When night actually begins depends on who you ask
There isn't a single answer to que horas comeca a noite because different systems define it differently. Astronomers, aviation, the military, and regular people all use slightly different cutoff points. The confusion comes from the fact that the sun doesn't just vanish below the horizon and suddenly it's dark. There's a transition period that lasts anywhere from 20 to 40 minutes depending on your latitude and the time of year.
que horas comeca a noite and the astronomical definition
The most technically precise answer comes from astronomy. Night officially begins when the sun reaches 18 degrees below the horizon. This is called astronomical twilight end, or civil and nautical twilight having passed completely. At this point the sky is as dark as it will get and you can see stars that would otherwise be washed out. For most Brazilian cities this happens roughly between 7:00 PM and 7:45 PM in summer, and between 6:00 PM and 6:30 PM in winter. But those are rough estimates. The exact time shifts every day. I spent years working with astronomical calculations for a client who needed precise sky darkness data for an observatory project in the sertão nordestino. The problem was that at lower latitudes near the equator, the sun sets almost vertically, which means those twilight phases compress into something like 25 minutes instead of the hour-long transitions you get at higher latitudes. One afternoon my calculations were off by about 12 minutes because I'd used a generic formula that assumed a mid-latitude sunset angle. I had to switch to a spherical trigonometry approach that accounts for the solar declination on that specific date and the exact geographic coordinates. Took me two hours to debug instead of the usual 20 minutes.
The practical definitions that actually matter
Civil twilight ends when the sun is 6 degrees below the horizon. At that point most outdoor activities still have enough ambient light, but streetlights need to turn on. Nautical twilight ends at 12 degrees below, where the horizon becomes difficult to distinguish at sea. Astronomical twilight ends at 18 degrees, which is what most people mean when they ask about night properly beginning. Here's something people miss: the duration of each twilight phase is not constant throughout the year. Near the equinoxes, the sun crosses the horizon at a steeper angle, so each phase passes quicker. Near the solstices at higher latitudes, the sun skims along at a shallow angle and you can have twilight lasting over an hour. In Fortaleza, for example, the difference between summer and winter twilight duration is maybe 5 to 8 minutes per phase. In cities like Porto Alegre, that gap widens significantly.
If you need exact times for a specific location, you should use the US Naval Observatory calculator or a tool like timeanddate.com. Both give you civil, nautical, and astronomical twilight times down to the minute. No app I've seen does better than these for accuracy, and some of the flashy weather apps actually interpolate badly between their data points, giving you times that are 3 to 5 minutes off depending on the season.
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Why most people get this wrong
The biggest mistake is treating night as a binary switch. It's not. From the moment the sun touches the horizon until astronomical twilight ends, you're in a gradient of light that changes every few minutes. If you're scheduling something light-sensitive, like astrophotography or outdoor film work, you need to account for the full transition, not just the final darkness point. Another issue is timezone confusion. Brazil spans three timezones and despite the 2018 abolition of daylight saving time in most of the country, some places still operate on informal adjusted time during certain periods. I once had a client schedule a rooftop event thinking night started at 7 PM based on São Paulo time, but the venue was in Manaus, which is two hours behind. The sky was still bright blue when everyone arrived. That one cost us a reshoot and about R$ 3.000 in wasted equipment rental.
If you want a quick rule of thumb: add roughly 30 to 40 minutes after sunset for full astronomical night at equatorial latitudes, and 45 to 60 minutes as you move south. This isn't exact but it's close enough for general planning. For anything precision-dependent, use the calculators and check the specific coordinates of your location.
