Axial tilt is an astronomical term regarding the inclination angle of Earth’s rotational axis in relation to a perpendicular to its orbital plane Solar Day

The celestial equator is a great circle on the imaginary celestial sphere, which is actually the plane of the terrestrial equator extended out into the universe i.e. it could be seen as a larger ring around the Earth's equator, at the same angle. The celestial equator is inclined by ~23.5°, with respect to the ecliptic plane; a result of axial tilt. Solar Day

Solar Year An equinox is the moment when the Sun is located right over the equator. The event occurs twice a year, around March 20 and September 23. solstice is either of the two times of the year when the sun is at its greatest distance from the equator: in technical words, when the celestial equator and ecliptic reach their largest separation.

At any location on Earth, a solar day may be simply defined as the time between one noon and the next. "noon" means the instant when the Sun crosses the meridian—an imaginary line on the celestial sphere through the north and south celestial poles, passing directly overhead at the given location. Meridian = line of longitude Solar Day

Unfortunately, this most direct measure of time has two serious drawbacks: the length of the solar day varies throughout the year the definition of noon varies from place to place on Earth. Solar Day

Recall that the solar day is the result of a "competition" between Earth’s rotation and its revolution around the Sun i.e. the Earth’s revolution means that it must rotate through about 361 degrees from noon to noon However, while Earth’s rotation rate is virtually constant, the rate at which the Sun traverses the celestial sphere as it moves along the ecliptic, is not, because: Solar Day However, while Earth’s rotation rate is virtually constant, the rate at which the Sun traverses the celestial sphere as it moves along the ecliptic, is not, because:

First, Earth’s orbit is not exactly circular and our orbital speed is not constant Earth moves more rapidly than average when closer to the Sun, more slowly when farther away  so the speed at which the Sun moves along the ecliptic varies with time. Solar Day

Second, due to Earth's axial tilt, the Sun moves along a great circle (the ecliptic) that is tilted to Earth's celestial equator. When the Sun crosses the equator at both equinoxes, it is moving at an angle to it, so the projection of this tilted motion onto the equator is slower than its mean motion. When the Sun is farthest from the equator at both solstices, it moves parallel to it, so the projection of this parallel motion onto the equator is faster than its mean motion. Solar Day

Normal terms: Equinox – sun is moving at an angle to equator  appears slower Solstice – sun moves parallel with equator  appears faster Solar Day

The combination of these effects means that the solar day varies by roughly half a minute over the course of the year not a large variation, but unacceptable for astronomical and many other purposes. The solution is to define a mean solar day, originally cast in terms of a fictitious mean Sun that moves around the celestial equator at constant speed Solar Day  but which in effect is just the average solar day over an entire year.  This is the day our clocks (atomic or otherwise) measure, and it is by definition constant.  One second is 1/24 1/60 1/60 = 1/86,400 mean solar days.

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