seems to be motionless from the star, and the earth's orbit has become a point relative to such a long distance.The light from these distant celestial bodies is parallel. No matter where the earth is in the orbit of revolution, the time interval between the two meridian planes of a place and a star remains unchanged.In comparison, the sun is much closer to the earth. Seen from the earth, the sunEclipticMoving from west to east, it moves about 1 degree in a day and night.For the meridian plane of a certain place, when a star day is completed, the sun has moved and the earth rotates from west to east, so the earth must turn another angle before the sun crosses this meridian plane again, that is, a true sun day is completed.
The stellar day is only used in astronomical work. The "day" we use in real life refers to the cycle of day night alternation, which is obviously closer to the true solar day.The time system established according to the true solar day is called“True solar time”。[1]
The length of a day and night
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The length of a day and night is not always equal to the rotation period.withcelestial bodies For example, the length of a day and night refers to the time when the sun rises to the sky twice in a row.The length of a day and night is generally based on the rotation period, but there are many factors that affect the time of a day and night.The revolution is the most important factor. For example, the revolution of the earth increases the time of a day and night by 3 minutes and 56 seconds.Generally, the revolution speed of large celestial bodies is much smaller than the rotation speed, so the impact on the time of day and night will be one to several orders of magnitude smaller.However, byTidal locking Later, the rotation period of the celestial body will be the same as the revolution period. At this time, the time of one day and night will depend on the position of the celestial body: if the central celestial body is the sun (such a celestial body has not been found), then half of the planet will always be day and the other half will always be night, and the relative position of the sun will never change;The central celestial body is a planet or other celestial body whose central celestial body is the sun. The length of a day and night is the superposition of the rotation period of the celestial body and the revolution period of the central celestial body.Mercury is close to the sun and rotates slowly due to the influence of the sun's tide generating force. Its revolution speed at the perihelion is faster than its rotation speed. Therefore, the sun will move eastward for a period of time and then reverse westward for a period of time.In addition to revolution, the motion fluctuation caused by other celestial bodies in the same system and the position of the same celestial body will also affect the length of a day and night.[1]
