please explain me RA & dec, alsoarc seconds and solid angle!!!!!!!!!!1?

Answer 1

don't understand what the first part you are talking bout is, but there are 60 minutes in a degree and 60 secons in a minute

Answer 2

Right ascencion (RA) and declination are ways of describing the position of objects in the sky, just as longitude and latitude are on the surface of the Earth.

The celestial sphere is an imaginary sphere surrounding the Earth. The celestial equator is above the Earth's equator, and the celestial poles are above the Earth's poles. So far, so good. We can describe the position of any object in the sky by giving two coordinates, just as we can describe the position of any object on the surface of the Earth. But wait - isn't space three-dimensional? It is indeed (as is the Earth!), but the third dimension is the distance the object is away from us, and we don't need to know that to find it in the sky.

So, OK. Why don't we just use longitude and latitude then? Well, latitude would be fine, but longitude has a problem - the Earth rotates. This means the stars don't stay in one position overhead; we'd like to have the coordinates of a star stay the same regardless of the Earth's rotation, so we need a coordinate system that, from our point of view, rotates along with the stars.

So that's our coordinate system! For historical reasons, latitude on the celestial sphere is called declination, and longitude is called right ascension. Just as with longitude, the "zero point" of right ascension is arbitrary; we use the position of the Sun at the vernal equinox.

One more complication - although declination is measured in degrees, just as latitude is, right ascension is measured in hours, minutes, and seconds. It's just a different way of dividing up a circle - instead of 360 degrees, you divide it into 24 hours, each of which has 60 minutes, each of which has 60 seconds (as usual). This is the only way that this "hour measure" is used, though; unless you are looking at a right ascension coordinate, angles are measured in degrees, arc-minutes, and arc-seconds.

Which brings up your second question - fortunately we've almost answered it already! The standard angular measure divides the circle into 360 degrees (you can thank the Babylonians for this). Each degree is divided into 60 arc-minutes, and each arc-minute is divided into 60 arc-seconds. That's it!

Finally, solid angles are just like regular angles, except they are three-dimensional. How does that work? Well, an angle in radians is defined to be the ratio of the arc length to the radius (note that radians are NOT degrees! A circle has 2 pi radians, but 360 degrees). How do I know a circle has 2 pi radians? The circumference of a circle is 2 pi r, so if I divide that by the radius, I get 2 pi.

The definition of a solid angle is analogous - it is the area subtended by the solid angle, divided by the square of the radius. A complete solid angle is a sphere, and subtends a solid angle of 4 pi steradians (this is the definition of a steradian).

Hope this helps!

Answer 3

Right ascension and declination are the equivalents of longitude and latitude on earth. Just imagine projecting latitude and longitude lines on earth into the sky and there you go. It's just a coordinate system on the "celestial sphere" to locate stars, planets, etc. in the sky.

1 arcsecond = 1/60 arcminute
1 arcminute = 1/60 degree

Solid angle measures the surface area of an object relative to a viewpoint ... i.e. how big does an object look to an observer. Just look it all up on wikipedia ... I'm sure it's all there.

Answer 4

Right ascension (RA) and declination (dec) are the astronomical equivalent of longitude. The Earth's equator and poles are projected onto the sky to align this system. So 0° dec is the celestial equator, and consists of all the points that pass directly overhead at the equator. 90° north dec is the point directly above the north pole. RA is usually measured not in degrees but in hours:minutes:seconds of time. The RA of an object on your meridian (i.e. due south of you) is equal to your local sidereal time. One hour of RA on the equator corresponds to 15° of arc, or 1° = 0:04:00 (4 minutes of RA). 0:00:00 RA is defined as the sun's position at the vernal equinox. Since this precesses over time relative to the stars, a star chart has to specify what "epoch" it uses for its positions. Modern charts are all referenced to the year 2000.

Degrees of arc are divided into 60 minutes of arc each, and each minute is divided into 60 seconds of arc. So an arc second is 1/3600th of a degree. Minutes and seconds of arc are written as ' and " respectively: 1° 10' 15" = one degree, ten minutes, fifteen seconds. The use of minutes and seconds to measure angles can be a little confusing in combination with RA measurements in time units: one minute of arc (0° 1') on the equator is equal to 4 seconds of RA (0:00:04), and one minute of RA (0:01:00) is equal to 15 minutes of arc (0°15').

Solid angle is a way of measuring areas in the sky. I'm just going to paste in the Wikipedia article ( http://en.wikipedia.org/wiki/Solid_angle ):

The solid angle, Ω, that an object subtends at a point is a measure of how big that object appears to an observer at that point. For instance, a small object nearby could subtend the same solid angle as a large object far away. The solid angle is proportional to the surface area, S, of a projection of that object onto a sphere centered at that point, divided by the square of the sphere's radius, R. (Symbolically, Ω = k S/R², where k is the proportionality constant.) A solid angle is related to the surface area of a sphere in the same way an ordinary angle is related to the circumference of a circle.

If the proportionality constant is chosen to be 1, the units of solid angle will be the SI steradian (abbreviated sr). Thus the solid angle of a sphere measured at its center is 4π sr, and the solid angle subtended at the center of a cube by one of its sides is one-sixth of that, or 2π/3 sr. Solid angles can also be measured (for k = (180/π)²) in square degrees or (for k = 1/4π) in fractions of the sphere (i.e., fractional area).

One way to determine the fractional area subtended by a spherical surface is to divide the area of that surface by the entire surface area of the sphere. The fractional area can then be converted to steradian or square degree measurements by the following formulae:

1. To obtain the solid angle in steradians, multiply the fractional area by 4π.
2. To obtain the solid angle in square degrees, multiply the fractional area by 4π × (180/π)², which is equal to 129600/π.