Conceptos basicos
Ancient astronomers perceived the sky as a large sphere with the Earth at its center.
They thought the stars were attached to the surface of this great sphere, and as it rotated once each
day, the stars would rise and set as they were carried across the sky.
We know today that this sphere is not real.
The stars and planets are at great distances from the Earth, and their apparent daily motion across
the sky results from the Earth's spinning on its axis.
But this image of a celestial sphere which surrounds the Earth at some great distance remains a
useful concept.
Astronomers have created a grid of reference lines and points on the celestial sphere to describe the
position of each star, planet, and galaxy.
Every object has a numerical address in the sky and that address is on the celestial sphere.
We use a coordinate system of longitude and latitude to locate any point on the surface of the Earth.
On the celestial sphere we use a similar system called equatorial coordinates, which are based on
the Earth's poles and equator.
The Earth's axis of rotation, extended outward from the North Pole, intersects the celestial sphere at
a point called the North Celestial Pole.
The star closest to this point, Polaris, is often called the North Star.
A similar extension from the South Pole marks the South Celestial Pole.
The Celestial Equator is the projection of the Earth's equator onto the celestial sphere.
All points along the celestial equator are equidistant from the north and south celestial poles.
To define the location of an object on the celestial sphere, we first determine its angular position
along the celestial equator.
This coordinate is right ascension, and it is analogous to longitude on the Earth.
As Greenwich marks the zero of longitude on Earth, the vernal equinox is the zero of right ascension
in the sky.
This point marks the Sun's position on the first day of spring, when the Sun Page 2 crosses the
celestial equator.
The vernal equinox is currently in the constellation of Pisces, but the position changes slowly over
time.
Right ascension is measured eastward in units of time (hours, minutes, and seconds), starting at 0
hours and continuing to 24 hours.
An hour of right ascension is equal to 15 degrees on the celestial equator (1/24 of 360 degrees) The
second equatorial coordinate is declination, and it measures an object's angular distance from the
celestial equator.
Objects above the celestial equator have positive declination, and those below have negative
declination.
The position of any object in the sky can be defined by the coordinates of right ascension
(abbreviated RA) and declination (abbreviated Dec).
All celestial objects have an address on the celestial sphere - their RA and Dec.
Altazimuth coordinates describe the position of an object with reference to the local horizon.
Points below the horizon have negative altitudes.
Altitude is sometimes called elevation, but in astronomy, elevation refers to the distance one is
above sea level (in feet or meters).
When you point directly upward, you are pointing toward the zenith.
Point downward, and you point toward the nadir.
The meridian is the line that passes through the celestial poles and the zenith.
It is the projection of the observer's Earth longitude on to the sky.
A transit occurs when an object passes across the meridian.
At this time, the object is at its maximum altitude in the sky.
The altazimuth coordinates of an object are local coordinates.
Stars very far south near the South Celestial Pole do not rise at all, and they remain unseen for
observers in the north.
As the Earth orbits the Sun, we see the Sun moving against the background stars.
The planets are always seen within about 18 degrees of the ecliptic, since they orbit the Sun in
nearly the same plane.
Ancient observers organized these groups into constellations and named them after particular
animals.
These constellations came to be known as the sign of the zodiac, and they are a center piece of
astrology.
During the summer months in the Northern Hemisphere, the axis leans towards the Sun resulting in
long days and short nights.
The first day of summer occurs about June 21 each year; this is the summer solstice.
At the same time in the Southern Hemisphere, the axis leans away from the Sun.
It is the elapsed time in hours, minutes, and seconds since the vernal equinox crossed the meridian.
The right ascension that is currently on the zenith is the local sidereal time.
Universal Time (UT) is based on Greenwich Mean Time.
UT is a 24 hour clock widely used by astronomers and navigators.
Universal Time is based on the rotation of the Earth.
It includes a correction for the slight wandering of the Earth's geographic pole.
The local time in New York is 12 hours behind the time in Singapore, but both cities report the same
Universal Time.
The Earth's rotation is slowing down by about 1 second per year, so the average length of the day is
increasing.
