Φ Factor is an analogue of the well known Drag Coefficient Cd=0,47 for smooth sphere in the parallel fluid flow.
And it is about the by sphere’s surface the portion of incident energy acceptance!
From Wikipedia, the free encyclopedia
“In fluid dynamics, the drag coefficient (commonly denoted as: Cd, Cx or Cw) is a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment, such as air or water. It is used in the drag equation in which a lower drag coefficient indicates the object will have less aerodynamic or hydrodynamic drag. The drag coefficient is always associated with a particular surface area.[3]
The drag coefficient of any object comprises the effects of the two basic contributors to fluid dynamic drag: skin friction and form drag.”
https://en.wikipedia.org/wiki/File:14ilf1l.svg
https://en.wikipedia.org/wiki/Drag_coefficient
Planet does not reflect and absorb as a disk. Planet reflects and absorbs as a sphere.
Φ factor explanation
The Φ  solar irradiation accepting factor  how it "works". It is not a planet specular reflection coefficient itself.
There is a need to focus on the Φ factor explanation.
Φ factor emerges from the realization that a sphere reflects differently than a flat surface perpendicular to the Solar rays.
Φ – is the dimensionless Solar Irradiation accepting factor
"Φ" is an important factor in the Planet Mean Surface Temperature Equation:
Tmean.planet = [ Φ (1a) S (β*N*cp)¹∕ ⁴ /4σ ]¹∕ ⁴ (K)
It is very important the understanding what is really going on with by planets the solar irradiation reflection.
There is the specular reflection and there is the diffuse reflection.
The planet's surface Albedo "a" accounts only for the planet's surface diffuse reflection.
Albedo is defined as the ratio of the scattered SW to the incident SW radiation, and it is very much precisely measured (the planet Bond Albedo).
So till now we didn't take in account the planet's surface specular reflection.
A smooth sphere, as some planets are, have the invisible from the space and so far not detected and not measured the specular reflection.
The sphere's specular reflection cannot be seen from the distance, but it can be seen by an observer situated on the sphere's surface. Thus, when we admire the late afternoon sunsets on the sea we are blinded from the brightness of the sea surface glare. It is the surface specular reflection what we see then.
Jsw.notreflected = Φ*(1a) *Jsw.incoming
For a planet with albedo a = 0 (completely black surface planet) we would have
Jsw.reflected = [1  Φ*(1a)]*S *π r² =
Jsw.reflected = (1  Φ) *S *π r²
For a planet which captures the entire incident solar flux (a planet without any outgoing specular reflection) we would have Φ = 1
Jsw.notreflected = Φ*(1a) *Jsw.incoming
Jsw.reflected = a *Jsw.incoming
And For a planet with Albedo a = 1 , a perfectly reflecting planet
Jsw.notreflected = 0 (no matter what is the value of Φ)
In general:
The fraction left for hemisphere notreflected is
Jnotreflected = Φ (1  a ) S π r²
We have Φ for different planets' surfaces varying
0,47 ≤ Φ ≤ 1
And we have surface average Albedo "a" for different planets' varying
0 ≤ a ≤ 1
Notice:
Φ is never less than 0,47 for planets (spherical shape). Also, the coefficient Φ is "bounded" in a product with (1  a) term, forming the Φ(1  a) product cooperating term.
So Φ and Albedo are always bounded together. The Φ(1  a) term is a coupled physical term.
The Φ(1  a) term "translates" the notreflected of a disk into the notreflected of a smooth hemisphere with the same radius.
When covering a disk with a hemisphere of the same radius the hemisphere's surface area is 2π r². The incident Solar energy on the hemisphere's area is the same as on the disk:
Jdirect = π r² S
But the notreflected Solar energy by the hemisphere's area of 2π r² is:
Jnotreflected = Φ*( 1  a) π r² S
It happens because a smooth hemisphere of the same radius "r" notreflects the Φ*(1  a)S portion of the directly incident on the disk of the same radius Solar irradiation.
In spite of hemisphere having twice the area of the disk, it notreflects only the Φ*(1  a)S portion of the directly incident on the disk Solar irradiation.
