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View Full Version : In the Arctic or Antartic circles--horizon lines and visibility



loiterer
05-26-2012, 12:36 PM
I have four characters travelling through an arctic region towards the pole. (This is a fantasy story so they have special beasts that they're riding.)

1) What, on almost perfectly flat terrain, would be the distance they could see to the horizon in each direction?

2) Assuming perfect weather, what kind of visibility is there at night at a) a new moon, and b) a full moon? Could four people riding one after the other see each other without using lights?

Kenn
05-26-2012, 01:43 PM
Generally, visibility is very good in polar regions, so the distance you can see is determined by the horizon (at least, it is up to about 40 or so miles) - this depends on their height above ground level. There is an Arctic haze which can reduce this, and this is thought to be caused by pollution. You can also get 'diamond dust', which is a suspension of ice crystals, and this can reduce visibility significantly.

The moon will appear low in the sky, in comparison to other places, although I doubt that'll make much difference. The things to remember are that snow will make things seem brighter and may improve the contrast (you wouldn't need lights unless it was overcast and moonless), and the polar night can last months.

Buffysquirrel
05-26-2012, 02:54 PM
When it's day in the polar regions, the sun shines so brightly off the snow that people need to protect themselves against the glare if they're not to experience snow blindness. Visibility is therefore a problem because there's too much light, not too little. There's also a phenomenon called white-out, where the land and the sky meld into each other and everything looks the same, making it difficult to find the horizon. Scale is also difficult and judging distance problematic.

If you look into the history of the NorthWest Passage, you'll find lots of sightings of land that proved not to be there. Clouds, maybe, mirages, white-out, delirium, who knows?

Navigation is tricky, partly because of white-out and also because, the closer you get to the magnetic pole, the less reliable compasses become, and the sun doesn't behave as you'd 'expect'. Add to that the tendency of people to travel in a circle, and you have potential for disaster.

Oh, not to mention that...I think it was on the way to the North Pole?...the ice you're walking on can move away from the Pole faster than you can travel towards it.

Polar explorers used to fix their position at the end of every day's journey to work out if they were even going in the right direction.

There's lots of first-hand accounts of polar exploration out there, from Cherry-Gararrd's Worst Journey in the World to Scott's diaries to Shackleton to your own Douglas Mawson. Well worth dipping into.

L M Ashton
05-27-2012, 04:15 AM
Is this story taking place on Earth? What time of year is it - ie summer or winter? How far north or south is it, ie how close to the pole?

Assuming Earth, then in the middle of summer, there would be light all the time - the sun doesn't set, whereas in winter, it would be dark all the time - the sun doesn't rise. (I spent a few weeks in Norman Wells, NWT in July and August one year, and at 11pm, my sister and I were in her kitchen chatting, no lights on, and it was as bright as it was at noon.)

Also, if it's summer, in the Arctic, depending on where you are, there may be no snow at all on the ground and the weather may be as warm as 10 or 20C, well above freezing.

I think you need to figure out where and when first before your questions can be properly answered.

loiterer
05-27-2012, 05:12 PM
Thank you for the replies so far, they have been most helpful.

LM Ashton, you are right. I should have given more information. It's an earth-like planet with a sun and moon and the magnetic north pole some way south of the pole itself. The story takes place at the start of autumn. At present the moon is a half moon. Two teams are competing against each other to make it to magnetic north and I wanted to know how close they had to be before they could see each other, and if they're moving at night, would they be able to navigate so as to see where they were going and also to not bump into other team-mates, or would they need artificial light.

Duncan J Macdonald
05-28-2012, 12:16 AM
Height of Eye / Distance to the Horizon Calculations

The math is very simple. Given a spherical planet of the approximate diameter of earth, then the distance to the horizon on a body of water is given by the following equation:

1.17 times the square root of your height of eye = Distance to the horizon in nautical miles.

A nautical mile is 2,000 yards (a statute mile is 1,760 yards).

So, with a height of eye of 9 feet (like riding on a beast), the horizon is 3.51 NM (3.98 statute miles).

To see another group, calculate their horizon distance and add the two together.


Since you're on sea-ice (or land-ice) you don't have a nice even surface -- terrain features will intervene.

Buffysquirrel
05-28-2012, 02:31 AM
You definitely want to read Cherry-Garrard's account of travelling during the polar night. It's horrifying.

Astronomer
05-28-2012, 07:03 PM
It's an earth-like planet with a sun and moon and the magnetic north pole some way south of the pole itself. The story takes place at the start of autumn. At present the moon is a half moon
What is the tilt of your planet's axis? This will determine the latitude of the arctic circle, which is critical to answering many of your questions.

WriterDude
05-29-2012, 03:25 AM
Slightly off topic, but this thread reminded me of Arctic mirages I read about as a child and the appearance of Bristol in the sky.

Linky (http://www.unmuseum.org/mirage.htm)

loiterer
05-29-2012, 10:23 AM
What is the tilt of your planet's axis? This will determine the latitude of the arctic circle, which is critical to answering many of your questions.

I hadn't even thought about it. Say it tilts a little less than earth, and they're approaching from the side it tilts towards, making the journey shorter. The pole is also on a landmass, I don't know if that makes any difference.

Buffysquirrel
05-29-2012, 04:20 PM
Well, being on a landmass means you don't have to worry about the ice taking you backwards :).

Kenn
05-29-2012, 08:34 PM
I hadn't even thought about it. Say it tilts a little less than earth, and they're approaching from the side it tilts towards, making the journey shorter. The pole is also on a landmass, I don't know if that makes any difference.
If they're approaching from the side tilting towards the sun, then they're approaching in summer. Planets don't have north poles permanently pointing towards the sun (it would be an unstable system). The Arctic Circle defines the limits on a planet where the polar night or midnight sun can be observed. It's got nothing to do with the north pole (only the level of axial tilt). If it happens at the start of autumn then, by definition, the lengths of day and night will be equal (unless the planet has very unearthlike eccentric behaviour).

I'm not sure why you think being on a land mass would make much difference.

I suspect you might use the stars to navigate at night.

Astronomer
05-30-2012, 12:39 AM
If its tilt is less than Earth's, then that would shrink the arctic circle. You say that the magnetic pole is somewhat south of the actual north pole. Does your magnetic pole lie outside the arctic circle? If so, then your explorers will still experience sunrise and sunset when they arrive, though the sun will never rise too far above the horizon.

If by the beginning of Autumn, you mean that the sun has just crossed the equatorial plane into the southern hemisphere (equinox), then the circle of perpetual darkness centered on the north pole will grow until the solstice, when it will encompass the entire arctic circle. Unless your explorers are extremely far north, they will still experience sunrise and sunset. But the farther north they go, and the longer it takes them, the less light they will have during the day. It's conceivable they could complete their journey in complete darkness.

If the moon orbits in the ecliptic (the plane of the planet's orbit around the sun), then a half-illuminated moon at equinox will either be X-degrees above the horizon, or X-degrees below the horizon, depending on whether it's waxing or waning. (X is the tilt of your planet's axis.) A full moon as seen from the north pole at equinox will always be right on the horizon, again assuming the moon orbits in the ecliptic.

Remember, too, that the moon's phase will change as the days go by. If your explorers take a week to reach their destination, then your half-moon should be either full or new when they arrive. (For example.)

I hope this helps.