सिद्धान्तशिरोमणि: गणिताध्याय (भास्कराचार्य - ग्रहगणित, मध्यमाधिकार व स्पष्टाधिकार सटीक)

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Fig. 22
(6) Let ϕ + δ = 20°. (Ref. fig. 22) ie. imagine
the Sun to rise at N, the north point so that RN + NP =
δ + ϕ = 90°. Then the Sun's diurnal path will be entirely
above the horizon, which means that what is called
'perpetual day' begins for that place on that day and lasts
as long as δ ⩾ 90 - ϕ. For the same place, let δ =

• (90 - ϕ) = QS. In this case the Sun sets at S, and
  what is called perpetual night begins and lasts till the
  southern declination of the Sun is greater than 90 - ϕ.
  The duration of perpetual day can be found as follows
  which applies to the perpetual night as well.
  Fig. 23
  (Ref. fig. 23). Let A be the point at which the
  declination is 90 - ϕ; let S be the summer solstice and let
  B be the point where again the declination is equal to
  90 - ϕ. So long as the Sun traces the arc AB = 2 AS =