Jnotreflected = Φ (1  a ) S π r² , where Φ = 0,47 for smooth without atmosphere planets.
and Φ = 1 for gaseous planets, as Jupiter, Saturn, Neptune, Uranus, Venus, Titan.
Gaseous planets do not have a surface to reflect specularly. The solar irradiation is captured in the thousands of kilometers gaseous abyss. Gaseous planets have only diffuse reflection which is expressed in planet energy balance with the albedo "a".
And Φ = 1 for heavy cratered planets, as Calisto and Rhea ( not smooth surface planets, without atmosphere ).
The heavy cratered planets have the ability to capture the incoming light in their multiple craters and canyons. The heavy cratered planets have only the albedo "a".
That is why the albedo "a" and the factor "Φ" we consider as different values.
Both of them, the albedo "a" and the factor "Φ" cooperate in the
Energy in = Φ(1  a)S
left side of the Planet Radiative Energy Budget.
Conclusively, the Φ Factor is not the planet specular reflection portion itself.
The Φ Factor is the Solar Irradiation Accepting Factor (in other words, Φ is the planet surface spherical shape and planet surface roughness coefficient).
*********************
It is not "a hypothetical solar irradiation accepting factor."
By "accepting" means how much energy a spherical shape body may "stop".
A spherical shape body may stop a flow of energy. Because of spherical shape a sphere
may stop Φ(1a)S of total incident energy.
Φ varies from 0,47 to 1.
The smoother the surface is the closer to 0,47 the Φ is.
Thus, a smooth surface sphere with Albedo a = 0
should "absorb" 0,47*S.
A sphere with rough surface (Φ = 1) and
a = 0 should "absorb" a 100% S.
And a sphere with Albedo a = 1 should not "absorb" EM energy, regardless the values of Φ.
At a first approach, when without the Rotational Warming phenomenon implementation, I use instead of Te, the Planet Corrected Effective Temperature Te.correct.
The formula is:
[ Φ(1a) /4σ ]¹∕ ⁴
Φ = 0,47
(the 0,47 is for smooth surface planets without atmosphere, the factor Φ accounts for the smooth planet surface specular reflection)
Table of results for Te and Te.corrected
Planet........ Te..........Te.correct
Mercury.....440 K.......364 K
Moon.........270 K......224 K
Earth.........255 K.......210 K
Mars,,,,,,,,,,210 K......174 K
We now have chosen Mercury for its very low albedo a=0,088 and for its very slow rotational spin N=1/175,938 rotations/day.
Mercury is most suitable for the incomplete effective temperature formula definition  a not rotating planet, or very slow rotating. Also it is a planet where albedo plays little role in energy budget.
These (Tmean, R, N, and albedo) parameters of the planets are all satellite measured. These parameters of the planets are all observations.
Planet….Mercury….Moon….Mars
Tsat.mean.340 K….220 K…210 K
R…......0,387 AU..1 AU..1,525 AU
1/R²…..6,6769….....1….…0,430
N…1 /175,938..1 /29,531..0,9747
a.........0,068......0,11......0,250
1a...…0,932……0,89…….0,75
Let’s calculate, for comparison reason, the Mercury’s effective temperature with the old incomplete equation:
Planet is a sphere and it reflects and absorbs like a sphere
.
The by a smooth spherical body solar irradiation absorption
Jabs = Φ (1  a ) S π r²
Φ = 0,47
.
.
.
Te.incomplete.mercury = [ (1a) So (1/R²) /4σ ]¹∕ ⁴
We have
(1a) = 0,932
1/R² = 6,6769
So = 1.362 W/m²  it is the Solar constant ( the solar flux on the top of Earth’s atmosphere )
σ = 5,67*10⁻⁸ W/m²K⁴, the StefanBoltzmann constant
Te.incomplete.mercury = [ 0,932* 1.362 W/m² * 6,6769 /4*5,67*10⁻⁸ W/m²K⁴ ]¹∕ ⁴ =
Te.incomplete.mercury = ( 37.369.999.608,40 )¹∕ ⁴ = = 439,67 K
Te.incomplete.mercury = 439,67 K
And we compare it with the
Tsat.mean.mercury = 340 K  the satellite measured Mercury’s mean surface temperature
Amazing, isn’t it? Why there is such a big difference between the measured Mercury’s mean surface temperature, Tmean = 340 K, which is the correct, ( I have not any doubt about the preciseness of satellite planets' temperatures measurements ) and the Mercury's Te by the effective temperature incomplete formula calculation Te = 439,67 K?
