How Perigee calculates apparent size
Nothing in Perigee is scaled by eye. Every object in the scene is placed at a fixed render distance and drawn at whatever radius reproduces its calculated angular diameter, so the size you see is a consequence of the geometry rather than a decision anybody made.
How is apparent size calculated?
Apparent size is the angle an object subtends at the eye. For an object of diameter d at distance r, the angular diameter θ is:
θ = 2 · arctan( d / 2r )
That is the whole calculation. It is applied identically to the Moon, to Saturn and to the Andromeda Galaxy, and it is the only thing that decides how much sky an object fills. If the fixed render distance in the scene ever changes, the rendered radius follows automatically — no object is ever scaled by hand to compensate.
How does Perigee compare sizes to the full Moon?
A figure in degrees is hard to picture, so Perigee restates it in full Moons: the object's angular diameter divided by the Moon's angular diameter at its familiar distance of 384,400 km. Saying something spans thirty-three Moons is the same statement as giving its angle, in units an eye can hold.
How does Perigee calculate light travel time?
The selected distance divided by the exact speed of light in vacuum. It is a statement about the distance, not about the rendering: the scene is not delayed by that amount.
Which constants does Perigee use?
- 299,792.458 km/s, exactly, by definition of the metre
- 149,597,870.7 km, exactly, per IAU 2012 Resolution B2
- 384,400 km, the familiar Earth–Moon distance
What does Perigee simulate, and what does it only draw?
Perigee labels every claim it makes with one of three boundaries, and shows that label in plain language beside the claim.
- A deterministic value from the same object and distance data as the scene.
- An authored visual treatment produced by the scene, not a physical result.
- A sourced physical effect that Perigee states but does not model.
What Perigee is not
Perigee is not a planetarium catalogue and not a physics simulator. It does not model heat, radiation, gravity or tides, it does not place objects on real orbits, and it does not claim that any of these skies could occur. Surface lighting, atmospheric treatment and the impossible-proximity effects are authored visualizations sitting on top of geometry that is exact.
Sources
Every figure on this site traces to one of these, each opened and dated at review.
- SI base unit: metre
- IAU 2012 Resolution B2: re-definition of the astronomical unit
- Saturn: Facts
- Moon Facts
- What is Betelgeuse? Inside the Strange, Volatile Star
- Messier 31 (The Andromeda Galaxy)
- Mars: Facts
- Jupiter: Facts
- Neptune: Facts
- SIMBAD basic query: Sirius (alpha Canis Majoris)
- SIMBAD basic query: Rigel (beta Orionis)