IS the horizon horizontal or perpendicular?
611133
Curved. It is the last of the land that the vision can see lining with the sky. Since earth is spherical, the horizon has to be curved.
767544
Let r=the earth's radius and h=your height.\n\nDraw a right triangle ABC, where AC = length r, BC = length r+h, and <A is the right angle and angle C is very small.  C is the center of the earth, and BA is tangent to the earth.  BA is the distance from your head to the horizon.  By Pythagoreas' theorem, \n\nlength(BA) = sqrt((r+h)^2 - r^2)\n= sqrt(2rh + h^2).
551537
The line that connects the two points of the crescent Moon is always almost at right angles to the path of the Moon along the sky. If the Moon goes up almost straight from the horizon (as it does when seen from the equator), then the crescent appears horizontal. If the Moon rises at a shallow angle (as seen far from the equator), then it moves as well along the horizon towards the west, and then the crescent is mostly vertical. The Moon rises almost vertically as seen from the equator because the orbit of the Moon stands approximately above the equator.
540627
Not an uncommon thing to see. The "sky" is laid out in compass points and most commercial air craft are assigned flight paths that are like "high ways" in the sky. Generally you will find those "trails" are on true points, 00%,45%,90% etc. The reason for the near perfect "X" is due to the vectors the two were heading on.
736872
It's an optical illusion. When it's near the horizon there are points of reference within your field of vision so your brain can correctly assess the size of it whereas high in the sky, there's nothing for the brain to use to assess size.
559805
9 miles\n\nThat's if you were standing at the edge of a calm stretch of water (and you're approximately 5 foot 8 inches tall) - then the horizon is approx. nine miles away.\n\nObviously, if you gain more height you'd see further, and you could actually see things beyond this horizon - if the things are tall enough!
1035386
Astronomers generally use either the equatorial or ecliptical reference frames. The standard reference plane for the equatorial reference frame is the earth's equator. The standard reference plane for the ecliptical reference frame is the plane of the earth's orbit around the Sun. The intersection of these two planes is a line that points in one direction towards the vernal equinox, and in the other direction towards the autumal equinox.\n\nIn both systems, the zero point of longitude (for ecliptic) or Right Ascension (for equatorial) is defined as the direction of the vernal equinox, also called the First point of Aries.
539862
Yes, but to a degree. The horizon line is going to be different, so you will see some of the stars, although a bit displaced, that the Northern hemisphere sees, and you will see a lot more that is only visible to the Southern hemisphere.
466473
The answer to this all depends on how high your head is off the ground.\n\nThis is an excellent page where you can type in how tall you are, and it will tell you what the straight line distance to the horizon is, and the curved distance.\n\nhttp://newton.ex.ac.uk/research/qsystems/people/sque/physics/horizon/
715400
Angle of line from horizontal position.
733444
horizontal is left to right \nvertical is up and down
473627
on a graph every point has an x and y coordinate.  The vertical(up-down) is the Y-coordinate, the horizontal (left-right) is the x-coordinate
1056603
http://www.wsanford.com/~wsanford/exo/sundials/horizontal_sundial_setup.html\n\nhere's how\n\nA horizontal sundial consists of the dial plate, marked off in hours, and the gnomon (including the style) that sits on the meridian (noon hour line) and sticks up from the dial plate at a right angle relative to the plane of the dial face. Place the sundial on a horizontal surface (hence the name, "horizontal sundial") and align the gnomon with true north (on the dial face, 12 noon should point north). \nFinding the Direction of True North \n\nUse a compass (compensate for magnetic declination). \nAlign with Polaris, the North Star.
667991
A horizontal line means that the x-coordinate can be any real number while the y-coordinate has to stay the same no matter what x value.  Thus, the point (-.5,2) has x-coordinate -.5 and y-coordinate 2.  Thus the horizontal line through that point has equation y=2.
59958
Could be the "Moon Illusion" - since often, the moon will appear closer to the horizon in the northern hemisphere than in the southern, the same reason it looks bigger near the horizon at any latitude applies.  Read the link below for more info on what the moon illusion is.
1014055
you would probably use spherical coordinates instead of cartesial coordinates.\n\nIn spherical coordinates, you map directions based on angles from a reference.  you measure an angle in your horizontal plane and then, from that vector, an angle in the vertical plane.
382880
Look at this page,\nhttp://mintaka.sdsu.edu/GF/explain/atmos_refr/horizon.html\nIt explains it very well mathmatically.\n\nThe answer would of course depend on the person's height above the Earth's surface and on the radius of the Earth, both of which are know/measurable.