If you’re north of the Arctic circle or south of the Antarctic circle, the Sun doesn’t set in their respective mid-summers.
However, even at latitudes where the Sun sets, it’s still a while before the night sky becomes dark enough to see the stars. What I want to know is, at what minimum latitude is it too light to see the stars in mid-summer, all through the night?
(Actually what I really want to know is at what minimum latitude would it be too light to see Aldebaran in the Antarctic mid-summer, but I figure northern hemisphere answers are more likely!)
Not really the answer you’re looking for, but for what it’s worth Ian Ridpath has a Guide to the Stars out which shows the latitudes at which each constellation are visible. It’s a geometric thing, though, and doesn’t cover the brightness of the sky – just which stars are obscured by the earth itself being in the way.
There are different degrees of twilight that have been officially defined.
Civil twilight: When the Sun is within 6 degrees of the horizon.
Nautical twilight: When the Sun is within 12 degrees of the horizon.
Astronomical twilight: When the Sun is within 18 degrees of the horizon.
Now, it’s very hard to say exactly at what stage of twilight a particular would be visible. It will depend on atmospheric conditions, how high the star is above the horizon, and who’s lookin’.
However, I would think that Aldebaran (a 1st magnitude star) would be visible during civil twilight. On the summer solstice, the Sun is at a declination of +23.5 degrees. During the darkest part of civil twilight, the Sun six degrees below the horizon. This will occur at midnight.
[Various arcane scribblings omitted due to lack of graphical editor.]
So whatcha want is to be at around 60.5 degrees latitude (north or south, doesn’t matter.) But this is only a rough estimates. At a higher latitude, it might still get dark enough to see Aldebaran. However, the higher your latitude, the lower Aldebaran will be in the sky, the closer it will be to the horizon, where the sky is brighter.
At an RA of 4 hours, Aldebaran will be on the opposide side of the sky from the Sun at 18 hours (for southern hemisphere summer solstice) so you have that working for you.
Nitpick: you’re assuming that Aldebaran is not visible during civil twilight (sun is 6 degrees or less below the horizon), but will become visible as soon as civil twilight ends and nautical twilight starts.
I do think that’s a very reasonable assumption, and I agree with your answer.
The game is Call of Cthulhu, and the PCs are having a bit of trouble with Hastur.
My players want to avoid being under a night sky containing Aldebaran, in September, 1926. They intend to do this by crossing the Antarctic Circle, thus avoiding night altogether. Not a solution I was prepared for, but it has so much potential for fun on my part that I’m running with it!
I was wondering whether they had to actually cross the Circle or whether they could get away with staying at one of the whaling stations in South Georgia. Looks like they need to go further south - Signey, South Orkney Islands might do.
The sky has lines of longitude and latitude, but we all them right ascension (RA) and declination (mentioned above.) To tell where anything in the sky is, like the Sun or a star, you just give its RA and Dec, the same way that you would give the longitude and latitude of a city on Earth.
Except to make things more fun, instead of 360 degrees of longitude, there hare 24 hours are right ascension. So if two objects are 12 hours apart, that means they’re on opposite sides of the sky. On the summer solstice, the Sun is at an RA of about 18 hours. At midnight it is below the horizon due north, and you want to look 14 hours of RA away, that is, to the south (where hopefully, the sky is a little darker) to find Aldebaran.
Ahhh, this is for a CoC campaign! If you wish to cite me, I absolutely insist you introduce an NPC astrologer named Frau Doktor Zelda LaGrange to deliver the information.
I’d be delighted! Hell, I might even put you on their boat…
I’m pleased to find that there’s no shortage of whaling stations, safe harbours, supply depots and various remote places to deposit the PCs. The Signey whaling station closed in 1926 - I may just make them responsible for that. I also note that the explosive whaling harpoon was well established by then, so I’ll give them one, and something worth shooting at with it.
I swear, sometimes I have as much fun researching these things as running them!
It will be dark at Signy Island from 10:31pm to 7:36am on Sept 1, and from 11:17pm to 6:39am on Sept 30. (This is from the US Naval Observatory’s sunrise/sunset calculator, at http://aa.usno.navy.mil/data/docs/RS_OneYear.html. I tried to post the table it generated for September 1926, but it didn’t come out right.)
(This assumes Signy Island and the Signey whaling station are the same place.)
Astronomical twilight, which is what I was using to compute this table, is a time when sky illumination is so faint that it is practically imperceptible. A 1st-magnitude star like Aldebaran would certainly be visible if astronomical twilight doesn’t last all night.
From another US Naval Observatory site, Aldebaran rises at 1:03am on September 15 on Signy Island, and transits (maximum distance above the horizon) at 5am.
That means Aldebaran would definitely be visible before the start of astronomical twilight. They’re out of luck. They would be OK there around Antarctic midsummer, but September is not around Antarctic midsummer.
Whoops, those are Greenwich, not local time. Local time is 3 hours earlier, so beginning of astronomical twilight ranges from 3:39am to 4:36am. (I thought those times looked off) Aldebaran rises at 1am local time, though…
Sure. In fact they’ll probably set off within a couple of days of the equinox -they’re in Boston at the moment.
What they’re facing is progressive insanity, affecting one of the PCs. (A point of SAN every few nights, in fact.) They can put up with a couple of months of it - they’re hoping that breaking the cycle with a summer Antarctic trip will stop it altogether. I haven’t yet decided if that’ll work, but they do have enough time to try it.
Or they can try to get within 16 degrees of the South Pole, an option I wasn’t aware of until Podkayne’s post. Although that would probably be more hazardous than confronting the Great Old One they’re running from. I expect they’ll come up with some unlikely scheme involving an aeroplane. They’ll predate Hubert Wilkins and Richard Byrd by a few years if they do.
Anne, thanks for the data. I think Signy is still too far North - I might try for Deception Island, or a harbour on the peninsula.
Oh, I think I understand now. They don’t have to be somewhere where the star isn’t visible because the sky is always light, they have to be somewhere where the star is only overhead when the sky is light. When it’s nighttime where they are, the other half of the sky would be visible.
May I suggest Sky & Telescope’s Interactive Sky Chart? It’s fun, versatile (time and location changes can be done) and does a pretty good job of transitioning from day to twilight to night [and vice-versa] on a minute by minute basis…if needed) and free! Have fun with it!
