Is it really darkest just before dawn?

Question

I would find a smooth, gaussian luminance profile much more credible.

Answer 1

No, it's not --- that's just a poetic metaphor.

While one's first impression is that it will be locally darkest about local midnight (ignoring daylight savings time effects), there are other effects than just the geometrical one involving the Sun's position.

For example, the darkest time will be affected by what time in the month it is. It should be darkest of all around local midnight at New Moon. At other times, particularly more than a night or two away from New Moon, the Moon will generally be contributing asymmetrically to the background light (time-wise, relative to midnight), both directly and through scattering of its light by Earth's atmosphere. This shifts the "truly darkest time" away from local midnight. In fact that effect is sometimes figured into observing schedules at major astronomical observatories. While observers of relatively nearby bright stars get the "bright time" (around Full Moon) all night, deep extragalactic observers get the "dark time" (around New Moon) all night. However, for a few shared nights in between (approaching 1st Quarter, or after 3rd Quarter), parts of those nights (the early or late parts) may be scheduled out separately to "bright time" or "dark time" observers according to when the darkest time will be during those particular nights.

Most other effects are probably negligible. The Moon can cast a relatively strong shadow, a measure of its relative brightness, of course. Venus can also cast a measurable shadow, too, though one's best chance of seeing it is only around New Moon. (I've noticed my "Venusian shadow" a few times.) However, that is either some time (but not too much) after sunset or some time (but not too much) before dawn, since Venus doesn't depart too far from the Sun's angular position. So even that effect probably wouldn't move the "darkest time" significantly from around midnight.

That moves us on to considering whether Jupiter, or stars like Sirius, are in the sky --- very small effects, I'd say. There could also be some lingering effects in the sky from dust kicked up by human activities, making for some asymmetrical scattering effects; but again they're probably very small.

Of course, all these astronomical considerations ignore local policies about switching late evening or early morning outdoor lighting off or on, or the hours for lighting special late-night public events, etc., and whether you're in a city or deep in the country so that this is irrelevant. But I presume from your question that you were mainly concerned about the astronomical effects.

Interesting question. Live long and prosper.

Answer 2

No! This is just an old wives tale. If you really want to know the answer, get up early someday, just before dawn is expected to occur, then compare it to how dark it was at midnight. You can find out what time sunrise is expected in your area either by consulting an almanac, by checking Yahoo! Weather, or by calling your local weather station.

Answer 3

it surely is and it gets afew degrees colder just as the sun rises also.