#celestial coordinates

[biosemiotica]

Animation showing how celestial and geographic coordinates are mapped on an astrolabe's tympan through a stereographic projection. Hypothetical tympan (40° north latitude) of a 16th-century European planispheric astrolabe.

[astroquizzical]

How much overlap is there between the night skies of someone in Sweden and South Africa?

"Hello and thank you for enlightening our minds. I have a very important question that triggers non stop my mind. Can someone standing in South Africa see the same sky of that one standing in Sweden?"

Their skies will never be exactly the same, but they’ll have more in common than you might guess! 

To understand this, we need to zoom out from the Earth a little, and remember that the Earth is (roughly speaking) a sphere, and so the skies above us are a little different depending on where you’re standing on it. The easiest and most extreme place to start is at the poles. 

Say you teleport yourself to the North Pole. “Up” is now due North (the technical term for “up” is the zenith), and the North Star, Polaris, will shine down on you from a point in the sky directly above you. Excluding variables like mountain ranges and trees, in an open space, everyone, regardless of where they stand, gets 180 degrees of sky. From a point directly above you, you can trace a 90 degree angle in any direction down before you hit the horizon. So our North Pole observer has as their horizon, 90 degrees away from North, before their vision is obstructed by the ground. 

Now, if you have a very adventurous friend willing to traverse Antarctica’s mountain ranges to reach the South Pole, they will have the amount of sky above them. Due “up” is still the zenith for this observer, though for them, an arrow pointing to their zenith will point exactly South. This southern observer also has 180 degrees of sky above them. 

If we compare these two folks from a very large distance (or from a convenient diagram), we can see that their heads are pointing away from each other, while their feet are respectively planted on solid ground (metaphorically speaking; the North Pole doesn't have any.). If we let the Earth turn for a 24 hour period, the sky above them would rotate smoothly around the northern and southern poles, spinning like a top above you. 

Since there are only 360 degrees in a circle, these two people have a set of 180 degrees each, with no overlap. They could both map the sky, and come back together, and compare maps, and find zero points in common. (They would, however, have fully mapped the sky.) 

Generally, however, people do not get to observe the sky from the Northern or Southern poles, so we can progress to a less extreme example. If you move away from the poles some, and more towards loci of human habitation, the general setup is the same; everyone gets 180 of sky above them, with their own zenith due “up” above them, and 90 degrees of sky in any direction to their own personal horizon. 

So let’s go to this picture of someone in Sweden (I’ve picked Stockholm) and South Africa (Cape Town), and what their locations are. Stockholm is about 59 degrees north of the Equator in the Northern Hemisphere, and Cape Town is 34 degrees below the equator in the Southern Hemisphere. Now when we look at their respective directions, their heads are both pointed out away from the Earth, but at an angle with respect to each other (indicated by the up arrows in the diagram below). 

The southern horizon for our observer in Stockholm extends 90 away from “up”. Up for them is 59 degrees above the equator. 59 - 90 is -31, or 31 degrees south. Our Cape Town observer can observe 90 degrees north of 34 degrees south, or 56 degrees north. 

In fact, if you add these two positions together, to find the total angle between them, you get 93 degrees. With 180 degrees of sky, that makes for 87 degrees of overlapping sky between them. This is very nearly 90, which is easier to think about.

This means that the southern half of the sky, to our Stockholm observer, will also be seen by our Cape Town observer — but as the northern half of their sky. (It will also look upside down, if they were to swap places.) The northern half of the sky in Stockholm won’t be visible to the Cape Town observer; Polaris would be beneath their feet. And likewise, the southern half of the sky to the Cape Town observer will be unique to them; just as the observer from the North Pole can’t see the stars near the south pole, a Stockholm observer won’t see the southern constellations. 

This is why astronomers value having telescopes in both the northern and southern hemispheres! Between the northern hemisphere facilities and their southern hemisphere counterparts, we can point a powerful tool at anything in the sky that we’re curious about.

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[mitchbattros]

Meteor Activity Outlook for June 22-28, 2013

Meteor Activity Outlook for June 22-28, 2013

http://earthchangesmedia.com/meteor-activity-outlook-for-june-22-28-2013

During this period the moon reaches its full phase on Sunday June 23th. At this time the moon is located opposite the sun and remains above the horizon for most of the night. As the week progresses the waning gibbous moon will rise later and later but will still be high in the sky during the…

During this period the moon…

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