Precise astronomical calculations require a dynamical time scale that is free of the irregularities
caused by the Earth's rotation.
Dynamical time is based on atomic clocks.
Universal time is continuously falling behind dynamical time.
In the year 1990, the difference was about 57 seconds.
In 2010, UT will lag behind dynamical time by 67 seconds.
Astronomcal events that occurred several thousand year ago, such as an ancient eclipse, can be
calculated with good precision.
The date of the event is in terms of dynamical time.
Knowing the UT time of the event requires careful estimates of how the Earth's rotation has slowed
over the centuries.
These estimates lead to time corrections of several hours for events in Roman times, and larger
corrections of more than a day for earlier epochs.
Astronomers use a date system that is independent of the civil calendar.
The Julian date is a continuous enumeration of days starting at Greenwich noon on January 1, 4713
B.C.
A particular Julian date is referred to as the Julian day number.
Since the starting Julian date is nearly 7000 years ago, any likely event under discussion will have a
positive Julian day number.
One advantage of using the Julian date is that the time interval between two events is found by
simple subtraction.
Moreover, there are direct algorithms to convert any Julian day number to a calendar date.
If you reckon time using the Julian date, there is no Y2K problem.
January 1, 2000 has no special designation or significance.
The brightness of the stars was first estimated by the Greek astronomer Hipparchus in the second
century B.C. Hipparchus' system divided the stars into five magnitude classes.
The brightest stars were first magnitude, and the faintest stars were fifth magnitude, with all others
falling somewhere in between.
This system has been formalized and extended in modern times to apply to all celestial bodies.
In general, the brighter an object, the smaller the number that represents its magnitude.
The brightest objects, like the Sun or the Moon, actually have negative magnitudes.
The world's largest telescopes are capable of detecting objects of magnitude 25, and the Hubble
Space Telescope reaches magnitude 28 and fainter.
Measuring the apparent magnitude of an object involves several factors: the object's distance
(objects farther away appear fainter); the wavelengths considered (some objects are brighter at
infrared or ultraviolet wavelengths than in visible light); the sensitivity of the detector (the eye is less
sensitive to blue light than standard photographic film).
With absolute magnitude, one can compare the "true" brightness of the stars, since the comparison
is always at the same distance.
The color is an indication of the star's temperature and spectra.
The spectrum of a star usually has a sequence of dark lines marking wavelengths that are absorbed
by the chemical elements in the star's atmosphere.
For example, you will see stars of spectral type B9, G4, and K3.
Principle Spectral Types type color temperature description example O blue 25,000 to 40,000 strong
lines of ionized helium Zeta Orionis and highly ionized metals B blue 11,000 to 25,000 lines of
neutral helium, Spica weak hydrogen lines Rigel A blue 7,500 to 11,000 strong lines of hydrogen,
Vega to white ionzed metals, weak helium lines F white 6,000 to 7,500 hydrogen lines weaker than
Canopus strong line of neutral metals G white 5,000 to 6,000 hydrogen lines weaker than Sun to
yellow type F, strong lines of calcium Capella and other neutral metals K orange 3,500 to 5,000
numerous lines of neutral metals Arcturus to red M red 3,000 to 3,500 numerous lines of neutral
metals, Antares strong molecular bands The O and B stars are hot and blue.
The brightest of these are the red giants such as the stars Antares and Betelgeuse.
Within a given spectral class, stars are divided into luminosity classes.
Over a star's lifetime these properties change as it evolves to a final fate determined by its mass.
In the first decade of the 20th century, the Danish astronomer Hertzsprung and American
astronomer Russell made the first study comparing the brightness (absolute magnitude) to the
spectral type of the stars.
In 1914 they published what has come to be known as the Hertzsprung-Russell (HR) diagram.
For nearly a century the HR diagram has been a powerful visual tool in understanding the properties
and evolution of stars.
Modern astronomy has revealed that the initial mass of a star is the key to understanding a star's
evolution.
A relatively low mass star like the Sun will eventually become a white dwarf.
------------Summarized by Copernic Summarizer