# How to calculate length of day and length of visible light?

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• Updated 7 months ago
We have sunrise and sunset info from astronomy api. Are those actual sunrise / sunset or civil sunrise/sunset data? Also How to calculate these two terms from the api?

Length Of Day:
This is defined to be the time of Actual Sunset minus the time of Actual Sunrise. The change in length of daylight between today and tomorrow is also listed when available.

Length Of Visible Light:
This is defined to be the time of Civil Sunset minus the time of Civil Sunrise.
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Posted 4 years ago

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The Fourier transform of the PSD is the time-domain autocorrelation function of the radiation. The autocorrelation function gives you a direct window on the correlation time. The FT will be peaked at zero (perfect correlation when the signal is on top of itself) and decline from there as the correlation reduces (imperfect correlation when the signal is offset from itself). Once you have the autocorrelation function you will have to make some judgement about how to get the correlation time from it. The time at which the correlation falls to 1/2 its peak might be a good choice. Multiply that by c and you have your correlation length.

Note that you will get slightly different answers if you choose some other criterion for the correlation time, for example if you choose 1/e of peak value. Also, the spectral sensitivity of your detector will change things a bit. Moral: at the end of the day you might do no better than c/Δf. It depends on what you want to do with the information once you have it. Check over here for more info.

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