At 11:49 PM 8/4/03, you wrote:
OK you did your cut and paste but what the hell is
magnitude of -2.9 and will appear 25.11 arc seconds wide?
Daniel
Magnitude is the brightness of a celestial object. It is based on star
brightness. Counter-intuitive, the lower the magnitude number the brighter
the object. The brightest stars are magnitude -1. It is 2.512 time
brighter than a magnitude 0 star. Thus, a magnitude of -2.9 means it will
be roughly six times brighter than the brightest star (-1 magnitude).
The "magnitude" system dates to Hipparcus and Ptolemy when they assigned
magnitude numbers of 1 through 6 to the stars they could see with the
unaided eye. They assigned first magnitude to a set of about 20 of the
brightest stars they could see from their location. Sixth magnitude stars
were just barely visible to the unaided eye under the most favorable
conditions (very dark sky). Today the magnitude system assigns -1 to the
brightest star(s).
An arc-second is an angular measure. There are 360 degrees to a circle, 60
minutes to a degree, and 60 seconds to a minute. To distinguish the use of
minutes and seconds of angular measure from time measure, they are referred
to as arc-seconds and arc-minutes. 25.11 arc-seconds is 0.006975
degrees. Angular measure is used to describe the diameter of a celestial
object (typically planet or planetary moon) as viewed from Earth. That and
its distance can be used to determine its absolute diameter using simple
trigonometry. The utility of describing size by angular measure is in
informing someone who wishes to observe the object how much magnification
will be required (if any) to make it visible, and how much magnification
will be required to make it appear a specific size. If only provided with
absolute diameter (e.g. in miles), one would have to know its distance to
calculate how big it will appear. It's much easier with direct information
about how big it appears to an Earth observer without regard to
distance. Furthermore, in very early astronomy it was much easier and more
immediate to measure size through a calibrated telescope using a reticle in
terms of angular measure (and still is). Measuring distance of an object
requires several measures of object location against the most distant
starfield over time as the Earth orbits the sun (e.g. one at the Vernal
Equinox and another at the Autumnal).
-- John
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