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A polar projection would be nice. OverMyHead 07:55, 28 February 2007 (UTC)[reply]

I suggest "northern solstice" and "southern solstice" or "solstice of Cancer" and "solstice of Capricorn". The only great circle mentioned is the equator; the rest are small circles. -phma

Why are you suggesting these terms when they're not the terms that the circles are known as? -- Zoe

It seems to me that "The other great circles are:" is the part that needs to be changed here, not the circles that are referenced. --Brion VIBBER

will major circles suffice? Lir 17:38 Nov 4, 2002 (UTC)

Lir, see great circle. --Ed Poor

On second thought, the circle of latitude really could be re-written for clarity. If I didn't already know what they were, I would hardly have understood any of them. --Ed Poor

What's wrong with circle of latitude? COuld you explain on that talk page, Ed? -- Tarquin 19:43 Nov 4, 2002 (UTC)

There's an apparent error in the Arctic_circle article. This ambiguity needs corrected (whichever article is in error) and the relevant data duplicated to Antarctic_circle.

This line "The main long-term cycle has a period of 41000 years and an amplitude of about 0.68°, or 76 km on the surface." does not jibe with this info "The Earth's axial tilt varies between 21.5° and 24.5° with a 41,000 year periodicity" from Axial_tilt.

That's a 3 degree difference, not .68 degree. If the 3 degree figure is correct, this article needs corrected. It would be nice to add the distance range for the Arctic and Antarctic circles to the Axial tilt article.

Please see Talk:Arctic Circle. Thanks. TerraGreen 00:00, 7 January 2006 (UTC)[reply]

Please see the discussion at wp:Reference desk/Science#A(nta)rctic_Circle Dbfirs 13:10, 20 October 2018 (UTC)[reply]

According to [ https://www.pmel.noaa.gov/arctic-zone/faq.html ], "The Arctic Circle is defined as the latitude above which the sun does not set on the summer solstice, and does not rise on the winter solstice", so I would assume that the antarctic circle has a similar definition. According to Sunrise, "Sunrise is the moment when the upper limb of the Sun appears on the horizon". According to Sunset, "The time of sunset is defined in astronomy as the moment when the upper limb of the Sun disappears below the horizon.".

Is there a citation supporting the "centre of the sun" claim? --Guy Macon (talk) 22:53, 20 October 2018 (UTC)[reply]

Burn, Chris. The Polar Night (PDF). The Aurora Research Institute. gives a good explanation. The difference is just under one degree of latitude for visibility of the sun so the NOAA definition is only an approximation. Dbfirs 07:25, 21 October 2018 (UTC)[reply]

I think I can see what has happened here. The official definition of the Arctic circle is

The Arctic Circle is defined as the latitude above which the sun does not set on the summer solstice, and does not rise on the winter solstice.

I never heard it defined any other way up to now. The approximate latitude is 66° 34'. It is fairly simple to work it out:

Obliquity of the ecliptic = 23° 26'.

On the solstice the sun is overhead at noon on lat. 23° 26'. It will be on the horizon at noon at lat. 90 - 23° 26' = 66° 34'. The upper limb will still be visible until the centre falls more than 50' below the horizon because the semidiameter is 16' and there is 34' of refraction. By that definition, therefore, the circle should be drawn at 67° 24'.

At the other pole, the sun will be on the horizon at midnight at lat. 90 - 23° 26' = 66° 34' as before. This time, the nearer you are to the equator the lower it gets. On latitude 66° 34' at midnight after allowing for refraction and semi diameter the upper limb is at an elevation of 50'. It will therefore just skim the horizon at latitude 65° 44'. The problem with the latest edits is that they do not make it clear that they are talking about the geometric altitude, not the apparent altitude. They are equating "zenith distance 90°" with "being on the horizon", which is an elementary error that, unfortunately, too many people make. 92.23.53.182 (talk) 14:11, 21 October 2018 (UTC)[reply]

The arctic and antarctic circles are defined geometrically at approximately 66° 34' as you state. That is the astronomical definition, to match the topics of Cancer and Capricorn. The "folk definition" of the upper limb of the sun on the horizon is a good approximation, but it happens at around 67° 24' as you also state and as observations confirm, though there is variation with atmospheric conditions. I agree that we need to clarify the articles to state that it is the geometric centre of the sun, not the centre of the refracted image on the horizon. I've added a note, but if you can make it clearer, please do so. Dbfirs 15:07, 21 October 2018 (UTC)[reply]