Transcript Hour Angle and Right Ascension

Finding Your Way In the Sky
Term: sky map
The sky map shows the entire sky as viewed from a given
location at a specified time and date. A stereographic
projection is used, as is the convention for printed star
maps.
Coordinates and Time
Astronomical coordinate systems are virtually all spherical
coordinate systems; defined by a great circle and its poles. A
latitude coordinate measures the angle above or below the circle,
a longitude coordinate measures the angle along the circle from
some arbitrarily defined point.
Terrestrial (latitude/longitude)
Horizon (altitude/azimuth)
The horizon system is a "local" system
centered at the individual observer. The
great circle is the observer's horizon.
The poles of that circle are the zenith
(directly overhead) and the nadir. The
reference point on the horizon is the
north point, defined by a great circle (the
meridian) from the zenith through the
north celestial pole to the horizon.
Extending the meridian in the oposite
direction establishes the south point. The
east and west points are defined by the
intersections of the celestial equator with
the horizon.
The latitude coordinate is the
altitude (or elevation), measured
from the horizon along a great circle
running through the zenith. The
longitude coordinate is azimuth
measured either from the north
point increasing towards the east
(geographic definition) or from the
south point increasing towards the
west (astronomical definition).
Because of this ambiguity in
definition, it is important to
determine which form is in use. The
geographic definition is most
common, even in use by astronomers.
Equatorial (declination/hour angle or declination/right ascension)
The Equatorial system is defined by the
celestial equator, the projection of the
Earth's equator onto to celestial
sphere. The poles ore the north
celestial pole and the south celestial
pole. The reference point is the vernal
equinox which is the point where the
ecliptic (the apparent path of the sun)
crosses the celestial equator with the
sun moving towards the summer
solstice. The latitude coordinate is
declination measured from the celestial
equator. The longitude coordinate is
right ascension (RA) - measured from
the vernal equinox increasing in the
direction of the sun's motion; ( 0 <= RA
< 24 hours ).
Hour Angle and Right Ascension
The introduction of hour angle and right ascension brings us to a
transition between coordinates and timekeeping. An observer has a
meridian running overhead from north to south. Objects rise in the
east, transit the meridian, and set in the west. An object on the
celestial equator (declination = 0) takes 12 hours to cross the sky
from east to west.
Let's actually build a
celestial sphere.
First, consider the
sky in relation to the
earth. Take the north
and south poles and
extend them into the
sky; these become
the north and south
celestial poles. The
earth's equator can
be projected outward
to form the celestial
equator.
Now consider yourself as
standing on top of the
earth. Your zenith is the
point directly above your
head; your horizon is a
circle perpindicular to the
line between you and the
zenith, and marks the
boundary of the hemisphere
of the sky which you can
actually see.
Now, to position the celestial poles with respect to your
location on the earth, you have to place them at an angle
that will match your lattitude.
When you're done, you should have a celestial sphere very
like the one at the top of this section.
Celestial Coordinates
The earth is spinning on its axis once every 24 hours. The
axis of the earth is also moving in a circular motion but
maintains a constant 23.4° tilt. This in effect makes the
earth like a wobbling gyroscope. Compounding things, the
earth is also moving around the Sun once a year and the
Sun is moving around our galaxy every 220 million years.
One more problem, the galaxy is moving through space. In
essence everything in the universe is moving constantly in
different direction and at different speeds
Because of the slow movement of just about everything in the
universe, we can catalog objects in the universe by devising a
system that allows us to locate and establish the objects
position.
Declination
Right Ascension
Alt-Azimuth
Angular Measure
Altitude
Zenith
Longitude and Latitude
Meridian
Nadir
Celestial Equator
Horizon
Celestial Pole
Angular Measure
You will need to know how to specify a direction by angular measure
Angles
Astronomy involves thinking about angles-often very tiny, tiny angles. A circle is
divided into 360 degrees (or 360°). Each
degree is divided into 60 arcminutes (or
60'). Each arcminute can be divided into
60 arcseconds (or 60"). Therefore there
are 360x60x60 = 1,296,000 ~ 1.3 x 106
arcminutes in a full circle. Yes, this seems
a crazy system and not very `metric'--it
goes back to the Babylonians who were
hung up on the number 60. No one has
been bothered to convince everyone else
to make a better system
Since we know the Earth has to rotate completely in 24
hours, and one entire rotation means circling 360°, the rate
in degrees that the Earth rotates per hour is:
360 degrees 15 degrees
Rotation rate = ------------- = -----------24 hour
hour
Or 15° per hour
There are angles like longitude and latitude on the
celestial sphere that describe the location of a star,
such as Betelguese. These angles are called Right
Ascension and Declination.
1) Go outside and find
Polaris and the celestial
pole in the night sky. The
easiest way is to look
roughly north and use the
2 "pointer stars" of the
Big Dipper.
2) Point with your left arm to
Polaris. Make a right angle to your
left arm with your right arm. Your
right arm is now pointing to
somewhere on the celestial equator.
Keeping your left arm pointing to
Polaris and your right arm at a
rightangle, rotate about your left
arm--your right arm will be sweeping
out the celestial equator.
(3) Point to the south celestial pole.
(4) Look for a bright star (preferable 2 or 3 different ones)
that is fairly close to the horizon or an object on the skyline-such as the roof of a house or a tree or a lampost. Note where
you are standing, where the star is located and the time. Come
back in about 1 hour and note how the stars have moved.