Sunny day

(1) - the initial position of the body; (2) - the position corresponding to one revolution around its own axis; (3) - position after one solar day

Solar days - the time interval during which a celestial body makes 1 turn around its axis relative to the center of the Sun. ^{[ clarify ]}

More strictly, this is the time interval between two similar (upper or lower) culminations (passing through the meridian) of the center of the Sun at a given point on Earth (or another celestial body).

Content

Sunny day on Earth

Sunny day duration variations

Due to the ellipticity of the Earth's orbit, the linear velocity of motion and the angular velocity of rotation of the Earth around the Sun changes during the year . The slowest of all the Earth moves in orbit, being in aphelion - the point of the orbit most distant from the Sun, the fastest being in perihelion . This is a significant reason for changing the duration of the solar day during the year. And the slope of the earth's axis leads to the movement of the Sun on the celestial sphere up and down from the equator during the year. At the same time, the right ascension of the Sun near the equinoxes changes more slowly (as the Sun moves at an angle to the equator) than at the time of the solstices , when it moves parallel to the equator.

The contribution of the ellipticity of the orbit to the fluctuation of the duration of solar days can be described by a sinusoid with an amplitude of +7.931 seconds at the perihelion and −7.92 seconds at the aphelion with a period of 1 year. There are other periodic effects that contribute to the length of solar days and depend on time, but they are small (disturbances from the moon and planets, etc.).

The fluctuation of the duration of solar days leads to the fact that the true solar time also fluctuates relative to the average solar time. Namely: as a result of the accumulation of deviations in the duration of solar days from 24 hours, the sundial is in a hurry or lags depending on the time of reference. Namely, if you start in autumn, when the oscillation amplitude passes through zero, it is 13 days later than the autumnal equinox, and then reaches above the mentioned maximum of +7.931 seconds, this is January 4, when the Earth passes the perihelion point, and before spring, when the amplitude again passes zero, it is also 13 days later than the day of the vernal equinox, the lag will be using the integral calculus S = 7.931 * 180 * 2 / π = 908.82565463651 s = 15 min. 8,825 .. sec. Now, if we divide the resulting sum in half, we get a positive amplitude of the equation of time , due to the ellipticity of the orbit, equal to 7 minutes and 34.41 seconds (see True Earth Motion around the Sun and the formula for true solar days).

Average Sunny Day

In order not to take into account this variability in everyday life, they use the average solar day tied to the so-called average Sun , a conditional point moving uniformly along the celestial equator (and not along the ecliptic , like the real Sun) and coinciding with the center of the Sun at the time of the spring equinox . The orbital period of the average Sun in the celestial sphere is equal to the tropical year .

The average solar day is not subject to periodic changes, like a true solar day, but their duration monotonously changes due to a change in the axial rotation period of the Earth and (to a lesser extent) with a change in the length of the tropical year, increasing by about 0.0017 seconds per century. Thus, the duration of the average solar day at the beginning of 2000 was equal to 86400.002 seconds. It should be noted that here it is the SI second specified using the intra-atomic periodic process, and not the average solar second, which is by definition equal to 1/86 400 parts of the mean solar day and, therefore, is also not constant.

Introduction of amendments

Although the average sunny day is not, strictly speaking, the constant unit of time, but the daily life of people is associated with them. Due to the accumulation of corrections to the duration of days in average solar time with respect to the uniform atomic time, sometimes it is necessary to add a so-called leap second to the atomic UTC scale in order to restore the reference of this scale to the solar time scale UT . It is theoretically possible to subtract a leap second, since the rotation of the Earth in principle does not have to constantly slow down.

Sunny day on other planets and satellites

Moon

The average solar day on the moon is equal to the average synodic month (the average interval between two identical phases of the moon , for example, full moons ) - 29 days 12 hours 44 minutes 2.82 seconds. True solar days can deviate from the average by 13 hours on both sides, which is connected both with the uneven movement of the Earth in orbit, and with the inclination of the Moon's orbit to the ecliptic, with the ellipticity of its orbit and with the inclination of the axis of rotation of the Moon to the orbit plane (see also Libration ).

Other planetary satellites

As in the case of the Moon, most of the satellites of the planets in the solar system, due to tidal resonance, have an orbital period equal to the period of axial rotation. Thus, for these satellites, the average solar day is close to the period of revolution around the planet. The exceptions are the most external satellites of giant planets (for example, Phoebe ), as well as Hyperion , which rotates around its axis randomly.

Planets

On gas giants that do not have a solid surface, the solar day depends on latitude - the atmosphere rotates at different speeds at different latitudes.

Mercury goes around the Sun in 87.97 days, and around its axis makes a full turn in 58.65 days (these periods are referred to as 3: 2). The average time interval between the two upper culminations of the Sun on this planet is 176 days, which is equal to its two years. Interestingly, when it is near perihelion, the sun for an observer on the surface of the planet can move in the opposite direction for 8 days, therefore, strictly speaking, the binding of the definition of solar days to a climax in this case is not quite correct.

On Venus , whose sidereal period of rotation around its axis is 243 days - more than the orbital period (224.7 days), the average solar day is approximately 116.7 days (due to rotation in the opposite direction).

On Mars, the average sunny day is only slightly more than the Earth. They are 24 hours 39 minutes 35.244 seconds.

On Jupiter, the day is 9 hours 55 minutes 40 seconds, on Saturn 10 hours 34 minutes 13 seconds. The day on Uranus is 17 hours 14 minutes 24 seconds, and on Neptune 15 hours 57 minutes 59 seconds.

In Pluto, due to its extreme distance from the Sun (and, consequently, the smallness of the angular orbital velocity), the average solar day is almost equal to the rotation period: 6 days 9 hours 17 minutes 36 seconds.