Axial tilt is an astronomical term regarding the inclination angle of Earth

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

• 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
  meridianan imaginary line on the celestial
  sphere through the north and south celestial
  poles, passing directly overhead at the given
  location.
• Meridian line of longitude

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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.

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Solar Day
Recall that the solar day is the result of a
"competition" between Earths rotation and its
revolution around the Sun i.e. the Earths
revolution means that it must rotate through
about 361 degrees from noon to noon However,
while Earths rotation rate is virtually
constant, the rate at which the Sun traverses the
celestial sphere as it moves along the ecliptic,
is not, because
However, while Earths rotation rate is virtually
constant, the rate at which the Sun traverses the
celestial sphere as it moves along the ecliptic,
is not, because
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Solar Day
First, Earths 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.
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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.
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Solar Day
Normal terms Equinox sun is moving at an angle
to equator ? appears slower Solstice sun moves
parallel with equator ? appears faster
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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

• ?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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