Let’s put these two temperatures together:
Te.incomplete.mercury = 439,67 K = 440 K
Tsat.mean.mercury = 340 K
Very big difference, a 100°C higher!
But why the incomplete effective temperature equation gives such a wrongly higher result?
The answer is simple – it happens because the incomplete equation assumes planet absorbing solar energy as a disk and not as a sphere.
We know now that even a planet with a zero albedo reflects the [1  Φ(1a)]S portion of the incident solar irradiation.
Imagine a completely black planet; imagine a completely invisible planet, a planet with a zero albedo. This planet still reflects the
[1  Φ(1a)]S portion of the incident on its surface solar irradiation.
The satellite measurements have confirmed it. The Mercury’s Φ = 0,47 Paradigm has confirmed it:
Φ  the dimensionless planet surface solar irradiation accepting factor.
Planet reflects the (1Φ + Φ*a) portion of the incident on the planet's surface solar irradiation.
Here "a" is the planet's average albedo. So we always have:
Jreflected = (1Φ + Φ*a)S
Jabsorbed = Φ(1a)S
**************
.
Let's see what the coupled term Φ(1  a)S produces:
Albedo =
=(satellite measured SW diffuselly reflected W/m²) /S W/m²
Earth's Albedo = 0,306
So = 1362 W/m²
(Earth's satellite measured SW diffuselly reflected W/m²) =
= Albedo * So = 0,306 *1362 W/m² = 416,8 W/m²
The Earth's surface the not reflected SW W/m² =
= Φ(1362 W/m²  416,8 W/m²)=
= 0,47*(945,2 W/m²) = 444,2 W/m²
*************
A planet reflects incoming short wave solar radiation.
A planet's surface has reflecting properties.
1. The planet's Albedo "a". It is a surface quality's dependent value.
2. The planet's spherical shape.
Φ is the planet solar irradiation accepting factor (the spherical shape and roughness coefficient).
Φ = 0,47 for a smooth surface sphere
What we had till now:
Jsw.incoming  Jsw.reflected = Jsw.absorbed
and
Jsw.absorbed = (1a) * Jsw.incoming
And
Jsw.reflected = a* Jsw.incoming
What we have now is the following:
Jsw.incoming  Jsw.reflected = Jsw.notreflected
And
Jsw.notreflected = Φ* (1a) * Jsw.incoming
For Planet Earth (smooth surface planet Φ = 0,47)
Jsw.notreflected = 0,47*(1a)*1.361 W/m² =
= 0,47*0,694*1.362W/m² = 444,26 W/m²
Averaged on the entire Earth's surface we obtain:
Jsw.notreflected.average = [ 0,47*(1a)*1.361 W/m² ] /4 =
[ 0,47*0,694*1.361W/m² ] /4 = 444,26 W/m² /4 =
= 111,07 W/m²
Jsw.notreflected.average = 111 W/m²
******************
Opponent:
"You need to show the flaws in the measurement methods that demonstrate that they are missing specular reflection."

Answer:
Yes, I have shown that the same (the specular reflection neglection) we can observe on six planets and moons with smooth surface, namely:
Mercury
Earth
Moon
Mars
Europa (Jupiter's satellite)
Ganymede (Jupiter's satellite)

Also, I invite you to observe the strong specular reflection the Earth's surface has everywhere you look.
Just stand facing towards the sun's direction, and then look down on ground.
What happens is your eyes unconsciously
narrow, because land exibits strong specular reflection.
Next, turn to the opposite of the sun direction and look at the same solar irradiated ground  you realize your eyes do not narrow unconsciously anymore, you feel your eyes pleasantly relaxed, because when in opposite direction you see ground in diffuse light only.

https://www.cristosvournas.com
The faster a planet rotates (n2>n1) the higher is the planet’s average (mean) temperature T↑mean:
Tmin↑→ T↑mean ← T↓max
.