The function estimates the projected (silhouette) area as a portion of the surface area of the organism as a function of zenith angle. The function is useful for estimating absorbed solar radiation.

`proportion_silhouette_area(psi, taxon, raz = 0, posture = "prostrate")`

- psi
`numeric`

zenith angle in degrees between`0`

and`360`

.- taxon
`character`

organism name. Current choices are`"lizard"`

,`"frog"`

, and`"grasshopper"`

.- raz
`numeric`

relative solar azimuth angle (in degrees). Required if`taxon = "lizard"`

. This is the horizontal angle of the sun relative to the head and frontal plane of the lizard and options currently include`0`

(in front),`90`

(to side), and`180`

(behind) degrees.- posture
`character`

value describing posture. Required if`taxon = "lizard"`

. Options include`"prostrate"`

(default) and`"elevated"`

.

`numeric`

silhouette area as a proportion.

Relationships come from

Lizards: Muth (1977)

Frogs: Tracy (1976)

Grasshoppers: Anderson et al. (1979)

Anderson RV, Tracy CR, Abramsky Z (1979).
“Habitat Selection in Two Species of Short-Horned Grasshoppers. The Role of Thermal and Hydric Stresses.”
*Oecologia*, **38**(3), 359--374.
doi:10.1007/BF00345194
.

Muth A (1977).
“Thermoregulatory Postures and Orientation to the Sun: A Mechanistic Evaluation for the Zebra-Tailed Lizard, Callisaurus draconoides.”
*Copeia*, **4**, 710 - 720.

Tracy CR (1976).
“A Model of the Dynamic Exchanges of Water and Energy between a Terrestrial Amphibian and Its Environment.”
*Ecological Monographs*, **46**(3), 293-326.
doi:10.2307/1942256
.

Other allometric functions:
`mass_from_length()`

,
`proportion_silhouette_area_shapes()`

,
`surface_area_from_length()`

,
`surface_area_from_mass()`

,
`surface_area_from_volume()`

,
`volume_from_length()`

```
proportion_silhouette_area(psi = 60,
taxon = "frog")
#> [1] 0.2902208
proportion_silhouette_area(psi = 60,
taxon = "grasshopper")
#> [1] 0.0862
proportion_silhouette_area(psi = 60,
taxon = "lizard",
posture = "prostrate",
raz = 90)
#> [1] 0.329857
proportion_silhouette_area(psi = 60,
taxon = "lizard",
posture = "elevated",
raz = 180)
#> [1] 0.176596
```