Definition of "nightsky"

Measures of the interstellar Ca ii K line and the mercury nightsky lines indicate that the spectrograph was stable during the run, and that the standard error of measurement of a single line is about 15 km s−1. […] Hγ may contain weak Hg nightsky lines, […]

1977, Contributions from the Dominion Astrophysical Observatory, Victoria, B. C., page 1

Solar roof ponds are an inexpensive and effective method of providing both heating at lower latitudes (i.e., 36 °NL or lower) and cooling in dry climates with clear nightskies. Furthermore, there are several modifications which can be made to the system to make it applicable to a variety of climates. For example, spraying or flooding the outside surface of the enclosed ponds to provide additional cooling by evaporation (up to 4 times the amount provided by nightsky radiation) can extend the system’s cooling capability to humid regions. […] Cooling—In contrast to heating, the optimal cooling configuration is a flat pond that is exposed to the entire hemisphere of the nightsky. […] However, unless carefully designed, it can decrease the cooling capability of the system by obscuring some of the nightsky and protecting the ponds from desirable airflow in cases where lower outside night temperatures would help cool the ponds by natural convection.

1979, Edward Mazria, The Passive Solar Energy Book: A Complete Guide to Passive Solar Home, Greenhouse, and Building Design, Emmaus, Pa.: Rodale Press, pages 114, 189 and 191

Another important problem in cosmology is: Why is the nightsky dark? It has, in fact, been said that we could use this observation to show that the universe is expanding. We now realize that this isn’t true. The well-known early astronomer Johannes Kepler was the first to see that there was a difficulty with the dark nightsky. To him, the problem centered around whether or not the universe was infinite. If it was infinite, Kepler was sure the nightsky should be bright.

1993, Barry Parker, “Overview of the Big Bang and Expanding Universe”, in The Vindication of the Big Bang: Breakthroughs and Barriers, Springer Science+Business Media, section “The Dark Nightsky Paradox”, page 85