Wikipedia:Reference desk/Archives/Science/2020 December 23

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December 23

Stars in winter

While the internet is full of statements that stars are brighter in winter and that this season is good for observation, I've noticed that the typical grey cloud cover, often before snowfalls or drizzle (at least in the Northern Hemisphere), precludes observing any stars at all. Unless one assumes clear cloudless nights in winter are more frequent than precipitation cloud cover? 212.180.235.46 (talk) 11:14, 23 December 2020 (UTC)

Where does anyone say stars are brighter in winter? The brightness of the vast majority of stars is relatively stable. Winter is good for observation simply because the hours of darkness are longer, and the sun goes further beneath the horizon, so the chances of a clear dark sky are much higher. Clouds can block the view of the sky at any time of year.--Shantavira|^{feed me} 13:32, 23 December 2020 (UTC)

Cold air is able to hold less moisture than warm air. The sky can be hazy in the winter, but when you get a cold snap you get really clear air and good viewing. Assuming you live in a place where atmospheric moisture is an issue.

And there is the way the Northern Hemisphere faces relative to the center of the galaxy which makes the night sky seem clearer in the winter. 85.76.79.191 (talk) 13:43, 23 December 2020 (UTC)

The Sagittarian Milky Way (where the center of the galaxy is) is still in a summer constellation everywhere. The geographic latitude makes the summer and southern sky more impressive in like the Outback than the northernmost deserts but the secondary Orion Arm is the closest galactic arm to Earth (we are slightly inside it in some interpretations) which causes the winter sky to have the highest concentration of objectively bright stars, the majorness of the summer sky's arm cannot compensate for its distance and it has many dim stars and fewer bright ones. Buttloads of stars. In the mid-northern hemisphere and light pollution where many astronomy writers live this bilateralness is emphasized to the eye. You can't even see the Milky Way anymore where many people live, so you can't tell that it's better in the summer, or at least the summer Milky Way is dim and unimpressive, that happens in some suburbs. Even most farmland does not show the Milky Way to its full potential (the unresolved summer side of the galaxy, which looks like veined marble in the remotest wilderness even at 40°N) Sagittarian Milky Way (talk) 14:45, 23 December 2020 (UTC)

The common wisdom, which seems to have been contorted, is that the astronomical seeing is better in winter (or more precisely, on nights with cold and dry atmospheric conditions). A key insight is that the atmosphere needs to be cold and dry - and stable - "all the way to the top." This is categorically not identical to a "cloudless night" - in fact, there are plenty of conditions that are considered "clear sky" that would be absolutely awful for star viewing, casual or otherwise. A great example would be moist, humid, warm, and turbulent air, common to the summer months in many parts of Earth.

Additionally, there are more dark hours per unit-of-time when Earth is so tilted that we call it "winter."

Whether the weather or climate in your geographic area meets these criteria will obviously vary; but in many places on Earth where optical astronomy is practiced, the winter months are the better months for seeing stars.

Nimur (talk) 17:22, 23 December 2020 (UTC)

Another interesting tidbit is the clearest sky can also be turbulent and the humidest sky can also be calm. If the stars twinkle a lot that is because the air is not stable and you're seeing the turbulence with the naked eye, if the air is dry it looks awesome though making people think it'd be great for a telescope when it's really just clear. If it's hazy but non-turbulent like a temperature inversion it looks horrible to the eye but good for bright small things in a telescope. Also I would like to note that astronomical seeing properly refers only to turbulence while dry clean air vs humid and/or polluted is called transparency.Sagittarian Milky Way (talk) 18:17, 23 December 2020 (UTC)

I would guess that the major factor is the one highlighted by Shantavira. If the sun does not go 18° below the horizon it's not properly dark. See Twilight#Astronomical twilight. 82.13.210.231 (talk) 17:35, 23 December 2020 (UTC)

At 40N and a place at the nominal longitude of its time zone it gets this dark at 10:36 daylight time on the latest day of the year, at Arctic circle minus <18 degrees (such as anywhere in your IP country of UK) there's one to many nights a year that never get this dark. Experimenters in the Egyptian desert stopped seeing twilight with the naked eye at 15 to 16 degrees though. In New York City 13 degrees is about as dark as it ever gets and there's always more than 4 hours below 18 degrees. Earth's tilt changes slowly so during Stonehenge times the twilight was even worse. Sagittarian Milky Way (talk) 17:55, 23 December 2020 (UTC)